object inheritance lecturer: kalamullah ramli electrical engineering dept. university of indonesia...

Object InheritanceObject Inheritance

Lecturer: Lecturer: Kalamullah RamliKalamullah Ramli

Electrical Engineering Dept.Electrical Engineering Dept.University of IndonesiaUniversity of Indonesia

Session-4Session-4

Slide - Slide - 22OOP Lecture 2004 Dr. –Ing. Ir. Kalamullah Ramli, OOP Lecture 2004 Dr. –Ing. Ir. Kalamullah Ramli, M.EngM.Eng

Extending classes & inheriting Extending classes & inheriting fields/methodsfields/methods Using this and super Constructor chaining Single and multiple inheritance

Class Extension and Inheritance


Inheritance: Natural, Hierarchical Way of Organizing Things

Staff Member

Employee Volunteer

Hourly Salaried

Consultant

Think in terms of “is a” relationships:An Employee is a Staff Member, An Hourly worker is a Employee.A Consultant is a(n) Hourly employee.

(subclass of Hourly)

(subclass of Employee)

(subclass of Staff)

(superclass)


class Animal { protected String strName = “”; protected String strNoise = “”; protected int iNumTimesPerformed = 0; // constructors, accessors & modifiers go here public void identifySelf( ) { System.out.println(“My name is “ + strName); } // of identifySelf public void perform ( ) { doYourThing( ); iNumTimesPerformed++; } // of perform public void doYourThing( ) { ; // ‘no-op’ method } // of doYourThing

} // of Animal

Example

Inheritance: SampleInheritance: Sample [1/4][1/4]


Subclasses(Dog extends Animal

i.e. “A dog is an animal” or“All dogs are animals”)

class Dog extends Animal { public Dog() { strNoise = “Woof”; } // of constructor

public void doYourThing ( ) { identifySelf(); System.out.println(“I am a dog”); System.out.println(strNoise); } // of doYourThing

} // of Dog

Recall: The Animal class had a no-op method for doYourThing()

Animal

Dog Cat Human



Subclasses(Cat extends Animal

i.e. “A cat is an animal” or“All cats are animals”)

class Cat extends Animal { public Cat() { strNoise = “Miaow”; } // of constructor

public void doYourThing ( ) { identifySelf(); System.out.println(“I am a cat”); System.out.println(strNoise); } // of doYourThing

} // of Cat

Animal

Dog Cat Human




Dog pickles = new Dog();pickles.setName(“Pickles”);pickles.doYourThing();// output:// “My name is Pickles”// “I am a dog”// “Woof”

Cat abby = new Cat();abby.setName(“Abby”);abby.doYourThing();// output:// “My name is Abby”// “I am a cat”// “Miaow”


Inheritance and Hierarchy

Variables (e.g. strNoise):• Java first examines current method, looks for local variable or parameter;• Java then examines current class (e.g. Dog);• Java then examines superclass (e.g. Animal); • Java continues up the class hierarchy until no more superclasses to examine.

Methods (e.g. doYourThing() or identifySelf()):• Java first examines current class;• Java then examines superclass;• Java continues up inheritance hierarchy until no more superclasses to examine.


Specifying ScopeSpecifying Scope [1/2][1/2]

Java allows you to override the scope rules by saying which variable/method you’re referring to:

Keyword super:keyword for specifying method or variable from superclass, e.g., super.doYourThing( )

Keyword this:keyword for specifying method or variable in current object, e.g., this.doYourThing( )


class Dog extends Animal { public Dog() { super.strNoise = “Woof”; } // of constructor

public void doYourThing ( ) { super.identifySelf(); System.out.println(“I am a dog”); System.out.println(strNoise); } // of doYourThing

} // of Dog

Same (in this case) asstrNoise = “Woof”;andthis.strNoise = “Woof”;

Same (in this case) asidentifySelf();orthis.identifySelf();

Specifying ScopeSpecifying Scope [2/2] [2/2]


Using Super

class Dog extends Animal { // constructor as before

public void doYourThing() {identifySelf();System.out.println(strNoise);

} // of doYourThing

public void identifySelf() { super.identifySelf();

System.out.println(“I am a dog”); } // of identifySelf

} // of Dog

Animal

Dog Cat

I.e.this.identifySelf()(newly defined below)

I.e. theidentifySelf()(defined in Animal)


class Shape {

public final double PI = 3.14159; protected String name; public String getName () { return (this.name); } // getName

public int area () { return (0); } // area

} // Shape

A Geometry ExampleA Geometry Example [1/2][1/2]

Shape

CircleRectangle


class Rectangle extends Shape { private int length, width; Rectangle () { this(0, 0); } // constructor

Rectangle (int l, int w) { this( l, w, “rectangle”); } // constructor

Rectangle (int l, int w, String n) { length = l; width = l; name = n; } // constructor

public int area () { return (length * width); } // area

public String getName () { if (length == width) return ("square"); else return (super.getName()); } // getName

public String toString () { String s; s = new String ("A " + getName() +

" with length " + length + " and width " + width);

return (s); } } // toString

} // Rectangle

A Geometry ExampleA Geometry Example [2/2][2/2]


Java’s rule:

• If first line of constructor is not an explicit call to a superclass constructor, Java will implicitly put super( ) as the first line, calling the superclass default constructor.

public Dog() { strNoise = “Woof”; } // of constructor

• An exception to this rule: chained constructor call tothis(params)

will defer super( ) call

• To use superclass constructors with params, call them explicitly, e.g., super(strName)

Constructors and Inheritance

implied call to Animal() here


Inheritance and Scoping Inheritance and Scoping [1/3][1/3]

Examples:super(xxx) // calls a superclass constructorsuper.xxx // accesses superclass’ variablesuper.xxx( ) // calls superclass’ method

this(xxx) // calls a current-class constructorthis.xxx // accesses current class’s variablethis.xxx( ) // calls current class’ method

Note: cannot do super.super<something>

(can achieve this effect via casting, but rarely should; details later...)


Inheritance and Scoping [2/3][2/3]

class StaffMember { String strName;

public StaffMember( ) { System.out.println (“in default StaffMem constr; No Name”); setName(“No Name”); } // of constructor

public StaffMember(String strName) { System.out.println (“in 2nd StaffMem constructior; have a Name”); setName(strName); } // of constructor

public void setName(String strName) { this.strName = strName; } // of setName

} // of StaffMember


class Employee1 extends StaffMember { public Employee1(String strName) { setName(strName); } // of constructor } // of Employee1

What happens???

Inheritance and Scoping [3/3][3/3]

Note: Employee has no local setName() method

class Employee2 extends StaffMember { public Employee2(String strName) { setName(strName); } // of constructor

public void setName(String strName) { super.setName(strName); System.out.println (“Name set”); }} // of Employee2


Class Object

• Java provides a base class, Object

• All classes that do not have an extends clause implicitly inherit directly fromclass java.lang.Object

Examples:

public boolean equals (Object o)

public boolean String toString ()

• When you create your own toString( ) method for a class, you are overriding the toString( ) provided by Object.


Object HierarchyObject Hierarchy

Animal

Dog Cat

Object

Employee

Salaried Hourly

class Object methods: String toString() boolean equals(Object obj) and a few others...

Animal

Dog Cat Human?

Object

Employee

Salaried Hourly

Or how about...


The EndThe End

QUESTIONS & QUESTIONS & COMMENTS ?COMMENTS ?

object inheritance lecturer: kalamullah ramli electrical engineering dept. university of indonesia...

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