Inheritence Java

8/2/2019 Inheritence Java

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Inheritance


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Single InheritanceThe class that is used as a basis

for defining a new class is called

aparentclass (or super class orbase class). The new class based

on the parent class is called a

childclass (or subclass or derived

class.)

In, children inherit characteristics

fromjust one parent. This is

called single inheritance. Some

languages allow a child to inherit

from more than one parent. This

is called multiple inheritance.

Java does not allow multiple

inheritance


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Syntax

A subclass inherits from a superclass using the

extends keyword

Inheritance is applicable to top-level classes,

nested top-level classes, member classes, localclasses and anonymous classes

classsubClassNameextendssuperClassName{

variable and method declarations}


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Example

public class Employee {

private String name;

private int id;

public String getName() {

return name;}

public int getId() {

return id;

}

}

public class SalariedEmployee extends Employee {

private int rate;

public int getRate() {

return rate;

}

}

public class HourlyEmployee extends Employee {

private int rate;

private int hours;


return rate;

}

public int getHours() {

return hours;

}

}


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Example

public class Payroll {

public static void main( String args[] ) {

SalariedEmployee lisa = new SalariedEmployee();

HourlyEmployee james = new HourlyEmployee();

System.out.println( lisa.getName() );

System.out.println( lisa.getId() );

System.out.println( lisa.getRate() );

System.out.println( james.getName() );

System.out.println( james.getId() );

System.out.println( james.getRate() );

System.out.println( james.getHours() );

}

}


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Example


public static void printEmployeeInfo( Employee e ) {

System.out.println( e.getName() );

System.out.println( e.getId() );

}

public static void main( String args[] ) {SalariedEmployee lisa = new SalariedEmployee();


printEmployeeInfo ( lisa );


printEmployeeInfo ( james );

System.out.println( james.getRate() );

System.out.println( james.getHours() );}

}


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Why Wont This Work?


public static void printEmployeeInfo( Employee e ) {

System.out.println( e.getName() );

System.out.println( e.getId() );

System.out.println( e.getRate() );

}


SalariedEmployee lisa = new SalariedEmployee();


printEmployeeInfo( lisa );


printEmployeeInfo( james );

System.out.println( james.getRate() );System.out.println( james.getHours() );

}

}


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Inheritance

A class can inherit from any class that is not final.

Objects of the subclass contain all the instance

variables and methods declared by the superclass.

The accessibility rules are still enforced a subclass cannot access the private parts of the

superclass.

Subclassing can be repeated as many times as

desired.

A class can have only one superclass, but may have

many subclasses.


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Why Wont This Work?


private int rate;

public SalariedEmployee( String n, int i, int r ) {

name = n;

id = i;rate = r;

}


return rate;

}

}


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Controlling Access to Class Members

Attribute Permitted accessNo access attribute From any class in the same package

public From any class anywhere. Enabled a class to

be accessed outside of its package. It also

enables a variable, method, or constructor to beaccessed anywhere its class may be accessed

Private No access from outside the class at all.

Prevents a variable, method, or constructor

from being accessed outside of he class inwhich it is declared

protected Intermediate level of protection. Enables a

variable, method or constructor to be accessed

by class or interfaces of the same package or

by sub-class of the class in which it is declared


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The access allowed between classes within the same

package

Class 2 Class 1

int a;

public int b;ptotected int c;

private int e;

Sub-Class 1

Package 1

Within a package, only the private members of the class Class 1can not be directly accessed by a method in another class in the same

package

No No


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The access allowed between classes within different

packages

Class 2 Class 1

int a;

public int b;ptotected int c;

private int e;

Sub-Class 1

Package 3 Package 1 Package 2

The only members of Class 1 that can be accessed from an ordinary class,

Class2 , in another package are those specified as public. Keep in mind thatthe class, Class 1, must also have been defined with the attribute public.


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Class designers use private data and public methods toenforce the notion of information hiding and the principle of

least privilege. If the client of a class needs access to data in

the class, provide that access through public methods ofthe class.


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Access to Inheriting Data Members

Sub Class

public int a;

protected int c;

Class

int a;

public int b;

ptotected int c;

private int e;

Sub-Class 1

int a ;

public int b;

protected int c;

Package 2 Package 1

No

No

No

A subclass that you define in the same package as its base inherits everything

except for private data member of the base public class. Members defined as

private in the base class are never inherited under anycircumstances.


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Constructors and Inheritance

The guarantee of proper initialization must bemaintained in the presence of inheritance.

Unless specified, java will call the default constructors

for each class in the inheritance hierarchy

Java provides syntax for explicitly controlling

which constructors are called.

The keyword super can be used to explicitly call

a superclass constructor super (argumentList) ;

supermust be the first statement in a constructor


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Example



private int id;

public Employee( String n, int i ) {

name = n;

id = i;}


return name;

}


return id;

}}


private int rate;

public SalariedEmployee( String n,

int i,

int r ) {super( n, i );

rate = r;

}


return rate;

}

}


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Why Wont This Compile?



private int id;

public Employee( String n, int i ) {

name = n;

id = i;}


return name;

}


return id;

}}


private int rate;


int i,

int r ) {rate = r;

}


return rate;

}

}


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Protected

Java provides a third access type calledprotected

Instance variables defined as protected can be

accessed by any subclass

by classes defined within the same package

Some feel that protected breaks encapsulation

Make everything private


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Example


protected String name;

protected int id;


return name;

}


return id;

}

}


private int rate;


int i,

int r ) {name = n;

id = i;

rate = r;

}


return rate;

}

}


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Scope Rules

Inheritance increases the number of scopes that

need to be searched (both static and instance

declarations are searched)

check the local scope check the class scope

check each superclass scope in turn up to the top of the

inheritance chain

If variables with the same identifier are declared inseveral scopes, the first one found is used.


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Method Overloading

Methods can beoverloaded, meaning that: Two or more methods in the same class:

can have the same name

provided they have different parameter lists, and

the return type is the same.

In a class with an overloaded method, all versionsof the method are available.

(Note that method overloading does not reallyhave anything to do with inheritance, but we willnext discuss overriding, which is easilyconfused with overloading.)


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Method Overriding

A subclass canoverridean inherited method byproviding a new method declaration that

has the same name,

has the same parameter list (in type and number) and

has the same return type as the one inherited.

Only the overridden method is directly accessible

The original method is still there, but you have to usesuper to access it

Method overriding relies on dynamic binding, sothe type of the object determines which methodgets called.


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Class Object

Every class in Java inherits from the class Object

Class Object declares the following methods

that can be overridden:

public boolean equals( Object obj );public String toString();

public final native int hashCode();

protective native Object clone();

protected void finalize();

public final Class getClass()


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Example



private int id;

public Employee( String n,

int i ) {

name = n;

id = i;

}


return name;

}


return id;

}

public String toString() {

return name +

", " +

id;

}}


private int rate;


int i,

int r ) {

super( n, i );

rate = r;

}


return rate;

}


return getName() +

", " +getId() +

", " +

rate;

}

}


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private int id;


int i ) {

name = n;

id = i;

}


return name;

}


return id;

}


return name +

", " +

id;

}}


private int rate;


int i,

int r ) {

super( n, i );

rate = r;

}


return rate;

}


return super.toString() +

", " +rate;

}

}


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Did You Ever Wonder

Did you ever wonder how println() or

print() work?

println() is an overloaded method

void println(boolean x) void println(char x)

void println(int x)

void println(double x)


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Final and Abstract Classes& Interfaces


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Restricting Inheritance

Parent

Child

Inheritedcapability

Fi l M b A f P ti


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Final Members: A way for PreventingOverriding of Members in Subclasses

All methods and variables can be overridden bydefault in subclasses.

This can be prevented by declaring them as

final using the keyword final as a modifier.For example: final int marks = 100;

final void display();

This ensures that functionality defined in thismethod cannot be altered any. Similarly, thevalue of a final variable cannot be altered.

Fi l Cl A f P ti


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Final Classes: A way for PreventingClasses being extended

We can prevent an inheritance of classes by otherclasses by declaring them as final classes.

This is achieved in Java by using the keyword final asfollows:

final class Marks{ // members

}

final class Student extends Person

{ // members

}

Any attempt to inherit these classes will cause an error.


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computePay()

For the Employee class we would like to add acomputePay() method

Where do we put it? Cant put it in the Employeeclass since we dont know how

to compute the pay for a generic employee

We really dont want to put it in the subclasses because wewont be able to say, for a generic Employee:

Employee x; x.computePay()

What we really want is:1. a placeholder in the Employee class, and

2. the method definition in each sublcass


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Abstract Methods

A method can be declaredabstract so that it must

be overridden by subclasses.

An abstract method does not have a method body;

the method declaration ends with a semi-colon,not a compound statement.

A class declaring one or more abstract methods

must be declared as anabstract class

Private and static methods cannot be abstract


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Abstract Classes

When we define a class to be final, it cannotbe extended. In certain situation, we want toproperties of classes to be always extended andused. Such classes are called Abstract Classes.

An Abstractclass is a conceptual class. An Abstractclass cannot be instantiated

objects cannot be created.

Abstract classes provides a common root for agroup of classes, nicely tied together in apackage:


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computePay()public abstract class Employee {


private int id;


int i ) {

name = n;

id = i;

}


return name;}


return id;

}

public abstract int computePay();


return name +

", " +

id;

}


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computePay()public class SalariedEmployee extends

Employee {

private int rate;


int i,

int r ) {

super( n, i );

rate = r;

}


return rate;

}

public int computePay() {

return rate;

}


return super.toString() +

", " +

rate;

}

}


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Abstract Class Syntax

abstract class ClassName{

...abstract Type MethodName1();Type Method2(){

// method body}

} When a class contains one or more abstract methods, it should be

declared as abstract class. The abstract methods of an abstract class must be defined in its

subclass. We cannot declare abstract constructors or abstract static

methods.


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Abstract Class -Example

Shape is a abstract class.

Shape

Circle Rectangle


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The Shape Abstract Class

Is the following statement valid? Shape s = new Shape();

No. It is illegal because the Shape class is an abstractclass, which cannot be instantiated to create its objects.

public abstract class Shape {public abstract double area();

public void move() { // non-abstract method

// implementation

}}


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Abstract Classes

public Circle extends Shape {

protected double r;

protected static final double PI =3.1415926535;

public Circle() { r = 1.0; )public double area() { return PI * r * r; }

}

public Rectangle extends Shape {

protected double w, h;

public Rectangle() { w = 0.0; h=0.0; }

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

}


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Abstract Classes Properties

A class with one or more abstract methods isautomatically abstract and it cannot beinstantiated.

A class declared abstract, even with no abstract

methods can not be instantiated. A subclass of an abstract class can be

instantiated if it overrides all abstract methodsby implementation them.

A subclass that does not implement all of thesuperclass abstract methods is itself abstract;and it cannot be instantiated.


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Summary

If you do not want (properties of) your class to beextended or inherited by other classes, define it as afinal class. Java supports this is through the keyword final. This is applied to classes.

You can also apply the final to only methods if you donot want anyone to override them.

If you want your class (properties/methods) to beextended by all those who want to use, then define itas an abstract class or define one or more of itsmethods as abstract methods. Java supports this is through the keyword abstract. This is applied to methods only. Subclasses should implement abstract methods; otherwise,

they cannot be instantiated.


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Interfaces

Design Abstraction and a way for

realizing Multiple Inheritance


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Interfaces

Interface is a conceptual entity similar to aAbstract class.

Can contain only constants (final variables) and

abstract method (no implementation) -Different from Abstract classes.

Use when a number of classes share acommon interface.

Each class should implement the interface.


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Syntax

An interface is declared as shown below:

The optional interfaceModifierallows aninterface to be declared public.

Any variables declared in an interface are: implicitly constants, and also

static

interfaceModifierinterfaceidentifier{

interfaceVariableDeclarations;

interfaceMethodDeclarations;

}

Interfaces: An informal way of


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Interfaces: An informal way ofrealising multiple inheritance

An interface is basically a kind of classit containsmethods and variables, but they have to be onlyabstract classes and final fields/variables.

Therefore, it is the responsibility of the class thatimplements an interface to supply the code formethods.

A class can implement any number of interfaces, butcannot extend more than one class at a time.

Therefore, interfaces are considered as an informal way

of realising multiple inheritance in Java.


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Implements

The implements keyword allows a class toimplement (conform to) one or more interfaces.

A class can implement any number of interfaces

(and also extend a class at the same time). Any variables defined in the interface become

static constants of the class.

A method declared in a public interface must bepublic in an implementing class.


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Interface - Example

speak()

Politician Priest

Speaker

speak() speak()

Lecturer

speak()


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Interfaces Definition

Syntax (appears like abstract class):

Example:

interface InterfaceName {

// Constant/Final Variable Declaration

// Methods Declaration only method body}

interface Speaker {

public void speak( );

}


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Implementing Interfaces

Interfaces are used like super-classeswhose properties are inherited by classes.This is achieved by creating a class that

implements the given interface asfollows:

class ClassName implements InterfaceName [, InterfaceName2, ]

{// Body of Class

}


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Implementing Interfaces Exampleclass Politician implements Speaker {

public void speak(){System.out.println(Talk politics);

}

}

class Priest implements Speaker {public void speak(){

System.out.println(Religious Talks);}

}

class Lecturer implements Speaker {

public void speak(){

System.out.println(Talks Object Oriented Design and Programming!);}

}


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Extending Interfaces

Like classes, interfaces can also be extended.The new sub-interface will inherit all themembers of the superinterface in the manner

similar to classes. This is achieved by using thekeyword extends as follows:

interface InterfaceName2 extendsInterfaceName1

{ // Body of InterfaceName2

}

Inheritance and Interface


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Inheritance and InterfaceImplementation

A general form of interface implementation:

This shows a class can extended another class whileimplementing one or more interfaces. It appears like a

multiple inheritance (if we consider interfaces as specialkind of classes with certain restrictions or specialfeatures).

class ClassName extends SuperClass implements InterfaceName [,

InterfaceName2, ]

{// Body of Class

}


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Example

public interface Calculator {public int add ( int x, int y );

public int sub ( int x, int y );

public int mult( int x, int y );

public int div ( int x, int y );

};

public class StandardCalculator implements Calculator {

public int add ( int x, int y ) { return x + y; }

public int sub ( int x, int y ) { return x - y; }

public int mult( int x, int y ) { return x * y; }

public int div ( int x, int y ) { return x / y; }

}


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Example

public interface StatCalculator {public double avg( int[] x );

public double sum( int x[] );

public double std( int[] x );

}

public class CalculatorWithStats implements Calculator, StatCalculator {

public int add ( int x, int y ){ return x + y; };

public int sub ( int x, int y ){ return x - y; };

public int mult( int x, int y ){ return x * y; };

public int div ( int x, int y ){ return x / y; };

public double avg( int[] x ) { return sum( x ) / x.length; }

public double sum( int[] x ) {

double s = 0;

for ( int i = 0; i < x.length; i++ ) { s = s + x[ i ]; }

return s;

}

public double std( int[] x ) {

double retVal = 0;

if ( x.length > 1 ) {

double sumDiff = 0;

double average = avg( x );

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

double diff = x[ i ] - average;

sumDiff = sumDiff + diff * diff;

}

retVal = Math.sqrt( sumDiff / ( x.length - 1 ) );

}

return retVal;

}

}

public interface Calculator {

public int add ( int x, int y );

public int sub ( int x, int y );

public int mult( int x, int y );

public int div ( int x, int y );

}


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Examplepublic class CalculatorWithStats2 extends StandardCalculator implements StatCalculator {

public double avg( int[] x ) { return sum( x ) / x.length; }

public double sum( int[] x ) {

double s = 0;

for ( int i = 0; i < x.length; i++ ) { s = s + x[ i ]; }

return s;

}

public double std( int[] x ) {

double retVal = 0;

if ( x.length > 1 ) {

double sumDiff = 0;

double average = avg( x );

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

double diff = x[ i ] - average;

sumDiff = sumDiff + diff * diff;

}

retVal = Math.sqrt( sumDiff / ( x.length - 1 ) );

}

return retVal;

}

}


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Testing the Calculator Class

public class TestCalculator {


int x = 4;

int y = 6;

int z = 7;

int[] numbers = {2,4,6};

CalculatorWithStats2 calc = new CalculatorWithStats2();

System.out.println( x + " + " + y + " = " + calc.add ( x, y ) );

System.out.println( x + " * " + z + " = " + calc.mult( x, z ) );

System.out.println( "avg of "

+ numbers[0] + ", " + numbers[1] + ", "

+ numbers[2] + " = + calc.avg( numbers ) );System.out.println( "std dev of "

+ numbers[0] + ", " + numbers[1] + ", "

+ numbers[2] + " = + calc.std( numbers ) );

}}


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Inheritance Among Interfaces

public interface StatCalculator extends Calculator

{

public double avg( int[] x );

public double sum( int x[] );

public double std( int[] x );}


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Static

Classes are mostly used to create objects, But a class has an existence. A class

and its objects are different types of things.

In the Java language, characteristics of a class definition (but not of its objects)

are called static. There is only one class definition for a given class, so when a

program is running, if something is static then there is only one of it.

You can think of the word static as meaning "no matter how many objects are

made, there will be only one of these."


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Static Methods

The methods that a class definition has are called staticmethods. (Sometimes they are called class methods, but

this is confusing.) A static method is a characteristic of a

class, not of the objects it has created.

Important:A program can execute a static methodwithout first creating an object!All other methods (not

static) must be part of an object, so an object must

exist before they can be executed.

Th t ti M difi


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The static Modifier

Static methods can be invoked through the class name ratherthan through a particular object

For example, the methods of the Math class are static:

Math.sqrt (25)

To write a static method, we apply the static modifier to themethod definition

The static modifier can be applied to variables as well

It associates a variable or method with the class rather thanwith an object


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Static Variables Static variables are also called class variables

Normally, each object has its own data space, but if a variable

is declared as static, only one copy of the variable exists

private static float price;

All objects created from the class share static variables

Changing the value of a static variable in one object changes it

for all others


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Static Methods

The order of the modifiers can be interchanged, but byconvention visibility modifiers come first

Recall that the main method is static; it is invoked by thesystem without creating an object

Static methods cannot reference instance variables,because instance variables don't exist until an object exists

However, a static method can reference static variables or

local variables

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Inheritence Java