Object Orientation

Object Orientation Yaodong Bi, Ph.D. Department of Computer Sciences University of Scranton January 4, 2020

Classes and objects for modeling • Classes • Real world concepts • Student, course, bicycle, etc • Objects • Instances of real word concepts • John Smith, Operating Systems, Mongoose Mountain S100, etc

Classes and objects for modeling “Customers John and Susan entered the MakeMoney bank and were served by teller Andy.” Classes Objects:

Classes and objects in software • Objects • Packaging both data and the procedures that operate on the data into one • Operations are the only way to change the data items – encapsulation • Classes • Template of objects of the same type • Write once and use many times • Implementation classes/objects • Some classes/objects do not match to concepts in the real world • List, stack, tree, queue – containers • Exception classes

Attributes and operations • Instance attributes (simply attributes) • Properties (internal state) of an object • Student: name, major, class, GPA • Course: title, # of credits, description • Each object has its own value for an attribute • Instance operations (simply operations) • Services of an object • Stack: push, pop, isEmpty, isFull • Encapsulation • Attributes (private and protected) are not accessible directly by client • Operations are used to read/write the values of attributes

Attributes and Operations • Class attributes (static attributes) • Represent properties that can be applied to all objects of the same class • Only one value for each class attribute shared by all objects of the class • All accounts of the CheckingAccount class have the same interest rate • Can be accessed in both instance and class operations • Class operations (static operations) • Used to access class attributes • Cannot access instance attributes and operations

Class member access modes • Public • Public attributes and public operations • Accessible to all clients or the world • Protected • Protected attributes and operations • Accessible to the class and its subclasses • Private • Private attributes and operations • Only accessible by the operations of the class

Member access modes in Java

Abstract and concrete classes • Abstract classes • Define a common interface for its subclasses • Defer some or all of its implementation of operations to its subclasses • Cannot be instantiated public abstract class ParentClass { public void foo() {…} public String toBeImplemented(); } • Concrete classes • That are not abstract classes • Can be instantiated • Concrete subclasses implement all the operations

Interfaces • Signature of an operation: • Operation name • Objects/classes it takes as parameters • Return value and type • Interface: • The set of signatures of related operations • Representing a capability or a set of services • A class may implement multiple interfaces • Its objects have more than one set of services

Interface & Inheritance • An interface may inherit from another interface using “extends” in Java • E.g., interface B extends A {} • The child interface (B) inherits all the operations defined in the parent interface (A). • The child may define (override) an operation with the same signature as one in the parent • A concrete class that implements the child must implement all the operations defined in A as well. • An interface may “extend” N > 1 interfaces • E.g., interface C extends B, A {} • The parents may have operations with the signature. • A concrete class that implements the child must implements all the operations defined in B and A.

Default Methods in Interfaces • Since Java 8, an interface may define default methods. • E.g., interface A { default public int foo() {…} } • A concrete class that implements the interface does not need to implement default methods • Default methods help in extending existing interfaces. • interface A is in a library (a jar file, e.g.,) • Class ClientB extends A • Adding a new operation to A would break ClientB • To add the new operation and not break ClientB, add the new operation as a default method.

Inheritance • Specifies is-a or a-kind-of relationship • Generalization and specialization • Superclasses and subclasses • Single and multiple inheritance • Class and interface inheritance

Superclasses and subclasses • Superclass • Also called base class or parent class • All of its members are inherited by its subclasses • Subclasses • Also called child classes • Inherit all the properties of its superclasses

Single and multiple inheritance • Single inheritance • A subclass cannot have more than one parent • Multiple inheritance • A subclass may have more than one parent • Java does not allow multiple inheritance • Java allows multiple interface inheritance • Through interface implementation

Person SSN Name getSSN() getName() Class inheritance lattice Faculty office getOffice() Student major getMajor() Staff dept getDept() Graduate PayRate getPayRate() Underg PayRate getPayRate() FullTime salary getSalary() PartTime PayRate getPayRate() TA labs getLabs()

Implementation inheritance • Also called class inheritance • Define an object’s implementation in terms of another object’s implementation • Pure class inheritance in C++ • Pure class inheritance in Java

Pure class inheritance with C++ Class BinaryTree { getRoot() {;} setRoot() {;} leftTree() {;} rightTree() {;} } Class BinSearchTree: private BinaryTree { insert() {;} remove() {;} find() {;} } The operations of BinaryTree are not accessible/visible to the clients of BinSearchTree because BinaryTree is private They use the operations ofBinaryTree

Pure class inheritance with Java • Java does not allow private parent • The parent and grandparents are public and accessible through the subclass • Java cannot implement pure implementation inheritance.

Interface inheritance • Describe when an object can be used in place of another • Pure interface inheritance in C++ • Superclasses are pure abstract classes • Pure interface inheritance in Java • Superclasses are Java interfaces

Interface inheritance with C++ Class Stack { virtual push() = 0; virtual pop() = 0; virtual isEmpty() = 0; virtual isFull() = 0; } A C++ Abstract class Class MyStack: public Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} } Class YourStack: public Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} }

Interface inheritance with Java Interface Stack { push(); pop(); isEmpty(); isFull() 0; } A Java Interface Class MyStack implements Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} } Class YourStack implements Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} }

Inheritance vs. Composition • Two common techniques for reuse • Class Inheritance • White-box reuse • Defined at compile-time, statically • Cannot change parent at rum-time • Object Composition • Black-box reuse • A reference/pointer to the object • Can be changed at run-time • Favor composition over inheritance

Delegation • A way to make composition as powerful as class inheritance • A receiving object delegates requests to its delegates == subclass defers request to parent class • In class inheritance • parent can use this to access the child • In delegation • Receiving object can pass itself to its delegate

Three techniques for reuse • Class Inheritance • can provide default operations and let subclass override them • Cannot be changed at run-time • Object Composition • Can change the behavior being composed at run-time • Requires redirection and so becomes less efficient • Generics • Changes the types that a class can use • Cannot change at run-time

Polymorphism • When a client sends a request to a reference, the method executed depends on the object behind the reference • Polymorphism and inheritance are very powerful tool in software design • So powerful that if they are used properly, they can hurt really bad • If something goes wrong, it is hard to tell which class/object caused the problem

Dynamic (late) binding • Dynamic binding is a way of implementing polymorphism • It means that a function call is not linked to the actual function until execution time • Since a reference may be used to point to different types (subtypes or subclasses) of objects at execution time, so the actual function to be executed may not be known when the program is compiled. • The opposite of dynamic binding is static binding which links a function call to the actual function at compilation time.

Method Overriding • A subclass overrides the operation(s) defined in the parent/grandparent classes. • This is for specialized subclasses to override the behavior defined in the super class(es). • For example: • Superclass Employee’s print method prints employee’s SSN, Name, Address. • Its subclass Engineer’s print method prints engineer’s type and level in addition to employee’s normal information.

Function overloading • The same name is used for more than one function • For example, a function of adding two integers may be named the same as the one of adding two floats: • Add(int x, int y); • Add(float x, float y); • Two functions use the same name for the same purpose with different data parameters

Nested Classes in Java • A class, nested class, that is defined in another class, enclosing class. • A nested class can be static and non-static • Static nested called static nested classes • Non-static class called inner classes • A nested class is a member of the enclosing class. • An inner class has an implicit reference to its enclosing class • A static nested class does NOT have any reference to its enclosing class

Static Nested Classes • A static nested class (SNC) is associated with its enclosingclass. • A SNC cannot refer to instance members of the enclosing class. • A SNC interacts with the instance members of its enclosing class and other classes just like any other top-level class • A SNC may be declared public, protected, private, and package private. • A SNC is behaviorally a top-level class.

Inner Classes • An inner class is associated with an instance of its enclosing class. • It has direct access to that instance’s members, even private members. • An inner class CANNOT define any static members • An inner class CANNOT define static initializers • An inner class CANNOT define interfaces • An inner class may define static final compile-time constants • Two special kinds of inner classes • Local classes • Anonymous classes

Local Classes • A local class is a class that is defined in a block. • A block is a group of statements between two matching parenthesis. • A local class is typically defined in a method. • A local class has access to the members of its enclosing class • A local class can only access local variables in the block, that are declared final. • Since SE 8, it can access local variables that are final or effectively final • Effectively final – whose value is never changed after initialized. • Since SE 8, it can access the parameters of the method in which (as the block) the class is declared.

Anonymous Classes • An anonymous class is a local class without a name • You can only use it once • Anonymous classes are used only in expressions • Declaration components • The new operator • The name of an interface or a class to extend • Parentheses that contain the arguments to a constructor • A constructor of the class to extend • No arguments if it is an interface • A declaration body of the anonymous class

Anonymous Classes • An anonymous class (AC) can access the members of the enclosing class • An AC can only access local variables what are final or effectively final • An AC cannot declare static members unless they are constant variables • An AC can declare: • Instance fields • Instance extra methods (not defined in the superclass) • They only be called inside the AC • Instance initializers • Local classes

Static and Instance Initializers • A static initializer may be used to initialize static variables with complex logic, e.g., JDBC connection. public class ClassWithStaticInitializer { static { // statements of static initializer } //other members } • A class may have any # of static initializers and they are executed in the order they appear in the class declaration • A class may define instance initializers that are executed before each constructor is executed. public class ClassWithInstanceInitializer { { // statements of instance initializer } //other members including constructors } • Instance initializers will be called in the order they appear. • Instance initializers may be used to define common parts for all constructors of the class.

Toolkits • A set of predefined classes • Provides useful, general-purpose functionality • Collection classes (list, stack, etc) • GWT (Google Web Toolkit) • Does NOT impose any design level decision • Code Reuse

Frameworks • A reusable design for a type of applications • Dictates the architecture of your app • Overall structure • Key components and their responsibilities • How classes/objects interact with each other • Design reuse • Inversion of Control • Dependency Injection

Frameworks • Framework examples: • Web Apps • J2EE, Spring Framework, .Net • ORM (object to relation mapping) • Hibernate, OpenJPA, MyBatis • Mobile Apps • Android • iOS

Object Orientation