Fall 2007 CS 225 1

Inheritance and Class Hierarchies

Chapter 3

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Chapter Topics

• Inheritance and how it facilitates code reuse• Polymorphism• Abstract classes• The Object class and its methods• Method overriding• Cloning and the difference between a true

clone (deep copy) and a shallow copy

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Chapter Topics (continued)

• Single versus multiple inheritance

• Interfaces and delegation to simulate multiple inheritance

• Object factory design pattern

• Packages and visibility

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Inheritance and Class Hierarchies

• One advantage 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

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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

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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

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Where would you add

• Cost

• Battery

• Time to discharge

• Number of expansion slots

• Wireless card present

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Initializing Superclass Data

• 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

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Protected Visibility

• 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

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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

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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(int inputA, int inputB, double

inputC)

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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

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Example

Computer [] comp = new Computer[3];comp[0] = new Computer( …);comp[1] = new LapTop( …);comp[2] = new Computer( …)for (int i=0; i<comp.length; i++) System.out.println(

comp[I].getRamSize() + "\n" + comp[I].toString();

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Interfaces

• An interface can declare methods but does not provide an implementation of those methods– Methods declared in an interface are called

abstract methods

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Abstract Classes

• An abstract class can have abstract methods, data fields, and concrete methods

• Abstract class differs from a concrete class in that– it cannot be instantiated– it can declare abstract methods, which must be

implemented in its subclasses

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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

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Abstract Class Number and the Java Wrapper Classes

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

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Class Object

• Object is the root of the class hierarchy

• All classes inherit the methods defined in class Object but may be overridden

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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

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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 strongly typed so the compiler needs to be able to verify that the type of the expression being assigned is compatible with the variable type

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Casting in a Class Hierarchy

• Java provides casting to enable us to process an 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

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Java 5.0 Reduces Need for Casting

• Two new features that reduce the need for casting:– Autoboxing/unboxing– Generics

• Autoboxing/unboxing eases the conversion between a primitive type and its corresponding wrapper type

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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

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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

• Will cause both e1.name and e2.name to reference “Jim”

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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

Will cause both e1.name and e2.name to reference “Jim”

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The Shallow Copy Problem

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• The statement e1.setAddressLine1("Room 224"); creates a new String object that is referenced by e1.address.line1 and e2.address.line1

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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

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Cloning

• After e1.setAddressLine1("Room 224"); only e1.address.line1 references the new String object.

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Employee.clone()

public Object clone() { try { Employee cloned = (Employee) super.clone(); cloned.address = (Address)address.clone(); return cloned; } catch (CloneNotSupportedException ex){ throw new InternalError(); }

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Address.clone()

public Object clone() { try { Address cloned = (Address) super.clone(); return cloned; } catch (CloneNotSupportedException ex){

throw new InternalError(); }

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Multiple Inheritance

• 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

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Multiple Interfaces to Emulate Multiple Inheritance

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Reuse Through Delegation• Delegation is used to reduce

– duplication of modifications • problems associated with version control

– A method of one class accomplishes an operation by delegating it to a method of another class

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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

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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

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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

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Visibility Summary

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Case Study: Problem Statement

• We want to draw some standard geometric shapes on the screen– rectangle, circle, right triangle

• Each shape has properties– position on the screen– interior color– border color

• Methods for– area, perimeter

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A Shape Class Hierarchy

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Rectangle Members

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A Shape Class Hierarchy (continued)

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Object Factories

• An object factory is a method that creates instances of other classes

• Object factories are useful when:– The necessary parameters are not known

or must be derived via computation– The appropriate implementation of an

interface or abstract class should be selected as the result of some computation

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Object Factory