Interfaces

Interfaces in JAVA
By Jai Vikas Marathe

Agenda
What is Interface?
Why Interface?
Interface as a Type
Interface vs. Class
Defining an Interface
Implementing an Interface
Implementing multiple Interface's
Inheritance among Interface's
Rewriting an Interface

What is an Interface?
It defines a standard and public way of specifying the
behaviour of classes
All methods of an interface are abstract methods

Example: Interface
// Note that Interface contains just set of
method
// signatures without any implementations.
// No need to say abstract modifier for each
method
// since it assumed.
public interface Relation {
public boolean isGreater( Object a, Object b);
public boolean isLess( Object a, Object b);

Example 2: Interface
public interface OperateCar {
// constant declarations, if any
// method signatures
int turn(Direction direction, double radius,
double startSpeed, double endSpeed);
int changeLanes(Direction direction, double
startSpeed, double endSpeed);
int signalTurn(Direction direction, boolean
signalOn);
int getRadarFront(double distanceToCar, double
speedOfCar);

Why do we use Interfaces?
Reason #1
To reveal an object's programming interface
(functionality of the object) without revealing its
implementation

Reason #2
To have unrelated classes implement similar methods
(behaviours)
Example:
Class Line and class MyInteger
Interface:
checkIsGreater(Object x, Object y)
checkIsLess(Object x, Object y)
checkIsEqual(Object x, Object y)

Reason #3
To model multiple inheritance

Interface vs. Abstract Class
All methods of an Interface are abstract methods while
some methods of an Abstract class are abstract methods
An interface can only define constants while abstract
class can have fields
Interfaces have no direct inherited relationship with any
particular class, they are defined independently

Interface as a Type
When you define a new interface, you are defining a
new reference type
You can use interface names anywhere you can use any
other type name
If you define a reference variable whose type is an
interface, any object you assign to it must be an instance
of a class that implements the interface

Example: Interface as a Type
Let's say Person class implements PersonInterface
interface
You can do
– Person p1 = new Person();
– PersonInterface pi1 = p1;
– PersonInterface pi2 = new Person();

Interface vs. Class:
Commonality
Interfaces and classes are both types
– For example:
// Recommended practice
PersonInterface pi = new Person();
Interface and Class can both define methods

Interface vs. Class:
DifferencesThe methods of an Interface are all abstract methods
You cannot create an instance from an interface
– For example:
PersonInterface pi = new PersonInterface();
//ERROR!
An interface can only be implemented by classes or
extended by other interfaces

Defining Interface
To define an interface, we write:
public interface [InterfaceName] {
//some methods without the body
}

Example
public interface Relation {
public boolean isGreater( Object a, Object b);
public boolean isLess( Object a, Object b);
public boolean isEqual( Object a, Object b);
}

Implementing Interface
public class Line implements Relation {
private double x1;
private double x2;
private double y1;
private double y2;
public Line(double x1, double x2, double y1, double y2){
this.x1 = x1;
this.x2 = x2;
this.y1 = y1;
this.y2 = y2;
} // More code follows

public double getLength(){
double length = Math.sqrt((x2-x1)*(x2-x1) +
(y2-y1)* (y2-y1));
return length;
}
public boolean isGreater( Object a, Object b){
double aLen = ((Line)a).getLength();
double bLen = ((Line)b).getLength();
return (aLen > bLen);}

public boolean isLess( Object a, Object b){
return (aLen < bLen);
}
public boolean isEqual( Object a, Object b){
return (aLen == bLen);
}}

Line.java:4: Line is not abstract and does not
override abstract method
isGreater(java.lang.Object,java.lang.Object) in
Relation
public class Line implements Relation
^
1 error

Implementing
Multiple
Interfaces

Relationship of an Interface to
a Class
A sub class can only extend one super class, but it can
implement multiple Interfaces
All abstract methods of all interfaces have to be
implemented by the sub class

Example
A sub class extends one super class but multiple
Interfaces:
public class ComputerScienceStudent
extends Student
implements PersonInterface,
AnotherInterface,
Thirdinterface{
// All abstract methods of all interfaces
// need to be implemented.}

Inheritance Among Interfaces
Interfaces can have inheritance relationship among
themselves
public interface PersonInterface {
void doSomething();
}
public interface StudentInterface extends
PersonInterface {
void doExtraSomething();
}

Problem
Consider an interface that you have developed called DoIt:
public interface DoIt {
void doSomething(int i, double x);
int doSomethingElse(String s);
}

Problem…
Suppose that, at a later time, you want to add a third
method to DoIt, so that the interface now becomes:
public interface DoIt {
void doSomething(int i, double x);
int doSomethingElse(String s);
boolean didItWork(int i, double x, String s);
}
8/16/13

Solution of Rewriting an
Existing Interface
For example, you could create a DoItPlus interface that
extends DoIt:
public interface DoItPlus extends DoIt {
boolean didItWork(int i, double x, String
s);
}

Interfaces

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