So You Think You Know Inheritance…

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You guys have already learned abstract, sub and super classes

Now lets take it up one more level!

Interfaces!

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What is an interface?-An interface is a type of an abstract

class that contains abstract methods. The methods in each interface provide a framework of behavior for any class.

-Interfaces are classes that may be adapted by more than one type of class.

-Interfaces usually represent something that “able” can be added to

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For Example

proSkaterObject

Rail Skateboard Curb

Grindable interface Each pro skater

object which is grindable,

implements the Grindable interface.

[little side note, this interface has “able” at

the end]

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How do we define interfaces?

public interface Grindable{

void showDamage();

Boolean isGrinded();

}

Declare it an “interface” class. Even though the class is abstract, for the class to be an interface, it needs to be declared

as one!

Create the methods and keep them un-instantiated. Since the class is an interface, the methods are understood to be abstract and do not need to be declared so. IMPORTANT

Interfaces do NOT contain instance

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How do we use interfaces?

public class Rail extends proSkaterObject implements Grindable

{

//variables and methods

}

We use an interface by using the key word

implements

The extends clause must precede the implements

clause

The class has its own methods and variables, as well as the methods

from the interface that are overridden

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An interface hierarchy usually looks like this:

Abstract Class

Subclass of

abstract class

Subclass of

abstract class

interface

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Let’s apply interfaces in the

real world

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Tony Hawk’s Pro Skater games are a great example of how interfaces are

used

Remember that pro skater hierarchy, lets revisit that…

Sick Grind

noob score!

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Refresher

pSObject

Rail Skateboard Curb

Grindable interface

This looks really confusing, but its not. Let’s break it down to

see where the interface is used

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Interface Hierarchy

Rail Curb

Grindable interface

This is just your ordinary inheritance hierarchy. Now, lets see how each class specifically uses the

interface

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A Closer Look[Gotta love those UML Diagrams]

interface Grindable

No instance variables allowed!

boolean isGrinded();

void showDamage();

Rail extends pSObject implements Grindable

//instance variables

boolean isGrinded();//check if object is grinded

void showDamage();//show sparks

Curb extends pSObject implements Grindable

//instance variables

boolean isGrinded();//check if object is grinded

void showDamage();//chipped pieces

Each subclass overrides the

methods of the interface

Interface is referenced by

using the implements

keyword

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interface Grindable

No instance variables allowed!

boolean isGrinded();

void showDamage();

Curb extends pSObject implements Grindable

//instance variables

boolean isGrinded();//check if object is grinded

void showDamage();//chipped pieces

Important:

Interfaces do NOT have instance

variables

Important:

The extends clause is ALWAYS before the implements clause

A Closer Look[Gotta love those UML Diagrams]

//instance variables

Rail extends pSObject implements Grindable

boolean isGrinded();//check if object is grinded

void showDamage();//show sparks

//instance variables

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-Interfaces are abstract, and their methods MUST remain un-instantiated

A couple of things to remember about interfaces

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-Interfaces are abstract, and subclasses that don’t override their methods become abstract too!

-Classes can implement as many interfaces as they want

-The extends clause must ALWAYS come before the implements clause

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The Comparable Interface

The standard java.lang package includes the Comparable interface, which contains a very

useful method for comparing objects

public interface Comparable

{

int compareTo(Object obj);

}

Any class that implements

Comparable must override the

compareTo method!

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Analysis of the compareTo

methodpublic interface Comparable

{

int compareTo(Object obj);

}

-This method compares an implicit object[this]

with the parameter object “obj”.

-This method will return a negative integer, zero, or a positive integer depending

on whether the implicit object is less than, equal, or

larger than the “obj”

Quick side note: If the two objects being compared are not type

compatible, then a ClassCastException is

thrown.

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