Java Classes

Chapter 1

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Chapter Contents

Objects and ClassesUsing Methods in a Java Class• References and Aliases• Arguments and Parameters

Defining a Java Class• Passing Arguments• Constructors• The toString Method• Static Fields and Methods

Packages – The Java Class Library

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Objects

An object is a program construct that• Contains data• Performs certain actions

The actions are called methods

The actions interact to form the solution to a given problem

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Classes

A class is a type or kind of object

Objects of the same class have• The same kinds of data• The same methods

A class definition is a general description of• What the object is• What it can do

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An Example of a Class

Fig. 1-1 An outline of a class

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Instantiation of the Class

Fig. 1-1 Three instances of the class Automobile

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Using Methods in a Java Class

Given: jName joe = new Name();

The new operator creates an instance of the class• Invokes the constructor method

Valued methods return a single value

void methods do not return a value

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Using Methods in a Java Class

Fig. 1-2 A variable that references an object.

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References and Aliases

Primitive types:byte, short, int, longfloat, double, char, boolean

All other types are reference or class typesString greeting = "Howdy";• greeting is a reference variable

When two variables reference the same instance, they are considered aliases

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References and Aliases

Fig. 1-3 Aliases of an object

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Arguments and Parameters

GivenName joe = new Name();joe.setFirst ("Joseph");joe.setLast ("Brown");

"Joseph" and "Brown" are arguments sent to the methods

Invocation of method must have same number of arguments as there are formal parameters in the declaration

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Defining a Java Class

Given

These are the data fields (instance variables)

Note they are private• They will require accessor and mutator

methods

public class Name

{ private String first; // first nameprivate String last; // last name

< Definitions of methods are here. >

} // end Name

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Method Definitions

Given

• This is a valued method• Returns a String

Given

• This is a void method

public void setFirst(String firstName)

{ first = firstName; } // end setFirst

public String getFirst()

{ return first; } // end getFirst

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Naming Methods

Start method name with lowercase letter• Use verb or action phrase

Start class name with uppercase• Use noun or descriptive phrase

Local variables• A variable declared within a method

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Passing Arguments

Call by value• For primitive type, parameter initialized to

value of argument in call

Call by reference• For a class type, formal parameter is initialized

to the address of the object in the call

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Passing Arguments

Fig.1-4 a & b The method giveLastNameTo modifies the object passed to it as an argument.

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Passing Arguments

Fig.1-4 c & d The method giveLastNameTo modifies the object passed to it as an argument.

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Passing Arguments

Figure 1-5 a & b A method cannot replace an object passed to it as an argument.

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Passing Arguments

Figure 1-5 c & d A method cannot replace an object passed to it as an argument.

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Constructors

A method that• Allocates memory for the object• Initializes the data fields

Properties• Same name as the class• No return type, not even void• Any number of formal parameters

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Constructors

Fig. 1-6 An object (a) after its initial creation; (b) after its reference is lost

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Static Fields and Methods

A data field that does not belong to any one object

One instance of that data item exists to be shared by all the instances of the class

Also called:static field, static variable, class variable

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Static Fields and Methods

Fig. 1-7 A static PI versus a non static field

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Packages

Multiple related can be conveniently grouped into a packageBegin each file that contains a class within the package

package myStuff;

Place all files within a directory• Give folder same name as the package

To use the package, begin the program with

import myStuff.*;

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The Java Class Library

Many useful classes have already been declared

Collection exists in Java Class Library

Example• The class Math is part of the package java.lang