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A Java interface is a collection of abstract methods and constants An abstract method is a method header without a method body An abstract method can be declared using the modifier abstract but because all methods in an interface are abstract usually it is left off An interface is used to establish a set of



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Java Interface • Method Prototypes - An interface defines a set of method prototypes - Does not provide code for implementation -- just the prototypes - Can also define final constants • Class implements interface - A class that implements an interface must implement all the methods in the interface The compiler



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the same type A Java class uses: variables to define data fields and methods to define behaviors

What is inheritance in Java?

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What is implementation inheritance (subclassing)?

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Java - Generics, Interface, and

Inheritance

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The ArrayList Class

The ArrayListclass is part of the java.util

package

Like an array, it can store a list of values and

reference each one using a numeric index

However, you cannot use the bracket syntax

with an ArrayListobject

Furthermore, an ArrayListobject grows and

shrinks as needed, adjusting its capacity as necessary

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The ArrayList Class

An ArrayListobject is created as follows:

ArrayList band = new ArrayList();

A list of methods supported by ArrayListclass is

given in Chapter 7 of the text book. Some examples of methods: void add(int index, Object obj)

Object get(int index)

Elements can be inserted or removed with a single method invocation. For example: band.add(2,"Paul"); bandMember = band.get(1);

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The ArrayList Class

When an element is inserted, the other elements "move aside" to make room

Likewise, when an element is removed, the

list "collapses" to close the gap

The indexes of the elements adjust

accordingly

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The ArrayList Class

An ArrayListstores references to the Object

class, which allows it to store any kind of object

We can also define an ArrayListobject to

accept a particular type of object

The following declaration creates an ArrayList

object that only stores Familyobjects

This is an example of generics and we can call

ArrayLista generic type

ArrayList reunion = new ArrayList();

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ArrayList Efficiency

The ArrayListclass is implemented using an

underlying array

The array is manipulated so that indexes remain

continuous as elements are added or removed

If elements are added to and removed from the

end of the list, this processing is fairly efficient

But as elements are inserted and removed from

the front or middle of the list, the remaining elements are shifted

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Collection Classes

The Java standard library contains several

classes that represent collections, often referred to as the Java Collections API

Their underlying implementation is implied in

the class names such as ArrayListand

LinkedList

The classes are implemented as generic types

It means that the type of object can be

established when an object of that collection type is instantiated.

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Generic Types

A class can be defined to operate on a generic

data type which is specified when the class is instantiated:

LinkedList myList = new LinkedList();

By specifying the type stored in a collection, only objects of that type can be added to it

Furthermore, when an object is removed, its type

is already established

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Interfaces

A Java interfaceis a collection of abstract

methods and constants

An abstract methodis a method header without

a method body

An abstract method can be declared using the

modifier abstract, but because all methods in an interface are abstract, usually it is left off

An interface is used to establish a set of

methods that a class will implement

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Interfaces

public interface Doable public void doThis(); public int doThat(); public void doThis2 (float value, char ch); public boolean doTheOther (int num); }interfaceis a reserved word

None of the methods in

an interface are given a definition (body)

A semicolon immediately

follows each method header

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Interfaces

An interface cannot be instantiated

Methods in an interface have public visibility by

default

A class formally implements an interface by:

stating so in the class header providing implementations for each abstract method in the interface If a class asserts that it implements an interface, it mustdefine all methods in the interface

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Interfaces

public class CanDo implements Doable public void doThis () // whatever public void doThat () // whatever // etc. }implementsis a reserved word

Each method listed

inDoableis given a definition

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Interfaces

A class that implements an interface can

implement other methods as well See Complexity.javaSee Question.javaSee MiniQuiz.java

In addition to (or instead of) abstract methods,

an interface can contain constants

When a class implements an interface, it gains

access to all its constants

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// Complexity.java // Represents the interface for an object that can be assigned an // explicit complexity. public interface Complexity public void setComplexity (int complexity); public int getComplexity();

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// Question.java // Represents a question (and its answer). public class Question implements Complexity private String question, answer; private int complexityLevel; // Constructor: Sets up the question with a default complexity. public Question (String query, String result) question = query; answer = result; complexityLevel = 1; // Sets the complexity level for this question. public void setComplexity (int level) complexityLevel = level;

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// Returns the complexity level for this question. public int getComplexity() return complexityLevel; // Returns the question. public String getQuestion() return question; // Returns the answer to this question. public String getAnswer() return answer;

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// Returns true if the candidate answer matches the answer. public boolean answerCorrect (String candidateAnswer) return answer.equals(candidateAnswer); // Returns this question (and its answer) as a string. public String toString() return question + "\n" + answer;

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// MiniQuiz.java // Demonstrates the use of a class that implements an interface. import java.util.Scanner; public class MiniQuiz // Presents a short quiz. public static void main (String[] args)

Question q1, q2;

String possible;

Scanner scan = new Scanner (System.in);

q1 = new Question ("What is the capital of Jamaica?", "Kingston"); q1.setComplexity (4); q2 = new Question ("Which is worse, ignorance or apathy?", "I don't know and I don't care"); q2.setComplexity (10);

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System.out.print (q1.getQuestion());

System.out.println (" (Level: " + q1.getComplexity() + ")"); possible = scan.nextLine(); if (q1.answerCorrect(possible))

System.out.println ("Correct");

else System.out.println ("No, the answer is " + q1.getAnswer());

System.out.println();

System.out.print (q2.getQuestion());

System.out.println (" (Level: " + q2.getComplexity() + ")"); possible = scan.nextLine(); if (q2.answerCorrect(possible))

System.out.println ("Correct");

else System.out.println ("No, the answer is " + q2.getAnswer());

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MiniQuiz.java - Sample Execution

The following is a sample execution of

MiniQuiz.class

cuse93> java MiniQuizWhat is the capital of Jamaica? (Level: 4)

Kingston

Correct

Which is worse, ignorance or apathy? (Level: 10)

sorry i dont know

No, the answer is I don't know and I don't care

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Interfaces

A class can implement multiple interfaces

The interfaces are listed in the implements

clause

The class must implement all methods in all

interfaces listed in the header class ManyThings implements interface1, interface2 // all methods of both interfaces

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Interfaces

The Java standard class library contains

many helpful interfaces

The Comparableinterface contains one

abstract method called compareTo, which is used to compare two objects

The Stringclass implements

Comparable, giving us the ability to put

strings in lexicographic order

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The Comparable Interface

Any class can implement Comparableto provide a

mechanism for comparing objects of that type if (obj1.compareTo(obj2) < 0)

System.out.println ("obj1 is less than obj2");

The value returned from compareToshould be negative is obj1is less that obj2, 0 if they are equal, and positive if obj1is greater than obj2 When a programmer designs a class that implements the

Comparableinterface, it should follow this intent

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The Comparable Interface

It's up to the programmer to determine what

makes one object less than another

For example, you may define the compareTo

method of an Employeeclass to order employees by name (alphabetically) or by employee number

The implementation of the method can be as

straightforward or as complex as needed for the situation

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Interfaces

You could write a class that implements certain

methods (such as compareTo) without formally implementing the interface (Comparable)

However, formally establishing the relationship

between a class and an interface allows Java to deal with an object in certain ways

Interfaces are a key aspect of object-oriented

design in Java

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Inheritance

Inheritanceallows a software developer to derive

a new class from an existing one

The existing class is called the parent class,or

superclass, or base class

The derived class is called the child classor

subclass

As the name implies, the child inherits

characteristics of the parent

That is, the child class inherits the methods and

data defined by the parent class

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Inheritance

Inheritance relationships are shown in a UML class diagram using a solid arrow with an unfilled triangular arrowhead pointing to the parent class &DU •Proper inheritance creates an is-arelationship, meaning the child is amore specific version of the parent

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Inheritance

A programmer can tailor a derived class as

needed by adding new variables or methods, or by modifying the inherited ones

Software reuseis a fundamental benefit of

inheritancequotesdbs_dbs20.pdfusesText_26
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