Interfaces in Java (1)

8/3/2019 Interfaces in Java (1)

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Interfaces in Java

Big Java - Chapter 09

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Some more design principles …

Code reuse is one of the major concern of softwareengineering

Not only saves time in terms of development, greatly

eases software maintenance cost

Programming for interface is one of the most

important strategy for software reuse

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Java Interfaces - Overview

Essentially Template Classes

Serves as design guideline

Typifies a class

Sketches out the outline (“skeleton”) of the class

Recommends class designers to follow the skeleton

What do we mean by “skeleton”?

What is then the class designer’s job?

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The Interface “Skeleton”

Two pieces Method

Field

Method skeleton

Method signature ONLY

NO code

Should be tagged public …. (Why??)

Field skeleton

Should be tagged public …. (Why??)

Should be tagged static … (Why??)

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Job of the class designer

Needs to create a class

Starts finding a matching template interface

If s/he finds one then implements the interface

ALL methods of the interface now get codes!

CANNOT directly use the interface

Interfaces DO NOT have objects … (why??)

ERROR:

Measurable x = new Measurable();2/13/2012 OOP 5



Looking more closely …

Consider DataSet class from Chapter 06

A simple class lets you find out MAX and AVG from a

set of values

getMaximum

getAverage method

Values are added one by one to it

Let’s see the code …

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Where’s the problem?

Imagine: Joe wants to use the DataSet class for BankAccount

objects

Harry wants to use the same DataSet class for Coin

objects

Both Harry and Joe want to have same statistics

(max and average) for a set of Coin and

BankAccount objects respectively

Problem:

DataSet is NOT coded for BankAccount/Coin

It only takes in double type variables (NOT object)

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So what to do? - I

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• Implementation DataSet classfor BankAccount Objects

• We have to do same for Coin

class as well!

OOPS!!! ……Lot of coding!!!

SHOULD WE?? … lets see



So what to do? - II

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Then what?

What is similar between Coin and BankAccount Class? Both needs to average out their amount

Bank: Amount is balance

Coin: Amount is value

Both needs to find their maximum amount

We should not duplicate the “behavior” of the primary

class (DataSet) To find maximum

To find average

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Towards a good strategy…

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Look at the places where we have beenchanging the code for having DataSet

for different types of objects




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You may suggest to overload the add method for different types.

Correct! But does that solve the problem?




What are all the things DataSet needs to knowabout the object (BankAccount/Coin)?

Only some measure of its “amount” (why??)

BankAccount: amount is balance (reqd. type:

double)

Coin: amount is value (reqd. type: double)

What should the object do?:

Object (BankAccount/Coin) should know how

to return its amount (balance/value) as double

Serves the purpose of DataSet

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Strategy

Assume: Objects (BankAccount/Coin) support an

operation getMeasure

Gives back the amount (hopefully in double)

Type of object is Measurable

Both Coin and BankAccount objects have decided

to come under one umbrella

Why? Both gets the same treatment!! – (max, avg.)

Then what about their own distinct characteristic?

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Strategy – Redesign DataSet

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What is Measurable? A Class?

BankAccount/Coin objects what to get under oneumbrella – Measurable

But they cant!!! why??

They have their own classes

Classes characterizes them as distinct entities

Do we add a method getMeasure to DataSet , that

returns double value of its amount?

If we do so, will compiler pass following statements-DataSet bankData = new DataSet();

bankData.add( new BankAccount(10000));

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The big question?

We want something like a Measurable object to passon to add ()

BUT, we have BankAccount/Coin objects

How do we make BankAccount/Coin Measurable?

Let us see …

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The answer – Type Substitution

Compiler is lucky!! … we have Type Substitution

Type substitution:

IF compiler expects type T

and

IF variable or expression of another type S is

“permitted”

THEN it is called type substitution

Note: this notion is much more powerful than simple

coercion

Ex:

a = 5/2.0 value stored in a as double 2/13/2012 OOP 20



You have already seen it!!

out.println( momsSaving );

type expected: Object

Permitted class of momsSaving: BankAccount

BankAccount is subtype of Object

g2.draw( rectangle1 );

type expected: Shape

Permitted class of momsSaving: BankAccount Rectangle is subtype of Shape

What’s the lesson? …

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Lesson A - Sub-Typing

Example subtyping (i.e. IS-A relation) BankAccount IS-A Object

SavingAccount IS-A BankAccount

Rectangle IS-A Shape

CONDITIONS:

Needs to guarantee that sub-type (Rectangle)

supports all the operations of its super-type (Shape)

Of course subtype can provide (and normally do) moreset of operations

Interface of subtype needs to be super-set of its

super type

Guess why this requirement?2/13/2012 OOP 22



Getting back to the problem …

How to make BankAccount Measurable? Two things:

Create a “type” Measurable

Make BankAccount as subtype of Measurable

But how??

Types are created by class and interface

constructs

Subtype are created by (two ways): Extending a Class: If class A extends class B,

then A is subtype of B

Implementing Interfaces: If class A implements

interface I, then A is subtype of I2/13/2012 OOP 23



End of sad days?

Well actually not!! How to decide which way to take:

Extending classes?

Implementing interfaces?

Imagine:

A situation where you really do not want to create

objects of the super-type

BUT you need to create objects of the sub-type Do we want Measurable objects? NO!!

Do we want BankAccount/Coin objects? YES!!

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Java Interfaces - I

Java Interface is just a set of operation specifications

Just the same as creating classes except

We do not (and actually cannot!!) provide

implementation (i.e. code) of any of method

Any class providing implementation of that interface

becomes subtype of the interface type

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Java Interfaces - II

Filename has to be same as of interface

Just like classes

Ex: interface Measurable needs to be stored in file Measurable.java

Public interfaces are available outside of the package inwhich they have been defined

Non-public interfaces have package level access

Methods of interface are always public – why??

A class implementing an interface necessarily need toprovide implementation for ALL methods of the

interface2/13/2012 OOP 26



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Next to Come ….

Polymorphism & interfaces

Event handling & interfaces

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Interfaces in Java (1)

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