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1Java

Interface

Part 1

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2Java

Result10

Heap class using Comparable objects9

Set Standard- Comparable example8

Set Standard7

Shared constants6

Uses5

Solution to student-teacher problem4Syntax3

Definition2

Dilemma1

contents

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3Java

instanceof14Cloneable13

clone() method of Object class12

Tagging11

contents

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4Java

Know

What interfaces are and why they are used

What a Marker interface is

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5Java

Be Able To

Write programs using interfaces where

appropriate

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6Java

Dilemma !

How will we represent a

teacher who is also a

student?

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7Java

StudentTeacherStudent

Teacher

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8Java

Definition

An interface is a special type of class which may have

b) some constants

c) some methods listed but with no implementations.

Always public, static and final

Always public and abstract

Interface cannot be instantiated.

They dont automatically inherit from Object class.

A class can inherit from more than one interface.

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9Java

Syntax

interface interface_name{datatype variable_name=value;

returntype method_name();}

class class_name implements

interface_name{returntype method_name(){}

}

public or static or

final can be

explicitly specifiedbut not necessary.

public or abstract

can be explicitly

specified but not

necessary.

public or default

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10Java

TeachingStaff

int getFactId()

void display()

Teacher

int getFactId()

void display()

StudentTeacherStudent

Step 1

Step 2

Step 3StudentTeacher

TeachingStaff

Solution to student-teacher

problem

Indirect relationship through the interface

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11Java

package teacher;

public interface TeachingStaff{

// important methods of Teacher class}

package teacher;

public class Teacher implements TeachingStaff{

/* implement all the methods of TeachingStaff*/ }

package student;

import teacher.*;

public class StudentTeacher extends Studentimplements TeachingStaff{

/*implement all the methods of TeachingStaff*/}

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12Java

Now before looking at the complete solution, let

me get this straight. Using an Interface will make

me implement all the important methods inside

both the StudentTeacher and Teacher classes. If

that is the case, then why do I need an interface at

all? The main reason we go for inheritance is for

reusability. And I dont see any reusability here. Ineed to copy and paste code from Teacher class

into StudentTeacher class anyway.

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13Java

Interface will help you relate classes. Why do I need to

relate ? - Polymorphism!

Lvoid listInvigilators(TeachingStaffs[])for(int i=0;i

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14Java

And we have formulated a betterway to implement methods of

interface than copy-paste. Look at

the code in the next slides.

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15Java

package teacher;

public interface TeachingStaff{

int getFactId();String getName(); }

-----------------------------------------

package teacher;

public class Teacher extendsgeneral.Personimplements TeachingStaff{

public int getFactId(){ return factId;}

} Note that it is an error if we omit public here

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16Java

package student;

import teacher.*;

public class StudentTeacherextendsStudent implements TeachingStaff

{private Teacher t;

public StudentTeacher(String name) {

super(name); t= new Teacher(name);

}

to help us call the teacher

class methods

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17Java

public int getFactId()

{return t.getFactId();}

public String getName(){

return t.getName();}

}

Notice how we

use private

reference of

teacher class tocall methods of

the teacher class

in order to avoid

retyping the samecode !

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18Java

import teacher.*;import student.*;import admin.*;class Test{static void listInvigilators(TeachingStaffs[]){for(int i=0;i

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19Java

Having said so much about interface being used to overcome

multiple inheritance problem, let me also tell you this.

What we have seen is only a small aspect of the benefits that

arise from using interfaces. You will be surprised to know that

interfaces are one of the most important design elements that

are used.

We will not deal with application designing here. There is much

more that we need to learn before we get there.

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20Java

Uses

Interfaces are used to

2. overcome the issues that arise because Java does not

support multiple inheritance.

3. used to share constants

4. used to set or define standards

5. used for tagging

6. used as callbacks

We have already seen this

We will see this use when we do

threads and AWT

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21Java

Shared constantspublic interface TeachingStaff {

int RETIREMENT_AGE=60;

}

import static teacher.TeachingStaff.

RETIREMENT_AGE;

class AnyClass{

System.out.println(RETIREMENT_AGE);

}

Do you remember this syntax?

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22Java

Set Standard

Standards must be set for the

bulb holder so that

b) any bulb can fit into anyholder.

c) it is easy for the bulb factory

to design bulbs.

I am not sure if I willfit into those non-

standard holders !

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23Java

Set Standard- Comparable example

Grade

int compareGrades(Grade g)

Will return 0 if grades are same , a value >0

if the current grade is greater than the the

grade object being passed.

Test

void rank(Grade[] g)

Student[] search(Grade g, String grade)

Will use

compareGrades() to

compare the grades

S t St d d C bl

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24Java

Set Standard- Comparable

example

Student

int compareTo(Object o)

java.lang.Comparable

int compareTo(Object o)

java.util.Arraysint binarySearch(Object[] a,Object key)

void sort(Object[] a)

Can be used to sort

and search grades

since Grade has

implemented

Comparable

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25Java

Why is java compelling me to

implement this interface ?

Think about it. How will the Array class

know how to sort your objects?

In order make sure that grades are sortedproperly java sets the standard by

compelling you to implement the

Comparable interface.

Like Arrays, there are many more classesthat have many more utility methods that

we can make use of if we implement

Comparable interface.

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package student;

public class Student extendsgeneral.Person implements Comparable{

public int compareTo(Object o) {

Student s=(Student)o;return

this.getName().compareTo(s.getName());

}}

Folder 2

compareTo() is already overridden in the String class

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27Java

Heap class using Comparable

objectspublic class Heap{

private Object heap[];

private int noOfElements;

private int currentPosition=1;

Heap(int noOfElements){

this.noOfElements=noOfElements;heap= new Object[noOfElements+1];

}

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28Java

public void insert(Object ele){if(currentPosition>noOfElements+1){System.out.println("Can't add any more!");

return;}else{

heap[currentPosition++]=ele;if(currentPosition-1!=1)

bubbleUp(currentPosition-1);return;

}}

void bubbleUp(int exchangeIndex){if(exchangeIndex==1){ return; }

int parentIndex=exchangeIndex/2;

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29Java

Comparablec1=(Comparable)heap[exchangeIndex];Comparable

c2=(Comparable)heap[parentIndex];if(c1.compareTo(c2)

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30Java

public void display(){for(int i=1;i

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31Java

void heapify(int index){int left=index*2;int right=index*2+1;

int temp=index;if(left0){exchange(index,temp);heapify(index);. }}}

smallest between left and right nodes

Exchange with smaller node

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32Java

public void heapSort(){int tot=currentPosition-1;

System.out.println("Heap Sort");for(int i=0;i

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heap.insert(45); heap.insert(21);heap.insert(18); heap.insert(14);heap.insert(60); heap.insert(32);

heap.insert(6); heap.heapSort();Heap sheap =new Heap(4);System.out.println("Inserting students

into heap");sheap.insert(new Student("Rama"));sheap.insert(new Student("Sita"));sheap.insert(new Student("Haunuman"));

sheap.insert(new Student("Ravana"));sheap.heapSort();}

}

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34Java

ResultInserting integers into heapHeap Sort

6141821

324560Inserting students into heapHeap SortHaunuman-3Rama-1Ravana-4Sita-2

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35Java

When we come to collection

classes, we will be learning

about Arrays class and many

more collection classes. There

make use of Comparable

interface extensively.

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36Java

Tagging

Interfaces which do not have any methods are

called Marker interfaces.

Marker interfaces are used to tag a class so that

the class is of the interface type. Some examples of marker interface in JDK are:

d) Cloneable

e) Serializablef) Remote

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37Java

clone() method of Object class

Teacher

factid =123

clone()

object

clone()

cloned object is returned

cloned object

Teacher

factid =123

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38Java

clone()

object

clone()

Teacher

factid =123

GradeEXCELLIENT1000

GradeEXCELLIENT1000

referenceAddress

1000

cloned object is returned

cloned object

returned

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39Java

Cloneable

That is the reason why the clone() method in

Object class is protected.

protected Object clone() throwsCloneNotSupportedException{}

If clone() method has to be given public access

then we need to override it.

To make other objects aware that the clone()method is implemented rightly in this class, this

class must implement the interface Cloneable.

package teacher;

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40Java

package teacher;public class Teacher extends general.Personimplements TeachingStaff,Cloneable{

public Object clone()throws

CloneNotSupportedException{return super.clone(); }}

public static void main(String str[]) throws

CloneNotSupportedException{Teacher t1=new Teacher("Gayatri");Teacher t2=(Teacher)t1.clone();t2.setName("Malini");

System.out.println(t1.getFactId()+""+t1.getName());System.out.println(t2.getFactId()+""+t2.getName());}

}

We are ok with the Object

classs clone()

implementation.

Output:

1 Gayatri

1 Malini

Folder 3

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41J

instanceof reference instanceof ClassName

We have seen this in the Classes Explored chapter. If the

actual type of referenceon the LHS has no directrelationship with the class on RHS then a compilation

error occurs. Rewriting the same syntax to include

interface also we have,

reference instanceofClassName/InterfaceName

The reference type can be of Interface or Class type.

If LHS or RHS have interface type, then there is no

compilation error that occurs.