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Inheritance, Polymorphism, Class Hierarchies and GENERICS

Inheritance, Polymorphism, Class Hierarchies and GENERICS. Introduction to Inheritance and Class Hierarchies. Popularity of OOP is that it enables programmers to reuse previously written code saved as classes

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Inheritance, Polymorphism, Class Hierarchies and GENERICS

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  1. Inheritance, Polymorphism, Class Hierarchies and GENERICS

  2. Introduction to Inheritance and Class Hierarchies • Popularity of OOP is that it enables programmers to reuse previously written code saved as classes • All Java classes are arranged in a hierarchy, starting with Object, which is the superclass of all Java classes • Inheritance in OOP is analogous to inheritance in humans • Inheritance and hierarchical organization allow you to capture the idea that one thing may be a refinement or extension of another

  3. Introduction to Inheritance and Class Hierarchies (continued)

  4. Is-a Versus Has-a Relationships • One misuse of inheritance is confusing the has-a relationship with the is-a relationship • The has-a relationship means that one class has the second class as an attribute • We can combine is-a and has-a relationships • The keyword extends specifies that one class is a subclass of another

  5. A Superclass and a Subclass • Consider two classes: Computer and Laptop • A laptop is a kind of computer and is therefore a subclass of computer

  6. Initializing Data Fields in a Subclass and the No-Parameter Constructor • Private data fields belonging to a base class must be initialized by invoking the base class’s constructor with the appropriate parameters • If the execution of any constructor in a subclass does not invoke a superclass constructor, Java automatically invokes the no-parameter constructor for the superclass • Initializes that part of the object inherited from the superclass before the subclass starts to initialize its part of the object

  7. Protected Visibility for Superclass Data Fields • Private data fields are not accessible to derived classes • Protected visibility allows data fields to be accessed either by the class defining it or any subclass • In general, it is better to use private visibility because subclasses may be written by different programmers and it is always good practice to restrict and control access to the superclass data fields

  8. Method Overriding • If a derived class has a method found within its base class, that method will override the base class’s method • The keyword super can be used to gain access to superclass methods overridden by the base class • A subclass method must have the same return type as the corresponding superclass method

  9. Method Overloading • Method overloading: having multiple methods with the same name but different signatures in a class • Constructors are often overloaded • Example: • MyClass(int inputA, int inputB) • MyClass(float inputA, float inputB)

  10. Polymorphism • A variable of a superclass type can reference an object of a subclass type • Polymorphism means many forms or many shapes • Polymorphism allows the JVM to determine which method to invoke at run time • At compile time, the Java compiler can’t determine what type of object a superclass may reference but it is known at run time

  11. Abstract Classes, Assignment, and Casting in a Hierarchy • An interface can declare methods but does not provide an implementation of those methods • Methods declared in an interface are called abstract methods • An abstract class can have abstract methods, data fields, and concrete methods • Abstract class differs from a concrete class in that • An abstract class cannot be instantiated • An abstract class can declare abstract methods, which must be implemented in its subclasses

  12. Abstract Classes and Interfaces • Like an interface, an abstract class can’t be instantiated • An abstract class can have constructors to initialize its data fields when a new subclass is created • Subclass uses super(…) to call the constructor • May implement an interface but it doesn’t have to define all of the methods declared in the interface • Implementation is left to its subclasses

  13. Abstract Class Number and the Java Wrapper Classes

  14. Summary of Features of Actual Classes, Abstract Classes, and Interfaces

  15. Class Object, Casting and Cloning • Object is the root of the class hierarchy; every class has Object as a superclass • All classes inherit the methods defined in class Object but may be overridden

  16. The Method toString • You should always override the toString method if you want to represent an object’s state • If you do not override it, the toString method for class Object will return a string…just not the string you want or are expecting

  17. Operations Determined by Type of Reference Variable • A variable can reference an object whose type is a subclass of the variable type • The type of reference, not the type of the object referenced, determines what operations can be performed • Java is a strongly typed language so the compiler always verifies that the type of the expression being assigned is compatible with the variable type

  18. Casting in a Class Hierarchy • Java provides casting to enable us to process one object referenced by one type through a reference variable of its actual type • Casting does not change the object referenced; it creates an anonymous reference to that object • Downcast: cast a higher type to a lower type • The instanceof operator can guard against ClassCastException errors • You can downcast an interface reference to the specific implementation type

  19. The Method Object.equals • The Object.equals method has a parameter of type Object • Compares two objects to determine whether they are equal • You must override the equals method if you want to be able to compare two objects of a class

  20. Cloning • The purpose of cloning in object-oriented programming is analogous to cloning in biology • Create an independent copy of an object • Initially, both objects will store the same information • You can change one object without affecting the other

  21. The Shallow Copy Problem

  22. The Object.clone method • Java provides the Object.clone method to help solve the shallow copy problem • The initial copy is a shallow copy as the current object’s data fields are copied • To make a deep copy, you must create cloned copies of all components by invoking their respective clone methods

  23. Multiple Inheritance, Multiple Interfaces, and Delegation • Multiple inheritance: the ability to extend more than one class • Multiple inheritance is a language feature that is difficult to implement and can lead to ambiguity • Therefore, Java does not allow a class to extend more than one class

  24. Using Multiple Interfaces to Emulate Multiple Inheritance • If we define two interfaces, a class can implement both • Multiple interfaces emulate multiple inheritance

  25. Implementing Reuse Through Delegation • You can reduce duplication of modifications and reduce problems associated with version control through a technique known as delegation • In delegation, a method of one class accomplishes an operation by delegating it to a method of another class

  26. Packages • The Java API is organized into packages • The package to which a class belongs is declared by the first statement in the file in which the class is defined using the keyword package followed by the package name • All classes in the same package are stored in the same directory or folder • All the classes in one folder must declare themselves to be in the same package • Classes that are not part of a package may access only public members of classes in the package Chapter 3: Inheritance and Class Hierarchies

  27. The No-Package-Declared Environment and Package Visibility • There exists a default package • Files that do specify a package are considered part of the default package • If you don’t declare packages, all of your packages belong to the same, default package • Package visibility sits between private and protected • Classes, data fields, and methods with package visibility are accessible to all other methods of the same package but are not accessible to methods outside of the package • Classes, data fields, and methods that are declared protected are visible to all members of the package Chapter 3: Inheritance and Class Hierarchies

  28. Visibility Supports Encapsulation • The rules for visibility control how encapsulation occurs in a Java program • Private visibility is for members of a class that should not be accessible to anyone but the class, not even the classes that extend it • Package visibility allows the developer of a library to shield classes and class members from classes outside the package • Use of protected visibility allows the package developer to give control to other programmers who want to extend classes in the package

  29. Visibility Supports Encapsulation (continued)

  30. A Shape Class Hierarchy

  31. A Shape Class Hierarchy (continued)

  32. A Shape Class Hierarchy (continued)

  33. Introduction • Generics • New feature of J2SE 5.0 • Provide compile-time type safety • Catch invalid types at compile time • Generic methods • A single method declaration • A set of related methods • Generic classes • A single class declaration • A set of related clases

  34. Motivation for Generic Methods • Overloaded methods • Perform similar operations on different types of data • Overloaded printArray methods • Integer array • Double array • Character array • Only reference types can be used with generic methods and classes

  35. Motivation for Generic Methods (Cont.) • Study each printArray method • Array element type appears in two location • Method header • for statement • Combine three printArray methods into one • Replace the element types with a generic name E • Declare one printArray method • Display the string representation of the elements of any array

  36. Generic Methods: Implementation and Compile-Time Translation • Generic method declaration • Type parameter section • Delimited by angle brackets ( < and > ) • Precede the method’s return type • Contain one or more type parameters • Also called formal type paramters

  37. Generic Methods: Implementation and Compile-Time Translation • Type parameter (Also known as type variable) • An identifier that specifies a generic type name • Used to declare return type, parameter types and local variable types • Act as placeholders for the types of the argument passed to the generic method • Actual type arguments • Can be declared only once but can appear more than once publicstatic < E > void printTwoArrays( E[] array1, E[] array2 )

  38. Generic Methods: Implementation and Compile-Time Translation (Cont.) • Compile-time translation • Erasure • Remove type parameter section • Replace type parameters with actual types • Default type is Object

  39. Additional Compile-Time Translation Issues: Methods That Use a Type Parameter as the Return Type • Application of Fig. 18.5 • Generic method • Use Type parameters in the return type and parameter list • Generic interface • Specify, with a single interface declaration, a set of related types • E.g., Comparable<T> • Method integer1.compareTo( integer2 ) • Compare two objects of the same class • Return 0 if two objects are equal • Return -1 if integer1 is less than integer2 • Return 1 if integer1 is greater than integer2

  40. Additional Compile-Time Translation Issues: Methods That Use a Type Parameter as the Return Type (Cont.) • Upper bound of type parameter • Default is Object • Always use keyword extends • E.g., T extends Comparable< T > • When compiler translates generic method to Java bytecode • Replaces type parameter with its upper bound • Insert explicit cast operation e.g., line 23 of Fig. 18.5 I preceded by an Integer cast (Integer) maximum( 3, 4, 5 )

  41. Overloading Generic Method • Generic method may be overloaded • By another generic method • Same method name but different method parameters • By non-generic methods • Same method name and number of parameters • When compiler encounters a method call • Search for most precise matching method first • Exact method name and argument types • Then search for inexact but applicable matching method

  42. Generic Classes • Generic classes • Use a simple, concise notation to indicate the actual type(s) • At compilation time, Java compiler • ensures the type safety • uses the erasure technique to enable client code to interact with the generic class • Parameterized classes • Also called parameterized types • E.g., Stack< Double >

  43. Generic Classes (Cont.) • Generic class declaration • Looks like a non-generic class declaration • Except class name is followed by a type parameter section • The –Xlint:unchecked option • Compiler cannot 100% ensure type safety

  44. Generic Classes (Cont.) • Generic class at compilation time • Compiler performs erasure on class’s type parameters • Compiler replaces type parameters with their upper bound • Generic class test program at compilation time • Compiler performs type checking • Compiler inserts cast operations as necessary

  45. Generic Classes (Cont.) • Creating generic methods to test class Stack< E > • Method testPush • Perform same tasks as testPushDouble and testPushInteger • Method testPop • Perform same tasks as testPopDouble and testPopInteger

  46. Wildcards in Methods That Accept Type Parameters • Data structure ArrayList • Dynamically resizable, array-like data structure • Method add • Method toString

  47. Wildcards in Methods That Accept Type Parameters • Motivation for using wildcards • Implement a generic method sum • Total the numbers in a collection • Receive a parameter of type ArrayList< Number > • Use method doubleValue of class Number to obtain the Number’s underlying primitive value as a double value

  48. Wildcards in Methods That Accept Type Parameters (Cont.) • Implementing method sum with a wildcard type argument in its parameter • Number is the superclass of Integer • ArrayList< Number > is not a supertype of ArrayList< Integer > • Cannot pass ArrayList< Integer > to method sum

  49. Generics and Inheritance: Notes • Inheritance in generics • Generic class can be derived from non-generic class e.g., class Object is superclass of every generic class • Generic class can be derived from another generic class e.g., Stack is a subclass of Vector • Non-generic class can be derived from generic class e.g., Properties is a subclass of Hashtable • Generic method in subclass can override generic method in superclass • If both methods have the same signature

  50. Collections

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