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Object-Oriented Programing in Java. Cheng-Chia Chen. Contents. Object and Class The contents of an object/class Creating and initializing Objects Accessing object data and methods Destroying and finalizing objects Subclass and inheritance Interfaces Java modifiers summary.

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  • Object and Class
  • The contents of an object/class
  • Creating and initializing Objects
  • Accessing object data and methods
  • Destroying and finalizing objects
  • Subclass and inheritance
  • Interfaces
  • Java modifiers summary
what is an object
What is an Object ?
  • Real-world objects:
    • Concrete objects: Apple1, Car1, TV2, Teacher2, Student3, …
    • Conceptual Objects: 1, 2.3, Date1, Meeting2, point2, …
  • Objects has:
    • Properties (attributes): color, weight, height, sex, name, speed, position,…
    • Capabilities (behaviors): can receive commands(, request, query) and respond (do actions) based on its internal states to change its internal state and/or external environment.
  • The properties of an object constitutes its current state.
what is a software object
What is a Software object ?
  • a software bundle of data and functions used to model real-world objects you find in everyday life.
  • In OOP, Software objects are building block of software systems
    • program is a collection of interacting objects
    • objects cooperate to complete a task
    • to do this, they communicate by sending “messages” to each other
  • Software objects can model
    • tangible things: School, Car, Bicycle,
    • conceptual things: meeting, date
    • Processes: finding paths, sorting cards
  • Note: Software objects are only abstraction of real-worldobjects; properties and behavior of irrelevance will not be modeled in software objects.
what is a java object
What is a Java Object?
  • In Java, an object consists of 0 or more fields and 0 or more methods.
    • Fields are used to model properties.
    • Methods are used to model capabilities.
  • Fields are variables.
    • like the fields of a C struct.
    • An object without methods is equivalent to a C struct.
  • A method is similar to a C function.
    • Normally, it will operate on the fields of the object.
    • These fields are accessible by name in the method.
  • Java variables can not hold objects, only references to them.
    • Object do not have a names.
    • Object are created only at runtime.
  • Given a reference r to an object, the syntax for accessing a field is: r.field_name, the syntax for accessing a method is: r.method()
classes and objects
Classes and Objects
  • Current conception:
    • a java/software object  a real-life object,
    • e.g., a Java car  a real car
  • Disadvantage: impractical to work with objects this way
    • may be indefinitely many (i.e., modeling all atoms in the universe)
    • do not want to describe each individual separately, because they may have much in common
  • Classifying objects into classes of similar properties/behaviors
    • factors out commonality among sets of similar objects
    • lets us describe what is common once
    • then “stamp out” any number of copies later
    • Ex: Student: { S1, S2, S3 } Course:{C1,C2,C3} Teacher:{ T1,T2}
    • but not {s1, t1}, {s2, t2}, {c1,c2,c3,s3}
  • Analog:
    • stamp 印章 (class)
    • Imprints 戳印 (objects)
what is a java class
What is a Java Class?
  • In Java, a class is a template (textual description) of a set of similar objects.
    • All objects in the class have the same types of properties and the same set of capabilities.
  • It defines the fields and methods that all objects in that class will have.
    • Classes have names.
    • Class appear in the text of your program.
    • A java program consists of a set of classes.
  • A defined class is a Java Type, so you can have objects or variables of that type.
an example the class of circles
An Example: the class of Circles:
  • Properties: a circle can be described
    • by the x, y position of its center and
    • by its radius.
  • Methods: Some useful operations on Circles:
    • compute circumference,
    • compute area,
    • check whether points are inside the circle,
    • etc.
the circle class
The Circle class
  • By defining the Circle class (as below), we create a new data type.

// The class of circles (partially defined)

class Circle

{// Fields

double x, y;

double r;

// Methods

double circumference() { return 2 * 3.14159 * r; }

double area() { return 3.14159 * r * r ; }

void scale(double multiplier) { r *= multiplier; }

void print() {

System.out.println("circle of radius "+ r +

" with center at (" + x + "," + y + ")“ ); }


creating objects
Creating Objects
  • In Java, objects are created by the new operator.
  • For example:

//define a variable to refer to Circle objects;no objects yet.

Circle c ; // c is null now

// create a circle object and make the variable refer to it

c = new Circle();

// define variable and create Circle object all at once

Circle d = new Circle();

Note: the fields in these objects are given default values at creation (0 for numbers; null for object references)

accessing object data
Accessing Object Data
  • We can access data fields of an object (subject to visibility restrictions -- see later).
  • For example:

// create a new Circle

Circle c = new Circle();

//initialize our circle to have center (2, 5) and radius 1.0.

c.x = 2.0;

c.y = 5.0;

c.r = 1.0;

// create another circle

Circle d = new Circle();

//initialize this circle to have center (10,7)and radius 4.0.

d.x = 10.0;

d.y = 7.0;

d.r = 1.0;

using object methods
Using Object Methods
  • To access the methods of an object, use same syntax as accessing the data of an object:

Circle c;

double a;


a = c.area(); // Not a = area(c);


  • Each method has a signature, which is defined by:
    • method name
    • types, number, and order of arguments
  • Each class can define several methods with same name and different arguments (overloading).
  • Every class in Java has at least one constructor method, which has the same name as the class.
  • The purpose of a constructor is to perform any necessary initialization for new objects.
  • Java provides a default constructor that takes no arguments and performs no special initialization (i.e. gives objects default values).
    • Note: Java compiler provide the default constructor: className() only if you do not provide any constructor at your class definition.
  • For example:

Circle c = new Circle();

defining a constructor
Defining a Constructor
  • Can define additional constructors for initialization.

// The circle class, with a constructor

public class Circle

{ public double x, y, r;

// Constructor method

public Circle(double x, double y, double r)


this.x = x;

this.y = y;

this.r = r;


// Other methods ... as above



defining a constructor cont
Defining a Constructor (cont.)
  • With the new constructor, we can create and initialize a Circle object as:

Circle c = new Circle(2.0, 5.0, 1.0);

  • A constructor is like a (static) method whose name is the same as the class name.
  • The return type is an instance of the class.
  • No return type is specified in constructor declarations, nor is the void keyword used.
multiple constructors
Multiple Constructors
  • A class can have any number of constructor methods.

public class Circle


public double x, y, r;

// Constructors

public Circle(double x, double y, double r)

{ this.x = x; this.y = y; this.r = r; }

public Circle(double r)

{ x = 0.0; y = 0.0; this.r = r; }

public Circle(Circle c)

{ this.x = c.x; this.y = c.y; this.r = c.r; }

public Circle()

{ x = 0.0; y = 0.0; r = 1.0; }

// Other methods ... as above

... }

multiple constructors cont
Multiple Constructors (cont.)
  • With the new constructors, we can initialize circle objects as follows:

Circle c1 = new Circle(2.0, 5.0, 1.0);

// c1 contains (2.0, 5.0, 1.0)

Circle c2 = new Circle(3.5);

// c2 contains (0.0, 0.0, 3.5)

Circle c3 = new Circle(c2);

// c3 contains (0.0, 0.0, 3.5)

Circle c4 = new Circle();

// c4 contains (0.0, 0.0, 1.0)

  • All uninitialized data receives default values.
invoking one constructor from another
Invoking one constructor from another
  • We can use this(…) in a constructor to invoke other constructor.

public class Circle {

public double x, y, r;

// Constructors

public Circle(double x, double y, double r)

{ this.x = x; this.y = y; this.r = r; }

public Circle(double r)

//{x =0.0; y = 0.0; this.r = r;} replaceable by

{this(0.0,0.0,r); }

public Circle()

//{x = 0.0; y = 0.0; r = 1.0;} replaceable by

{this(1.0); }

... }

Note: do not result in recursion.

object and object references


Object and Object References
  • A java object is a memory structure containing both data and methods
  • An object reference holds the memory address of an object
  • Rather than dealing with arbitrary addresses, we often depict a reference graphically as a “pointer” to an object

ChessPiece bishop1 = new ChessPiece();

assignment revisited











Assignment Revisited
  • The act of assignment takes a copy of a value and stores it in a variable
  • For primitive types:

int num1 = 5, unm2 = 12;

num2 = num1;

reference assignment







Reference Assignment
  • For object references, assignment copies the memory location:

bishop2 = bishop1;

  • Two or more references that refer to the same object are called aliases of each other
  • One object (and its data) can be accessed using different variables
  • Aliases can be useful, but should be managed carefully
  • Changing the object’s state (its variables) through one reference changes it for all of its aliases
destroying objects
Destroying Objects
  • When an object no longer has any valid references to it, it can no longer be accessed by the program
    • It is useless, and therefore called garbage
    • Java performs automatic garbage collection periodically, returning an garbage object's memory to the system for future use
    • Hence it is needless to explicitly destroy objects.
  • How can references to objects '' go away'‘ ?
    • Re-assigning object variables (a = b; a = null) or
    • object variables going out of scope.
  • no more malloc/free bugs
object finalization
Object Finalization
  • A constructor method performs initialization for an object; a Java finalizer method performs finalization for an object.
  • Garbage collection only help freeing up memory.
  • But there are other resources needed to be released.
    • file descriptors, sockets, lock, database connection.
example a finalizer method from the java fileoutputstream class
Example: A Finalizer Method from the Java FileOutputStream class.


* Closes the stream when garbage is collected.

* Checks the file descriptor first to make sure it is not already closed.


protected void finalize() throws IOException {

if (fd != null) close();


notes about finalize
Notes about finalize()
  • invoked before the system garbage collects the object.
  • no guarantees about when a finalizer will be invoked, or in what order finalizers will be invoked, or what thread will execute finalizers.
  • After a finalizer is invoked, objects are not freed right away.
    • because a finalizer method may "resurrect" an object by storing the this pointer somewhere.
  • may throw an exception; If an uncaught exception actually occurs in a finalizer method, the exception is ignored by the system.
  • No ‘class Finalization’ method defined.
classes v s objects
Classes v.s. Objects
  • two of the most frequently occurring terms in the OO programmer's vocabulary.
  • A class An object...
  • exist at compile time exists at runtime only
  • a template/pattern created/instantiated from

for objects a class' specification

  • "only exists once" can be created many times

from one class

  • a .java file returned by the new


  • dress pattern dress
  • architectural plans house
  • stamp imprints
types of variables and methods in a java class
Types of variables and methods in a Java class
  • There are two main types of variables/fields:
    • instance variables
    • class variables
    • Instance variables store information pertaining to one particular object's state
    • Class variables store information pertaining to all objects of one class
  • Likewise, there are two types of methods:
    • Instance methods
    • Class methods
    • Instance methods belong to individual objects; whereas class methods belongs to the whole class.
    • Note: In class method, you cannot use this and instance variables.(why?)

Class diagram of an Account class.

Class Name

Instance Methods


Class Methods

declare class field method with the static modifier
Declare class field/method with the ‘static’ Modifier
  • makes methods and variablesbelong to the class rather than instances of the class.
  • Example: counting how many circles:

public class Circle

{ public double x, y, r; // instance variables

// ncircles is class variable

public static int ncircles = 0;

// Constructors

public Circle(double x, double y, double r)

{ this.x = x; this.y = y; this.r = r;

ncircles++; }

public Circle(double r)

{ x = 0.0; y = 0.0; this.r = r;

ncircles++; }


the static modifier cont
The static Modifier (cont.)
  • In the above example, there's only one instance of the ncircles variable.
  • Diff. ways to referencencircles:
    • Circle.ncircles // ClassName.classVarName
    • ncircles; this.ncircles, // used inside the Circle class definition only
    • c.ncircles // where c is a Circle variable
  • Similar approach for static methods.
  • Examples of static methods (or called class method):
    • Math.cos(x) Math.pow(x,y) Math.sqrt(x)
notes on class methods
Notes on class methods
  • Must be declared with the static keyword
    • Also called static method
  • Can only operate on class variables (e.g., static)
    • Cannot use ‘this’
    • Cannot use instance variables
  • To access a class method: same as to access class vars:
    • Circle.countCircles() // ClassName.classVarName
    • countCircles(); this.countCircles(),
    • // legal only when inside the Circle class definition
    • c.countCircles() // where c is a Circle variable
  • Lots of examples of class methods in the JDK (e.g., String)
example instance member v s class member
Class B { int x; static int y;

static int b1() { … }

int b2 () { … }

static int b3() { … // class method

int c;

c = x; c = this.x //error

c = y; c = B.y; //ok!

A a = new A();

B b = new B();

c = a.a1(); //ok!

c = a.a2() ; // ok!

c = A.a2() ; // error!

c = A.a1() ; // ok!

c = b.b1() ; //ok!

c = b.b2() ; // ok!

c = B.b2() ; // error!

c = B.b1() ; // ok!

c = b1(); // ok

c = b2(); // error }

Int b4() { // instance method

c = x; c = this.x //ok!

c = y; c = B.y; c = this.y //ok!

c = b1(); c = this.b1() // ok

c = b2(); c=this.b2() // ok



Class A {

public static int a1(){…}

public int a2() {…} … }

Example: instance member v.s. class member
class and instance initializers
Class and Instance initializers
  • Both class and instance variables can have initializers attached to their declarations.
    • static int num_circles = 0;
    • float r = 1.0;
  • Class variables are initialized when the class is first loaded.
  • Instance variables are initialized when an object is created.
  • Sometimes more complex initializationis needed.
    • For instance variables, there are constructor methods ,and instance initializer.
    • For class variables static initializers are provided
an example static instance initializer
An example static/instance initializer

public class TrigCircle { // Trigonometric circle

// Here are our static lookup tables, and their own simple initializers.

static private double sines[] = new double[1000];

static private double cosines[] = new double[1000];

// Here's a static initializer "method" that fills them in.

// Notice the lack of any method declaration!

static {

double x, delta_x;

int i;

delta_x = (Circle.PI/2)/(1000-1);

for(i = 0, x = 0.0; i < 1000; i++, x += delta_x) {

sines[i] = Math.sin(x);

cosines[i] = Math.cos(x);

} … // The rest of the class omitted.

an example static instance initializer continued
An example static/instance initializer (continued)

// instance field and methods

Private int[] data = new int[100]; // data[i] = i for i = 0..99

// instance initializer as an unnamed void method

{ for(int I = 0; I <100; I++) data[I] = I; }


notes on initializers
Notes on initializers
  • can have any number of static/instance initializers;
  • can appear anywhere a field or method can appear.
  • Static initializer behaves like class method and cannot use this keyword and any instance fields of the class
  • The body of each instance initializers (alone with field initialization expressions) is executed in the order they appear in the class and is executed at the beginning of every constructor.
  • The body of each static initializers (alone with static field initialization expressions) is executed in the order they appear in the class and is executed while the class is loaded.
inheritance in oop
Inheritance in OOP
  • Inheritance is a form of software reusability in which new classes are created from the existing classes by absorbing their attributes and behaviors.
  • Instead of defining completely (separate) new class, the programmer can designate that the new class is to inherit attributes and behaviours of the existing class (called superclass). The new class is referred to as subclass.
  • Programmer can add more attributes and behaviors to the subclass, hence, normally subclasses have more features than their superclasses.
  • Inheritance relationships form tree-like hierarchical structures.
subclasses and inheritance
Subclasses and Inheritance
  • An important aspect of OO programming:
  • the ability to create new data types based on existing data types
  • Example ... a class of drawable Circles:
    • we'd like to be able to draw the circles we create, as well as setting and examining their properties.
    • for drawing, we need to know the color of the circle's outline and its body
    • In Java, we implement this by defining a new class that extends the behavior of the Circle class.
    • This new class is a subclass of Circle.
subclass example
Subclass Example
  • The class GraphicCircle:

public class GraphicCircle extends Circle

{ // Extra fields

Color outline, fill;

// Extra constructors

public GraphicCircle(Color edge, Color fill)

{ x = 0.0; y = 0.0; r = 1.0;

outline = edge; this.fill = fill; }

public GraphicCircle(double r, Color edge, Color fill)

{ super(r); outline = edge; this.fill = fill; }

// Extra methods

public void draw(Graphics g)

{ g.setColor(outline); g.drawOval(x-r, y-r, 2*r, 2*r);

g.setColor(fill); g.fillOval(x-r, y-r, 2*r, 2*r); }


subclass inheritance
Subclass Inheritance
  • A subclass inherits fields and methods from its parent class.
  • A subclass methodoverridesa superclass method if they have the same signature.
  • A subclass field shadows a superclass field if they have the same name.
  • Refer to the superclass field via super.field
  • Note: you can also use super.method(…) to refer to overridden superclass method.
using subclasses
Using Subclasses
  • Subclasses are just like ordinary classes:

GraphicCircle gc = new GraphicCircle();


double area = gc.area();

  • We can assign an instance of GraphicCircle to a Circle variable.
  • Example:

GraphicCircle gc;



Circle c = gc; //widening conversion is

// always safe; explicit cast is not needed.

superclasses objects and the class hierarchy
Superclasses, Objects, and the Class Hierarchy
  • Every class has a superclass.
  • If a class has no extends clause, it extends the Object class.
  • Object Class:
    • the only class that does not have a superclass
    • methods defined by Object can be called by any Java object
abstract classes
Abstract Classes
  • Abstract classes let us define the ``appearance'' of a group of classes.
  • We subsequently implement specific classes according to this pattern.
  • declare classes that define only part of an implementation, leaving extended classes to provide specific implementation of some or all of the methods.
  • The benefit of an abstract class is that
    • methods may be declared such that the programmer knows the interface definition of an object,
    • however, methods can be implemented differently in different subclasses of the abstract class.
abstract classes cont
Abstract Classes (cont.)
  • Any class containing anabstract methodis automatically abstract itself
  • But an abstract class need nothave abstract methods in it!
  • an abstract class can not be instantiated
  • a subclass of an abstract classcan be instantiatedif it overrides each of the abstract methods of itssuperclass andprovides an implementation for all of them
  • if a subclass of an abstract classdoes not implement all of the abstract methodsit inherits, thatsubclass is itself abstract
abstract classes cont an example
Abstract Classes (cont.): an example

public abstract class Shape

{ public abstract double area(); // abstract methods

// to be implemented by subclasses

public abstract double circumference();


public class Circle extends Shape

{ protected double r;

protected static final double PI=3.141592653;

public double Circle() { r = 1.0; }

public double Circle(double r) { this.r = r; }

// implementation of two abstract methods of shape class

public double area(){ return PI*r*r; }

public double circumference() { return 2*PI*r; }

public double getRadius() { return r; }


abstract classes an example cont
Abstract Classes : an example (cont.)

public class Rectangle extends Shape

{ protected double w,h;

// two constructors

public Rectangle() { w=1.0; h=1.0; }

public Rectangle(double w, double h)

{ this.w = w; this.h = h; }

// implementation of two parent methods

public double area() { return w*h; }

public double circumference()

{ return 2*(w + h); }

// methods for this class

public double getWidth() { return w; }

public double getHeight() { return h; }


  • the concept of inheritance
  • the protected modifier
  • adding and modifying methods through inheritance
  • creating class hierarchies


  • Inheritance allows a software developer to derive a new class from an existing one
  • The existing class is called the parent class, or superclass, or base class
  • The derived class is called the child class or subclass.
  • The child class inherits characteristics (data & methods) of the parent class
  • That is, the child class inherits the methods and data defined for the parent class





  • Inheritance relationships are often shown graphically in a class diagram, with the arrow pointing to the parent class
  • Inheritance relationships:
    • base class: Vehicle
    • derived class: Car
    • Car inherits data & methods
    • from Vehicle

Inheritance should create an is-a relationship, meaning the child is a more specific version of the parent

deriving subclasses
Deriving Subclasses
  • The reserved word extends is used to establish an inheritance relationship

class Car extends Vehicle {

// class contents


  • See Words.java



class Book {

protected int pages = 1500;

public void pageMessage () {

System.out.println ("Number of pages: " + pages);

} // method pageMessage

} // class Book

class Dictionary extends Book {

private int definitions = 52500;

public void definitionMessage () {

System.out.println ("Number of definitions: "+definitions);

System.out.println ("Definitions per page: " +definitions/pages); // inherited var

} // method definitionMessage

} // class Dictionary

inheritance example
Inheritance Example

class Words { // Test Driver

public static void main (String[] args) {

Dictionary webster = new Dictionary ();

webster.pageMessage(); // inherited method

webster.definitionMessage(); } } // class Words

controlling inheritance by the protected modifier
Controlling Inheritance by the protected Modifier
  • The protected (and public) visibility modifier allows a member of a parent class to be inherited into the child
  • But protected visibility provides more encapsulation than public does
  • However, protected visibility is not as tightly encapsulated as private visibility
  • Note: Inheritance does not change the visibility of the parent class members when used through instances of child classes.


visibility modifiers and their usage
Visibility modifiers and their usage

The visibility modifiers determine which class members can be referenced from where and which cannot.

  • public members:
    • all classes
  • protected members
    • all classes in the same package +
    • all subclasses (and subsubclasses …)
    • Note: Java has no notions of public, private or protected inheritance as in C++; all inheritances are public.
  • package members [default visibility]
    • all classes in the same package
  • private members
    • can only be used in the same class where the member is defined.

Visible regions of members of class A

visible region for pubic members [of class A]

visible region for protected members

visible region for package members

package a.b.c

package a.b.c;

public class A {

public int p1…

protected int p2

private int p3 … }

package a.b.c

… extend A

visible region for

private members

of class A

… extend A

… extend A

… extend A

protected members are accessible to subclass instances

public class A {

public int p1;

protected int p2;

int p3

private int p4;

… }

public class B extends A{

// p1 and p2 can be used


Protected members are accessible to subclass instances

class C [extend B] {

int p;

B b = new B();

p = b.p1; // ok since p1 is public

p = b.p2 ; // ok

p = p2; // ok ! protected p2 is inherited

A a = new A();

p = a.p2// error!!

// protected field (p2) can be accessed from subclasses only through subclass instances


the super reference
The super Reference
  • Constructors are not inherited, even though they are declared to have public visibility
  • Yet we often want to use the parent's constructor to set up the "parent's part" of the object
  • The super reference can be used to refer to the parent class, and is often used to invoke the parent's constructor



class Book {

protected int pages;

public Book (int pages) { this.pages = pages; }

public void pageMessage () {

System.out.println ("Number of pages: " + pages); }

} // class Book

class Dictionary extends Book {

private int definitions;

public Dictionary (int pages, int definitions) {

super (pages); // construct Book part of a Dictionary

this.definitions = definitions;

} // constructor Dictionary

public void definitionMessage () {

System.out.println ("Number of definitions: " + definitions);

System.out.println ("Definitions per page: " + definitions/pages);

} // method definitionMessage

} // class Dictionary


class Words2 {

public static void main (String[] args) {

Dictionary webster = new Dictionary (1500, 52500);



System.out.println(webster); // try println object

} // method main

} // class Words2

single vs multiple inheritance
Single vs. Multiple Inheritance
  • Java supports single inheritance, meaning that a derived class can have only one parent class
  • Multiple inheritance allows a class to be derived from two or more classes, inheriting the members of all parents
  • Collisions, such as the same variable name in two parents, have to be resolved
  • In most cases, the use of interfaces gives us the best aspects of multiple inheritance without the overhead
indirect access
Indirect Access
  • An inherited member can be referenced directly by name in the child class, as if it were declared in the child class
  • But even if a method or variable is not inherited by a child, it can still be accessed indirectly through parent methods
  • See Eating.java and School.java



class Food {

final private int CALORIESPERGRAM = 9;

private int fat;

protected int servings;

public Food (int numFatGrams, int numServings) {

fat = numFatGrams;

servings = numServings;

} // constructor Food

private int calories() { return fat * CALORIESPERGRAM; }

public int caloriesPerServing() { return (calories() / servings); }

} // class Food

class Pizza extends Food {

public Pizza (int amountFat) { super (amountFat, 8); }

} // class Pizza

pubic class Main{ …

Pizza special = new Pizza (275);

System.out.println ("Calories per serving: " + special.caloriesPerServing());


public class Student {

protected String name;

protected int numCourses;

public Student (String studentName, int classes) {

name = studentName; numCourses = classes;

} // constructor Student

public void info () {

System.out.println ("Student name: " + name);

System.out.println ("Number of courses: " + numCourses);

} // method info

public static void main(String[] argv) {

System.out.println("Student Main"); }

} // class Student


class GradStudent extends Student {

private String source; private double rate;

public GradStudent (String name, int classes,

String supportSource, double hourlyRate) {

super (studentName, classes);

source = supportSource;

rate = hourlyRate;

} // constructor GradStudent

public void support () {

System.out.println ("Support source: " + source);

System.out.println ("Hourly pay rate: " + rate);

} // method support

} // class GradStudent


public class School {

  • public static void main (String[] args) {
  • Student s1 = new Student ("Sammy", 5);
  • GradStudent g1 = new GradStudent ("Pete", 3,
  • "Teaching Assistant", 8.75);
  • s1.info(); System.out.println();
  • g1.info(); g1.support();
  • }// method main
  • } // class School
  • g1 call public info(), which use protected name and numCourses of class Student
overriding methods
Overriding Methods
  • A child class can override the definition of an inherited method in favor of its own
  • A child can redefine a method it inherits from its parent
  • Overriding method:
    • has the same signature as the parent's method
    • has different code in the body
  • The actual type (not casted type) of an object determines which method is invoked
  • See Messages.java



class Messages {

public static void main (String[] args) {

Message m = new Message();

Advice a = new Advice();

m.message(); a.message();

(Message a).message() // same as a.message()

} } // class Messages

class Message {

public void message() {

System.out.println (”Message");

} } // class Thought

class Advice extends Message {

public void message() { // overriding method

System.out.println (”Advice");

} } // class Advice

overloading vs overriding
Overloading vs. Overriding
  • Don‘t confuse the concepts of overloading (多載) and overriding(覆蓋)
  • Overloading deals with multiple methods in the same class with the same name but different signatures
  • Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature
  • Overloading lets you define a similar operation in different ways for different data
  • Overriding lets you define a similar operation in different ways for different object types


the super reference revisited
The super Reference Revisited
  • Inherited parent methods/fields can be explicitly invoked using the super reference
  • If a method/field is declared with the final modifier, it cannot be overridden
  • The concept of overriding can be applied to data (called shadowing variables), but shadowing behaves quite differently from overriding.
  • The syntax is:

super.method (parameters)


  • See Firm.java


shadowing superclass fields vs overriding superclass methods
Shadowing superclass fields vs overriding superclass methods

class A { int x ; int m() …}

class B extends A { int x; int m() …}

class C extends B {

int x; // x in B and A are shadowed

// by this.x

int m() {…} // m() overrides m() in A & B

C c = new C(); …

x, this.x // field x in C

super.x, ((B) this).x // field x in B

((A) this).x // filed x in A

super.super.x // syntax error!!

c.x // field in C

((B)c).x // fields in B

((A)c).x // fields in A

((A) c).m() ; // m() in A ? no !!

super.m(); // m() in B

((B) this).m(); // m() in C

((A) this).m(); // m() in C

((A) c).m();

((B) c).m();

m(); // m() in C


class Firm {

public static void main (String[] args) {

Manager sam = new Manager ("Sam", "123 Main Line",

"555-0469", "123-45-6789", 1923.07);

Employee carla = new Employee ("Carla", "456 Off Line",

"555-0101", "987-65-4321", 846.15);

Employee woody = new Employee ("Woody",

"789 Off Rocker", "555-0000", "010-20-3040", 769.23);

woody.print(); System.out.println ("Paid: " + woody.pay());


carla.print(); System.out.println ("Paid: " + carla.pay());


sam.print(); sam.awardBonus (2000);

System.out.println ("Paid: " + sam.pay());

System.out.println(); } }


class Employee {

protected String name, address, phone, ID;

protected double salary;

public Employee (String name, String address,

String phone, String ID, double salary) {

this.name = name; this.address = address;

this.phone = phone; this.payRate = payRate; this.ID = ID;

} // constructor Employee

public double pay () { return salary; } // method pay

public void print () {

System.out.println (name + " " + ID);

System.out.println (address);

System.out.println (phone);

} } // class Employee


class Manager extends Employee {

private double bonus;

public Manager (String name, String address, String phone,

String ID, double pay) {

super (name, nddress, phone, ID, pay);// call parent’s constructor

bonus = 0; // bonus yet to be awarded


public void awardBonus (double bonus) { this.bonus = bonus; }

public double pay () { // managers need special way to count pay!

double pay = super.pay() + bonus; // call parent’s method

bonus = 0;

return pay;

} }

class hierarchies







Class Hierarchies
  • A child class of one parent can be the parent of another child, forming class hierarchies
class hierarchies1
Class Hierarchies
  • Two children of the same parent are called siblings
  • Good class design puts all common features as high in the hierarchy as is reasonable
  • Class hierarchies often have to be extended and modified to keep up with changing needs
  • There is no single class hierarchy that is appropriate for all situations
  • See Accounts2.java



class Accounts2 {

public static void main (String[] args) {

SavingsAccount savings =

new SavingsAccount (4321, 5028.45, 0.02);

BonusSaverAccount bigSavings =

new BonusSaverAccount (6543, 1475.85, 0.02);

CheckingAccount checking =

new CheckingAccount (9876, 269.93, savings);

savings.deposit (148.04);

bigSavings.deposit (41.52);

savings.withdrawal (725.55);

bigSavings.withdrawal (120.38);

checking.withdrawal (320.18);

} // method main

} // class Accounts2


class BankAccount {

protected int account; protected double balance;

public BankAccount (int accountNum, double initialBalance) {

account = accountNum; balance = initialBalance; }

public void deposit (double amount)

{ balance += amount; } // method deposit

public boolean withdrawal (double amount) {

boolean result = false;

if (amount > balance) System.out.println ("Insufficient funds.");

else { balance -= amount;

System.out.println ("New balance: " + balance);

result = true; }

return result; } } // class BankAccount


class CheckingAccount extends BankAccount {

private SavingsAccount overdraft;

public CheckingAccount (int accountNum,

double initialBalance, SavingsAccount protection) {

super (accountNum, initialBalance);

overdraft = protection; } // constructor CheckingAccount

public boolean withdrawal (double amount) {

boolean result = false;

if ( ! super.withdrawal (amount) ) {

System.out.println ("Using overdraft...");

if ( ! overdraft.withdrawal (amount - balance) )

System.out.println ("Overdraft source insufficient.");

else { balance = 0;

System.out.println ("New balance on account " +

account + ": " + balance);

result = true; }

} return result; }

} // class CheckingAccount


class SavingsAccount extends BankAccount {

protected double rate;

public SavingsAccount (int accountNum,

double initialBalance, double interestRate) {

super (accountNum, initialBalance);

rate = interestRate;

} // constructor SavingsAccount

public void addInterest () {

balance += balance * rate;

} // method addInterest

} // class SavingsAccount


class BonusSaverAccount extends SavingsAccount {

private final int PENALTY = 25;

private final double BONUSRATE = 0.03;

public BonusSaverAccount (int accountNum,

double initialBalance, double interestRate) {

super (accountNum, initialBalance, interestRate);

} // constructor SuperSaverAccount

public boolean withdrawal (double amount) {

return super.withdrawal (amount+PENALTY);

} // method withdrawal

public void addInterest () {

balance += balance * (rate + BONUSRATE);

} // method addInterest

the object class
The Object Class
  • A class called Object is defined in the java.lang package of the Java standard class library
  • All objects are derived from the Object class
  • If a class is not explicitly defined to be the child of an existing class, it is assumed to be the child of the Object class
  • The Object class is therefore the ultimate root of all class hierarchies
  • The Object class contains a few useful methods, such as toString(),equal(), which are inherited by all classes
  • You may choose to override equals and/or toString to define equality/toString in your way.



import java.awt.Point;

  • class TestToString {
  • public static void main (String[] args) {
  • Integer n = new Integer (25);
  • Point p = new Point (0, 0);
  • A a = new A();
    • System.out.println ( n.toString() );
  • System.out.println ( p.toString() );
  • System.out.println ( a.toString() );
  • } // method main
  • } // class TestToString
  • class A {
  • public String toString() { return "I am AnyClass"; } // method toString
  • } // class AnyClass
abstract classes1
Abstract Classes
  • An abstract class is a placeholder in a class hierarchy that represents a generic concept
  • An abstract class cannot be instantiated
  • We use the modifier abstract on the class header to declare a class as abstract
  • An abstract class often contains abstract methods (like an interface does), though it doesn’t have to
abstract classes2
Abstract Classes
  • The child of an abstract class must override the abstract methods of the parent, or it too will be considered abstract
  • An abstract method cannot be defined as final (because it must be overridden) or static (because it has no definition yet)
  • The use of abstract classes is a design decision; it helps us establish common elements in a class that is too general to instantiate
references and inheritance



References and Inheritance
  • An object reference can refer to an object of its class, or to an object of any class related to it by inheritance
  • For example, if the Holiday class is used to derive a child class called Christmas, then a Holiday reference could actually be used to point to a Christmas object

Holiday day;

day = new Christmas();

references and inheritance1
References and Inheritance
  • Assigning a descendant class instance to an ancestor reference is considered to be a widening conversion, and can be performed by simple assignment
  • Assigning an ancestor object to a subclass reference can also be done, but it is considered to be a narrowing conversion and must be done with a cast
  • The widening conversion is the most useful
  • A polymorphic reference is one which can refer to one of several possible methods
  • Suppose the Holiday class has a method called celebrate, and the Christmas class overrode it
  • Now consider the following invocation:


  • If day refers to a Holiday object, it invokes Holiday's version of celebrate; if it refers to a Christmas object, it invokes that version


  • In general, it is the type of the object being referenced, not the reference type, that determines which method is invoked
  • Note that, if an invocation is in a loop, the exact same line of code could execute different methods at different times
  • Polymorphic references are therefore resolved at run-time, not during compilation


  • Note that, because all classes inherit from the Object class, an Object reference can refer to any type of object
  • A Vector is designed to store Object references
  • The instanceOf operator can be used to determine the class from which an object was created
  • See Variety.java



import java.awt.Point; import java.util.Vector;

class MyVariety {

public static void main (String[] args) {

Vector collector = new Vector();

Integer num1 = new Integer (10); collector.addElement (num1);

Point origin = new Point (0, 0); collector.addElement (origin);

Integer num2 = new Integer (37);collector.addElement (num2);

Point corner=new Point (12, 45);collector.addElement (corner);

int temp; Object something;

for (int count=0; count < collector.size(); count++) {

something = collector.elementAt (count);

if (something instanceof Integer) {

temp = ((Integer)something).intValue() + 20;

System.out.println (something + " + 20 = " + temp); }

else System.out.println ("Point: " + something);


} }

polymorphism via inheritance






Polymorphism via Inheritance
  • Consider the following class hierarchy:

class Firm2 {

public static void main (String[] args) {

Staff personnel = new Staff();





class Staff {

StaffMember[] staffList = new StaffMember[6];

public Staff() {

staffList[0] = new Executive ("Sam", "123 Main Line", "555-0469",

"123-45-6789", 1923.07);

staffList[1] = new Employee ("Carla", "456 Off Line",

"555-0101", "987-65-4321", 846.15);

staffList[2] = new Employee ("Woody", "789 Off Rocker",

"555-0000", "010-20-3040", 769.23);

staffList[3] = new Hourly ("Diane", "678 Fifth Ave.",

"555-0690", "958-47-3625", 8.55);

staffList[4] = new Volunteer ("Norm", "987 Suds Blvd.", "555-8374");

staffList[5] = new Volunteer ("Cliff", "321 Duds Lane", "555-7282");

((Executive)staffList[0]).awardBonus (5000);

((Hourly)staffList[3]).addHours (40);

} // constructor Staff


public void payday() {

double amount;

for (int count=0; count < staffList.length; count++) {


amount = staffList[count].pay();

if (amount == 0.0)

System.out.println ("Thanks!");

else System.out.println ("Paid: " + amount);

System.out.println ("**********************");


}// method payday

}// class Staff


class StaffMember {

protected String name, address, phone;

public StaffMember (String empName, String empAddress,

String empPhone) {

name = empName;

address = empAddress;

phone = empPhone;

} // constructor StaffMember

public double pay() { return 0.0; } // default pay method

public void print() {

System.out.println ("Name: " + name);

System.out.println ("Address: " + address);

System.out.println ("Phone: " + phone);

} } // class StaffMember


class Volunteer extends StaffMember {

public Volunteer (String empName, String empAddress,

String empPhone) {

super (empName, empAddress, empPhone);

} // constructor Volunteer

public double pay() {

return 0.0;

} // method pay

} // class Volunteer


class Employee extends StaffMember {

protected String ID; protected double payRate;

public Employee (String empName, String empAddress,

String empPhone, String empSsnumber, double empRate) {

super (empName, empAddress, empPhone);

this.ID = ID; payRate = empRate;

} // constructor Employee

public double pay () { return payRate; } // method pay

public void print () {


System.out.println (“ID number: " + ID);

System.out.println ("Pay rate: " + payRate);

} // method print

} // class Employee


class Executive extends Employee {

private double bonus;

public Executive (String name, String addr, String phone,

String ID, double pay) {

super (name, addr, phone, ID, pay);

bonus = 0; // bonus yet to be awarded

} // constructor Executive

public void awardBonus (double bonus) {

this.bonus = bonus; } // method awardBonus

public double pay () {

double pay = super.pay() + bonus;

bonus = 0; return pay; } // method pay

public void print () {


System.out.println ("Current bonus: " + bonus);

} // method print



class Hourly extends Employee {

private int hoursWorked;

public Hourly (String name, String addr,

String phone, String ID, double hrRate) {

super (name, address, phone, ID, hrRate);

hoursWorked = 0; }

public void addHours (int moreHours) {

hoursWorked += moreHours;

} // method addHours

public double pay () {

return payRate * hoursWorked;

} // method pay

public void print () {


System.out.println ("Current hours: " + hoursWorked);

} // method print

} // class Hourly

summary for inheritance
Summary for inheritance
  • Inheritance: reuse the existing objects (is-a relation)
  • Protect modifier: better encapsulation
  • Use super to invoke parent’s methods.
  • Overriding methods and overloaded methods
  • All Java classes inherit from object class
  • Polymorphism: which overriding method is invoked
  • based on the object’s type
  • Widening & narrowing
  • A Java interface is a collection of abstract methods and constants
  • An abstract method is a method header without a method body (i.e., no implementation)
  • An abstract method in an interface can be declared using the modifier abstract, but because all methods in an interface are abstract, it is usually left off.
    • cf: abstract methods in an abstract class must be declared explicitly using the abstract modifier.
  • An interface is used to formally define a set of methods that a class will implement

interface is a reserved word

A semicolon immediately

follows each method header


None of the methods in an

interface are given

a definition (body)

public interface Doable


public void doThis();

public int doThat();

public void doThis2 (float value, char ch);

public boolean doTheOther (int num);


  • An interface cannot be instantiated
    • Doable d = new Doable(); // error
  • Like a class, a user-defined interface can be used as the type of variables.
    • Doable a, b;
  • Methods in an interface have public visibility by default
  • A class formally implements an interface by
    • stating so in the class header
    • providing implementations for each abstract method in the interface
  • If a class asserts that it implements an interface, it must define all methods in the interface or the compiler will produce errors.

implements is a

reserved word

Each method listed

in Doable is

given a definition


public class CanDo implements Doable


public void doThis ()


// whatever


public void doThat ()


// whatever


// etc.


  • A class can implement more than one interfaces
  • See Speaker.java (page 236)
  • See Philosopher.java (page 237)
  • See Dog.java (page 238)
  • The interfaces are listed in the implements clause, separated by commas
  • The class must implement all methods in all interfaces listed in the header
  • An interface can be implemented by multiple classes
  • Each implementing class can provide their own unique version of the method definitions
  • An interface is not part of the class hierarchy
  • A class can be derived from a base class and implement one or more interfaces


interface constants
Interface constants
  • Unlike interface methods, interface constants require nothing special of the implementing class
  • Constants in an interface can be used in the implementing class as if they were declared locally
  • This feature provides a convenient technique for distributing common constant values among multiple classes


extending interfaces
Extending Interfaces
  • An interface can be derived from another interface, using the extends reserved word
  • The child interface inherits the constants and abstract methods of the parent
  • Note that the interface hierarchy and the class hierarchy are distinct
  • Unlike class hierarchy, an interface can extend more than one interfaces.
    • public interface Transformable extends Scable, Translatable, Rotatable { }
  • A class that implements an interface must define also all methods in all ancestors of the interface.



An interface Example

interface Printable {

public String name();

public String print(); // public can be omitted

} // interface Printable

class PrintLogger {

public void log (Printable file) {

System.out.println (file.name() + " : " + file.print());

} // method log

} // class PrintLogger


class File {

protected String id; protected int size;

public File (String id, int size) {

this.id = id; this.size = size;

} // constructor File

public String name() { return id; }// method name

} // class File

class TextFile extends Fileimplements Printable {

protected String text;

public TextFile (String id, int size, String contents) {

super(id, size); text = contents;

} // constructor TextFile

public String print() { return text; }

} // class TextFile


class BinaryFile extends File {

protected byte[] data;

public BinaryFile (String id, int size, byte[] data) {

super(id, size); this.data = data;

} // constructor BinaryFile

} // class BinaryFile

class ImageFile extends BinaryFile implements Printable {

public ImageFile (String id, int size, byte[] data) {

super(id, size, data);

} // constructor ImageFile

public String print() { return new String (data); }

} // class Image_File


public class Printer {

public static void main (String[] args) {

byte[] logoData = {41, 42, 49, 44 };

TextFile report = new TextFile

(“Reprot 1", 1024, "One two three …");

ImageFile logo = new ImageFile(“Picture 1", 4, logoData);

PrintLogger daily = new PrintLogger();

daily.log (report);

daily.log (logo);



marker interface
Marker interface
  • An interface without including any method.
    • useful for providing additional information about an object.
    • EX:
    • java.lang.Serializable
    • java.lang.Cloneable
    • java.rmi.Remote


Object obj;

Object copy;

copy = o.clone() // may raise CloneNotSupportedExceptionexception

if(obj instanceof Cloneable) copy = o.clone();

else copy = null;

polymorphism via interfaces
Polymorphism via Interfaces
  • An interface name can be used as the type of an object reference variable

Doable obj;

  • The obj reference can be used to point to any object of any class that implements the Doable interface
  • The version of doThis that the following line invokes depends on the type of object that obj is referring to:


polymorphism via interfaces1
Polymorphism via Interfaces
  • That reference is polymorphic, which can be defined as "having many forms"
  • That line of code might execute different methods at different times if the object that obj points to changes
  • See PrinterLogger.java(slide 106)
  • Note that polymorphic references must be resolved at run time; this is called dynamic binding
  • Careful use of polymorphic references can lead to elegant, robust software designs
some interfaces used in core java classes
Some interfaces used in core java classes
  • The Java standard class library contains many interfaces that are helpful in certain situations
  • The Comparable interface contains an abstract method called compareTo, which is used to compare two objects

pubilc iterface Comparable {

public abstract int comparedTo(Object); }

Ex:int rlt = x.comparedTo(y);

if(rlt < 0) {… } // x < y

else if (rlt>0) { …} // x > y

else {…} // rlt = 0 means x is equal to y.

  • The String class implements Comparable which gives us the ability to put strings in alphabetical order
the iterator and enumeration interface
The Iterator and Enumeration interface

The java.util.Iterator/Enumeration interface contain methods that allow the user to move through a collection of objects easily

public interface Iterator {

public abstract boolean hasNext();

public abstract Object next();

public abstract void remove(); }

pubic interface Enumeration {

public boolean hasMoreElements();

pubic Object nextElement(); }

Ex: Object obj ; // obj is an object implementing Iterator

for(Iterator i = (Iterator)obj; i.hasNext(); )


events skipped
Events [skipped]
  • An event is an object that represents some activity to which we may want to respond
  • For example, we may want our program to perform some action when the following occurs:
    • the mouse is moved
    • a mouse button is clicked
    • the mouse is dragged
    • a graphical button is clicked
    • a keyboard key is pressed
    • a timer expires
  • Often events correspond to user actions, but not always
  • The Java standard class library contains several classes that represent typical events
  • Certain objects, such as an applet or a graphical button, generate (fire) an event when it occurs
  • Other objects, called listeners, respond to events
  • We can write listener objects to do whatever we want when an event occurs
events and listeners




This object may

generate an event

This object waits for and

responds to an event

Events and Listeners

When an event occurs, the generator calls

the appropriate method of the listener,

passing an object that describes the event

listener interfaces
Listener Interfaces
  • We can create a listener object by writing a class that implements a particular listener interface
  • The Java standard class library contains several interfaces that correspond to particular event categories
  • For example, the MouseListener interface contains methods that correspond to mouse events
  • After creating the listener, we add the listener to the component that might generate the event to set up a formal relationship between the generator and listener
mouse events
Mouse Events
  • The following are mouseevents:
    • mouse pressed - the mouse button is pressed down
    • mouse released - the mouse button is released
    • mouse clicked - the mouse button is pressed and released
    • mouse entered - the mouse pointer is moved over a particular component
    • mouse exited - the mouse pointer is moved off of a particular component
  • Any given program can listen for some, none, or all of these
  • See Dots.java (page 246)
  • See DotsMouseListener.java (page 248)
mouse motion events
Mouse Motion Events
  • The following are called mouse motion events:
    • mouse moved - the mouse is moved
    • mouse dragged - the mouse is moved while the mouse button is held down
  • There is a corresponding MouseMotionListener interface
  • One class can serve as both a generator and a listener
  • One class can serve as a listener for multiple event types
  • See RubberLines.java (page 249)
key events
Key Events
  • The following are called key events:
    • key pressed - a keyboard key is pressed down
    • key released - a keyboard key is released
    • key typed - a keyboard key is pressed and released
  • The KeyListener interface handles key events
  • Listener classes are often implemented as inner classes, nested within the component that they are listening to
  • See Direction.java (page 253)
  • An animation is a constantly changing series of pictures or images that create the illusion of movement
  • We can create animations in Java by changing a picture slightly over time
  • The speed of a Java animation is usually controlled by a Timer object
  • The Timer class is defined in the javax.swing package
  • A Timer object generates an ActionEvent every n milliseconds (where n is set by the object creator)
  • The ActionListener interface contains an actionPerformed method
  • Whenever the timer expires (generating an ActionEvent) the animation can be updated
  • See Rebound.java (page 258)
summary of java modifiers
Summary of Java Modifiers
  • Modifiers used in java:
    • for accessibility: public, [package], protected, private
    • abstract,
    • final,
    • static
    • native,
    • strictfp
    • synchronized
    • transient
    • volatile
example of transient fields
Example of transient fields

class Point {

int x, y;

transient float rho, theta;

} // rho and theta are not persistent data

example of synchronized method and volatile field
Example of synchronized method and volatile field

class Test {

static int i = 0, j = 0;

static void one() { i++; j++; }

static void two() { System.out.println("i=" + i + " j=" + j);

} }

example of synchronized method and volatile field1
Example of synchronized method and volatile field

public class Main {

pubic static void main(String[] args){

new Thread1().start();

new Thread2().start();


class Thread1 extend Thread {

public void run(){ for(;;) Test.one();} }

class Thread2 extend Thread {

public void run(){ for(;;) {Test.two(); sleep(500); } }

// it is possible that Thread2 prints a result with

// j > i, since i,j may be updated out of order in MM.

example of synchronized method and volatile field2
Example of synchronized method and volatile field

class Test {

static int i = 0, j = 0;

static synchronized void one() { i++; j++; }

static synchronized void two() {

System.out.println("i=" + i + " j=" + j);

} } // i and j must be equal

class Test {

static volatile int i = 0, j = 0;

static void one() { i++; j++; }

static void two() {

System.out.println("i=" + i + " j=" + j);

} } // i always >= j.

nested classes
Nested Classes
  • In addition to a class containing data and methods, it can also contain other classes
  • A class declared within another class is called a nested class (or called inner class)

Outer Class



why nested classes
Why Nested Classes
  • A nested class has access to the variables and methods of the outer class, even if they are declared private
  • Nested classes can be hidden from other classes in the same package.
  • Anonymous classes are handy when defining callbacks on the fly.
  • Convenient when writing event-driven programming
nested classes1
Nested Classes
  • A nested class produces a separate bytecode file
  • If a nested class called Inside is declared in an outer class called Outside, two bytecode files will be produced:



  • Nested classes can be declared as static, in which case they cannot refer to this, instance variables or methods
  • A nonstatic nested class is called an inner class
kinds of java classes interfaces
Kinds of Java classes /interfaces

Top-level classes /interfaces

  • Non-nested top-level classes/interfaces
    • are ordinary classes/interfaces that are direct members of a package.
  • Nested top-level classes / interfaces
    • are static members of other top-level classes/interfaces
    • nested interfaces are implicitly static (hence top-level).

Inner classes:

  • Member classes
    • are non-static nested classes
  • Local classes
    • are classes defined inside method body
  • Anonymous classes
    • are classes defined within method body without given a class name
nested top level classes interfaces
Nested top-level classes /interfaces
  • also called static member classes/interfaces
  • behave like an ordinary top-level class/interface except that
    • it can access the static members of all of its direct or indirect containing classes.
    • can be public, protected, package or private.
    • must use the name A.B.C t o reference to a class C enclosed by class B enclosed by class A.






example of a static member interface
Example of a static member interface

public class LinkedStack {

public interface Linkable { // interfaces are static by default.

public Linkable getNext();

public void setNext(Linkable node); }

// The top of the stack is a Linkable object.

Linkable top;

public LinkedStack() {};

pubic boolean empty() { return (top == null) ;}

public void push(Linkable node) { node.setNext(top); top = node; }

public Object pop(Linkable node) throw EmptyStackException {

if(empty()) throw new EmptyStackException();

Object r = top; top = top.getNext(); return r; } }

// This class defines a type of node that we'd like to

// use in a linkedStack.

class IntegerNode implements LinkedStack.Linkable


// Here's the node's data and constructor.

private int i;

public IntegerNode(int i) { this.i = i; }

// implementation of LinkedStack.Linkable.

private LinkedList.Linkable next;

public LinkedList.Linkable getNext() { return next; }

public void setNext(LinkedList.Linkable node)

{ next = node; }


public class test {

pubic static void main(String[] args) {

// declare an array of 10 IntergerNodes

IntergerNode[] n = new IntergerNode[10];

LinkedStack s = null;

for (int i = 0; i < n.length; i++) {

n[i] = new IntergerNode(i);

s.push(n[i]) ;


while(! s.empty()) System.out.println(s.pop());


features of static member classes
Features of static member classes
  • obey the same rules of other static members:
    • can access only static members (using simple or full name)
    • accessible to other classes according to the used visibility modifier.
    • note: useful for compiler only.
    • as to interpreters:
    • pubic or protected nested/member classes
    • => visible to all classes,
    • package or private nested/member classes
    • => visible to containing package
How to reference a nested static class C inside [static] class A of [static] class B of package a.b :
    • outside package a.b => a.b.A.B.C
    • if import a.b.A.B.C or a.b.A.B.* => C // not recommended
    • if import a.b.A.B or a.b.A.* => B.C // not recommended
    • inside package a.b => A.B.C (or a.b.A.B.C)
    • inside class A => B.C (or A.B.C or a.b.A.B.C)
    • inside class B => C ( or any of the above)
  • Note: All static fields, methods, and classes of a top level class are accessible to all code [even inside a static class] within the class no matter they are private or not.
import LinkedStack.Linkable;

// or import LinkedStack.*;

class IntegerNode implements Linkable


// Here's the node's data and constructor.

private int i;

public IntegerNode(int i) { this.i = i; }

// implementation of LinkedStack.Linkable.

private LinkedList.Linkable next;

public Linkable getNext() { return next; }

public void setNext(Linkable node)

{ next = node; }


access references in nested classes
access references in nested classes

public class A { static int a; static private int ma();

static class B1 { static int y1; static private int mb1();

static class C1 { static int c1; static int mc1(); }

static class B2 {

static class B1 { … }

static int b2; static private int mb2();

static class C2 { // various ways to access other members

// a, A.a, ma(), A.ma(), B1, A.B1 reference A’s members

// b2, B2.b2, A.B2.b2, mb2, B2.mb2(), A.B2.mb2(), B2.B1, B1.

// A.B1.y1, A.B1.C1.c1, B1.C1.c1, C1.c1


member classes
Member classes
  • static class  static fields/methods
  • member class  instance field/method
    • beside referring to all static fields/methods, can also reference this, instance field/method of all enclosing classes, even they are private.
    • associated with an instance of each of the enclosing classes
  • Member class v.s. Static class
    • static class and its enclosing classes are staticclass-class relationship
    • member class and its enclosing classes are instance-instancerelationship.
    • 1. Each member class instance must be created/accessed through instances of the containing class.
    • 2. Each member class instance is associated with an unique instance of each of its containing classes.
example a linkedlist enumerator as a member class
Example: A LinkedList Enumerator, as a Member Class

import java.util.Enumeration;

public class LinkedStack { // those from old LinkedStack

public interface Linkable { ... }

private Linkable top;

public void push(…) { ... } public Object pop() { ... }

// This method returns an Enumeration object for this inkedStack.

// Note: no LinkedStack object is explicitly passed to the

// constructor.

public Enumeration enumerate() { return new Enumerator(); }

example a linkedlist enumerator as a member class1
Example: A LinkedList Enumerator, as a Member Class

// the implementation of the Enumeration interface.

protected class Enumerator implements Enumeration {

Linkable current;

public Enumerator() { current = top; }

public boolean hasMoreElements() { return (current != null); }

public Object nextElement() {

if (current == null)

throw new NoSuchElementException("LinkedStack");

Object value = current;

current = current.getNext();

return value;

} } }

  • Note: Enumerator is only accessible to subclasses or the package of LinkedStack.
restrictions on member classes
Restrictions on member classes
  • A member class cannot have the same name as any containing class or package.
  • Member classes cannot contain any static fields, methods or classes (with the exception of constant fields).
    • since member class is associated with object instances, it is nonsense/needless to have static members.
  • Interfaces cannot be defined as member classes.
    • since interfaces cannot be instantiated, there is no way for an object to create an interface instance.
    • A nested interface is by default static, even if the modifier ‘static’ is not given in the header.
new syntax for member classes
New syntax for member classes
  • member class can access instance field/method of containing class.
    • public Enumerator() { current = top; }
  • How to make the reference explicit ?
    • public Enumeration() { this.current = this.top;}
    • // this.current ok ; but this.top err!!
    • // since there is no top in class Enumeration
  • Solution:
    • public Enumeration() { this.current = LinkedStack.this.top;}
  • New syntax: C: a containing class name
    • C.this is used to reference the associated C instance.
    • needed only when using this incurs ambiguity.
accessing superclass members of the containing class
accessing superclass members of the containing class
  • Recall that we use super.f (or super.m(…)) to reference shadowed or overridden member of parent class of this.
  • Likewise, we use
    • C.super.f
    • to reference the f field of the parent class of C, which is a containing class of this, and use
    • C.super.m()
    • to reference the method m() of the parent class of C.
    • Note: not implemented by java 1.1.
using containing class instance to invoke constructors of member class
using containing class instance to invoke constructors of member class
  • Every instance of a member class is associated with an instance of its containing class.
    • pubic Enumeration enumerate(){return new Enumeration();}
    • can also be written as
    • pubic Enumeration enumerate() { return this.new Enumeration;}
  • More useful case:
    • LinkedStack stack = new LinkedStack();
    • Enumeration e1 = stack.enumerate();
    • // could create one without invoking enumerate()!!
    • Enumeration e1 = stack.new Enumeration();
  • syntax: C: a containing class of member class D with constructor D(…); s : this or var of type C
    • s.new D(…) will invoke D(…) of class D in instance of C referenced by s.
some special case
Some special case
  • It is possible that a class extends a member class.
    • public class A { …
    • public class B { …} …}
    • class C extend A.B {
    • pubic C( … ) { ??? } … }
  • problem: what is the instance of the containing class A of the parent class B of C
  • Solution:
    • pubic C( A a, … ) { a.super(…); }
containing hierarchy vs inheritance hierarchy
Containing Hierarchy vs Inheritance Hierarchy

class A extends A1 { int x;

class B extends B1 { int x;

class C extends C1 { int x; …} } }

class A1 { int x; …} class B1 { int x; …}

class C1 extends C2 { int x; …}

class C2 { int x; … }

Problem: how to reference different x in class C.

  • this.x, C.this.x // x in C1 B.this.x // x in B
  • A.this.x // x in A super.x, ((C1)this).x // x in C1
  • ((C2)this).x // x in C2 ((B1) (B.this)).x, B.super.x // x in B1
  • ((A1) (A.this)).x, A.super.x // x in A1

Problem: how about overridden methods ?

local classes
Local Classes
  • class declared locally within a block of Java code.
    • within method body
    • within instance/static initialization block
  • Feature of Local class:
    • Local class is to member class what local variable is to instance variable.
    • Properties similar to that of local variables:
    • 1. invisible outside the containing block.
    • 2. cannot use accessibility or static modifiers
    • Properties similar to member classes
    • 1.can access any member of any containing classes.
    • 2. no local interfaces
    • can use any final local variables or method parameters that are visible from the scope in which they are defined.
example defining and using a local class
Example: Defining and using a Local Class

// This method creates and returns an Enumeration object for this LinkedStack.

public Enumeration enumerate() {

// Here's the definition of Enumerator as a local class.

class Enumerator implements Enumeration {

Linkable current;

public Enumerator() { current = top; }

public boolean hasMoreElements() { return (current != null); }

public Object nextElement() { … } // omitted


// Create and return an instance of the Enumerator class defined here.

return new Enumerator();


fields and variables accessible to a local class
Fields and variables accessible to a local class

class A { protected char a = 'a'; }

class B { protected char b = 'b'; }

pubic class C extends A {

public static void main(String[] args) {

// Create an instance of the containing class, and invoke the

// method that defines and creates the local class.

C c = new C();

c.createLocalObject('e'); // pass a value for final parameter e.


private char c = 'c'; // Private fields visible to local class.

public static char d = 'd';

public void createLocalObject(final char e) {

final char f = 'f'; int i = 0; // i not final; not usable by local class.

class Local extends B { char g = 'g';

public void printVars() { // All of these fields and variables are accessible.

System.out.println(g); // (this.g) g is a field of this class.

System.out.println(f); // f is a final local variable.

System.out.println(e); // e is a final local argument.

System.out.println(d); // (C.this.d) d -- field of containing class.

System.out.println(c); // (C.this.c) c -- field of containing class.

System.out.println(b); // b is inherited by this class.

System.out.println(a); // a is inherited by the containing class. } }

Local l = this.new Local(); // Create an instance of the local class

l.printVars(); // and call its printVars() method.


typical uses of local classes tba
Typical uses of local classes [TBA]
  • used to implement adapter classes
anonymous classes
Anonymous classes
  • a local class without a name
  • very commonly used as adapter classes.
  • created through another extension to the syntax of the new operator.
  • defined by a Java expression, not a Java statement.
example implementing an interface with an anonymous class
Example: Implementing an Interface with an Anonymous Class

import java.io.*;

// A simple program to list all Java source files in a directory

public class Lister {

public static void main(String[] args) {

File dir = new File(args[0]); // f represents the specified directory.

// List the files in the directory, using the specified filter object.

// The anonymous class is defined as part of a method call expression.

String[] list = dir.list( new FilenameFilter() {

public boolean accept(File f, String name) {

return name.endsWith(".java");


for(int i = 0; i < list.length; i++) // output the list



anonymous class vs local class
Anonymous class vs local class
  • when to use anonymous class:
    • The class has a very short body.
    • Only one instance of the class is needed.
    • The class is used right after it is defined.
    • The name of the class does not make your code any easier to understand.
  • Restrictions on anonymous classes:
    • An anonymous class has no name and hence cannot be used to create more than one instance for each execution.
    • It is not possible to define constructors for anonymous classes.
example enumeration implemented as an anonymous class
Example: Enumeration implemented as an anonymous class

public Enumeration enumerate() {

// Instantiate and return this implementation.

return new Enumeration() {

Linkable current = top; // This used to be in the constructor, but

// anonymous classes don't have constructors.

public boolean hasMoreElements() { return (current != null); }

public Object nextElement() {

if (current == null) throw new NoSuchElementException("LinkedList");

Object value = current;

current = current.getNext();

return value; }

}; // Note the required semicolon. It terminates the return statement.


new java syntax for anonymous classes
New Java Syntax for Anonymous Classes
  • syntax:
    • new class-name ( [ argument-list ] ) { class-body }
    • new interface-name () { class-body }
  • Syntax 1 return an instance of a anonymous subclass of class-name
    • the subclass does not provide additional methods but overrides or implements existing super class methods.
  • Syntax 2 return an instance of a class implementing interface-name.
use initializer to help construct anonymous class instances
use initializer to help construct anonymous class instances

public class InitializerDemo { public int[] array1;

// This is an instance initializer.

// It runs for every new instance, after the superclass constructor

// and before the class constructor, if any.

{ array1 = new int[10];

for(int i = 0; i < 10; i++) array1[i] = i; }

// another instance initializer. The instance initializers run in the order in

// which they appear.

int[] array2 = new int[10]; { for(int i=0; i<10; i++) array2[i] = i*2; }

static int[] static_array = new int[10];

// By contrast, the block below is a static initializer. Note the static

// keyword. It runs only once, when the class is first loaded.

static {

for(int i = 0; i < 10; i++) static_array[i] = i; }


some other notes
Some other notes
  • Blank finals
    • a field or variable can de declared final without specifying an initial value in the declaration.
    • public class Test
    • { final int x ; { x = 1; }
    • public static void main(String[] args)
    • { Test t = new Test(); t.x = 2; } // error!! even {x=1;} is removed!!
    • }
  • Class Literals:
    • Each user or system defined class is represented by an object of the class java.lang.Class at runtime.
    • Given a fully qualified class name a.b.C, there are two ways to reference to the Class object representing a.b.C:
    • 1. Class.forName(“a.b.C”)
    • 2. a.b.C.class
    • 2. is useful for inner classes as well; as to 1.it requires special knowledge of how inner classes is translated into top level classes.
    • class.forName(a.b.C$D); a.b.C.D.class