Topic 3 Part 1 Class Object

7/30/2019 Topic 3 Part 1 Class Object

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OBJECT ORIENTED

PROGRAMMING IN JAVAUnderstand Objects and Classes


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COURSE LEARNING OUTCOMES :

Upon completion of this course, the students

should be able to:

1. Describe the features and basic concepts of Java

language.

2. Write, compile Java source code and interpret Java byte

code using Java Development Kit (JDK).

3. Implement the key of OOP concepts: classes and objects,

inheritance and polymorphism.

4. Incorporate exception handling in Java Programming.


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OBJECTIVES :

1. Create classes in Java program

2. Identify built-in classes in Java program

3. Create an object in Java program

4. Explain the concepts of accessing objects via reference

variables

5. Explain the concepts of accessing objects data and

method6. Explain the scope of variables in classes


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Introduction

Java is an object-oriented programming

language.

The concept behind object-oriented

programming is implemented in Java usingclasses and objects.

Classes A class is a user-defined data

type. Class is a container which holds

variables and methods.

Objects The Objects are created to

access the variables and methods from

classes.


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Create Classes in Java Program

class Book

{

//body of the class Book

}


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Create Classes in Java Program

Variables and methods are declared in the

body of class.

Methods are necessary for operating with

the data contained in the class. Methoddeclarations have four basic parts:

The name of the method (methodname).

The type of the value the method returns(type).

A list of parameters (parameter-list).

The body of the method.


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Build-in classes in Java library

A Java class library specification is a very

detailed document outlining the classes in

the API (Application Programming

Interface).

Every API includes documentation

describing the use of classes, variables and

methods in the programs.

To view the Java SE (Standard Edition) API

specification online, go to the following web

site :

http://docs.oracle.com/javase/7/docs/api/in

dex.html
http://docs.oracle.com/javase/7/docs/api/index.htmlhttp://docs.oracle.com/javase/7/docs/api/index.htmlhttp://docs.oracle.com/javase/7/docs/api/index.htmlhttp://docs.oracle.com/javase/7/docs/api/index.html


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Create an Object in Java Program

Object is an entity that consists of variables

and methods to manipulate those variables.

The class members can be accessed through

objects..

For example,

Book b1;

b1 = new Book( );


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Create an Object in Java Program

Primitive Variable :

Store value

Eg : int a =7;

Reference variable : Also call object references

Store address

Eg:

Shirt myShirt = new Shirt();


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To declare, instantiate and initialize

an object variable :

1. Declare a reference to the object by

specifying its identifier and the type ofobject that the reference points to class

of the object

2. Create the object instance using new

keyword.3. Initialize the object reference variable

by assigning the object to the object

reference variable.


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Declare Object Reference Variables

To declare an object reference variable, state theclass that you want to create an object from.

Select the name you want to use to refer to the

object.

The syntax is :

classname identifier;

Eg :

Shirt myShirt();

Where :

The classname is the class or type of object

referred to with the object reference

The identifier is the name you assigned to

the variable of type classname


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Instantiating an Object

After declare the object reference, you can

create the object that you refer to.

The syntax for instantiating an object is :

new classname(); Eg :

new Shirt();

Where :

The new keyword creates an object instance froma class

The classname is the class or type of object being

created


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Initializing Object Reference Variable

Use sign (=) to initialize the object

reference.

The syntax for initializing an object to an

object reference variable is :identifier = new classname();

Eg :

myShirt = new Shirt();

When you have created a valid objectinstance, you can use it to manipulate an

objects data or call an objects methods.


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Using an Object Reference Variable to

Manipulate Data To manipulate the value or to invoke themethods of a specific object used (.) dot

operator.

Eg :myShirt.colorcode=B;

myShirt.displayData();


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Storing Object Reference Variables in

Memory Primitive Variable : Store value

Eg : int a =7;

Reference variable : Also call object references

Store address/location(memory address)

Eg:

Shirt myShirt = new Shirt();

Memory address is written in hexadecimal

notatiom (eg : 0x334009)

The memory address is assigned while a program

runs.


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How reference variables are stored in

memory??public static void main(String args[])

{

int counter;

counter = 10;Shirt myShirt = new Shirt();

}


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counter

myShirt

yourShirt

0x34009

0x99f311

Stack Memory Heap Memory

10

0

0.0

U

0

0.0

U

0x34009

0x34009

ShirtID

price

colorCode

ShirtID

price

colorCode


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Assigning a Reference Fron One

Variable To Another Example : ThemyShirt and yourShirt reference

variables contain the same address to the

same object.Shirt myShirt = new Shirt();

Shirt yourShirt = new Shirt();

myShirt = yourShirt;

The value of the item on the right isassigned to the item on the left.

The line of code in the figure would appear

in a method, such as the main method.


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Variable To Another

counter

myShirt

yourShirt

0x34009

0x99f311

0x99f311

Stack Memory Heap Memory

10

0

0.0

U

0

0.0

U

0x34009

0x34009

ShirtID

price

colorCode

ShirtID

price

colorCode

Unreference

memory >>garbage collected


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Variable To Another

The address in the reference variableyourShirt (0x99f311), is assigned to the

variable myShirt.

Both variable now point to the same object,even though the other object.

The one that the variablemyShirt once

pointed to, still exists.

Unless another reference variable waspointing to the second Shirt object, the

object is garbage collected.


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METHOD IN JAVA PROGRAM


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Preview Questions

State TRUE or FALSE

1. A class can contain ONLY ONE method

2. The basic form of method must return a

string

3. Worker methods can also caller methods4. Main method accepts an array of strings

5. A methods can only have one parameter

6. Methods cannot have the same name as the

class.7. Methods can be declares outisde a class as

a standalone method.


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Define Method

In a class is contained within one or more

methods.

The syntax of all method declarations is :

[modifiers] return_type method_identifier([arguments])

{

//method code block

}


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Where :

[modifiers] represent several Java

technology keywords that modify the way

methods are used. Modifiers are OPTIONAL

return_type is the type of value return from amethod that can be used elsewhere in the

program. Methods can return only ONE item

(literal value, variable, object reference and soon). If nothing is be returned, the keyword void

must be specified as the return type. Method_identifier is the name of the

methods


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The ([arguments]) represent a list of

variables whose values are passed to the method

for use by the method. Arguments are OPTIONAL

(indicated by the square brackets); many method

do not accept arguments.

Themethod_code_block is a sequence of

statements that the method performs. A wide

variety of tasks can take place in the code block

or body of a method.


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Basic Form of a Method

The basic form of a method accepts no arguments and

returns nothing.

The following display method from Shirt class is a basic

method :public void displayInformation() {

System.out.println(Shirt ID : + shirtID);

System.out.println(Shirt description : +

description);

System.out.println(Color Code: + colorCode);System.out.println(Shirt Price: + price);

System.out.println(Quantity in stock: +quantityInStock);

} // end of display method

This method prints several lines of information.


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Invoking a Method From a Different

Class

To invoke/execute a method in a different class,

use the dot (.) operator with an object reference

variable to do to access the public variables of

an object.

E.g :public class ShirtTest {

public static void main(String args[]) {

Shirt myShirt;

myShirt = new Shirt();

myShirt.displayInformation();

}

}


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In this e.g, an object reference variable callmyShirt is declared and initialized to aShirt object (on line 3 and 4).

ThemyShirt object reference variable

then invokes the display method within theShirt object (line 6).


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Calling and Worker Methods

In the previous example, the ShirtTest classcall the display method from within anothermethod (mainmethod).

Therefore, themainmethod is referred to as the

calling method because it is invoking or calling

another method to do some work.

Conversely, the display method is referred to as

the worker method because itdoes some work for

the main method.

Many methods are both calling and worker

methods because they not only do some work, but

call other methods too.


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When a calling method calls a worker

method, the calling method stops execution

until the worker method is done.

After the worker method has completed,

program flow is said to return to the point

after the method invocation in the calling

method (return to line 6 in theShirtTest.java class)


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Invoking a Method in the Same Class

Calling a method in the same class is easy,

justinclude the name of the worker method

and its arguments, if any.


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Guidelines for Invoking Methods

1. There is no limit to the number of method calls

that a calling method can make.

2. The calling method and the worker method can

be in the same class or in a different class.

3. The way to invoke the worker method is

different, depending on whether it is in the same

class or a different class from the calling method.

4. You can invoke methods in any order. Methods

do not need to be completed in the order in

which they are listed in the class where they are

declared (the class containing the worker

methods)


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Passing Arguments and Returning

Values

Methods can be invoked by a calling method

with a list of arguments (variables or values

to be used by the worker method).

Additionally, methods can return a value to

the calling method that can be used in the

calling method.


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1

2

3

910..

4

5

6

7

8

Object 1 Object 2

V1

V2

Caller method Walker method

Value 1 being

passed from

object 1 toobject 2

Object2

returns value

2 to object 1


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Topic 3 Part 1 Class Object

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Transcript of Topic 3 Part 1 Class Object