lecture 5

Lecture-5 & 6Instructor Name:

Object Oriented Programming

Today’s Lecture

Components of Class

Fields

Methods Main() Method, Setter Method, Getter Method, immutator.

Calling Method in same class Main method

Calling Method in other class. Creating Objects from class

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Class & Methods

Messages to Object

3

Object<7_series_BMW>

“Start the engine of the BMW”

Start_Engine

Class & Methods

Method of Class

4

Class<CAR>

Start_Engine

Class & Methods

Behavior Behavior is how an object acts and reacts, in terms of state

changes and interactions with other objects.

An operation is some action that one object performs upon another in order to elicit a reaction.

We will use the word method to describe object behavior in java.

Invoking a method causes the behavior to take place.

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Class & Methods

What is a Method? Method defines the behaviour or basic functionality of the

class.

Methods have two parts: a header and a body. Methods belong to a class

– Defined inside the class Heading

– Return type (e.g. int, float, void)– Name (e.g. nextInt, println)– Parameters (e.g. println(…) )– More…

Body– enclosed in braces {}.– Declarations and/or statements. 6

Class & Methods

Writing your own methods The general form of method definition is

scope type name(argument list) { statements in the method body}

where scope indicates who has access to the method, type indicates what type of value the method returns, name is the name of the method, and argument list is a list of declarations for the variables used to hold the values of each argument.

The most common value for scope is private, which means that the method is available only within its own class. If other Class need access to it, scope should be public instead.

If a method does not return a value, type should be void. Such methods are sometimes called procedures.

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Class & Methods

Returning value from methods You can return a value from a method by including a return

statement, which is usually written as

return expression; where expression is a Java expression that specifies the value

you want to return.

As an example, the method definition

private double feetToInches(double feet) { return 12 * feet;}

converts an argument indicating a distance in feet to the equivalent number of inches, relying on the fact that there are 12 inches in a foot.

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Class & Methods

Method Examplespublic class Dog {

private String name;private int age;

public setName(String dogName){name=dogName;

}

public String getName(){return name;

} }

public class CreateDog {public static void main (String args[]){

Dog myDog = new Dog();}

} 9

Class & Methods

Methods Invocation Invoked as operations on objects/Class using the dot ( . )

operator

reference.method(arguments) “Reference” can either be the class name or an object

reference belonging to the class Inside the class: “reference” can be ommitted

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Class & Methods

Methods Overloading A class can have more than one method with

the same name as long as they have different parameter list.

public class Pencil {. . .public void setPrice (float newPrice)

{price = newPrice;

}

public void setPrice (Pencil p) {price = p.getPrice();

}}

How does the compiler know which method you’re invoking? — compares the number and type of the parameters and uses the matched one

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Class & Methods

Methods – Parameter Value Parameters are always passed by value.

public void method1 (int a) { a = 6;}

public void method2 ( ) { int b = 3; method1(b); // now b = ?

// b = 3}

• When the parameter is an object reference, it is the object reference, not the object itself, getting passed.

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Method Example – (Parameter is Object Reference)

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- What is passed is the object reference, and it’s passed in the manner of “PASSING BY VALUE”!

class PassRef{public static void main(String[] args) { Pencil plainPencil = new Pencil("PLAIN"); System.out.println("original color: " + plainPencil.color);

paintRed(plainPencil);

System.out.println("new color: " + plainPencil.color);}

public static void paintRed(Pencil p) { p.color = "RED"; p = null;}

}

plainPencil

plainPencil

plainPencil p

plainPencil p

color: PLAIN

- If you change any field of the object which the parameter refers to, the object is changed for every variable which holds a reference to this object

color: PLAIN

color: RED

color: RED NULL

p

- You can change which object a parameter refers to inside a method without affecting the original reference which is passed

Class & Methods

Accessor & Mutator Methods Accessor methods are those which returns information to the

caller about the state of the object; they provide access to information about the state of object

Accessor usually contains the return statement in order to pass back that information.

getName()

Mutator method is one that change the state of an object.

The most basic form of object is one that takes a single parameter whose value is used to directly overwrite what is stored in the object’s field.

setName()

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Class & Methods

Accessor Methods

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Class & Methods

Mutator Methods

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Class & Methods

Keyword - thisCan be used only inside methodWhen call a method within the same class,

don’t need to use this, compiler do it for you.

When to use it?– method parameter or local variable in a method

has the same name as one of the fields of the class

– Used in the return statement when want to return the reference to the current object.

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Class & Methods

Keyword – this Example 1

When a method parameter or local variable in a method has the same name as one of the fields of the class, you must use this to refer to the field.

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class A{int w; public void setValue (int w) {

this.w = w; //same name!}

}

Class & Methods

Keyword – this Example 2

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class Exp{public int i=0; public Exp increment () {

i++;return this; // return current object

}

public static void main (String[] args){Exp e = new Exp();int v = e.increment().increment().i; // v=2!!

}}

Object Initialization

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When objects are created, the initial value of data fields is unknown unless its users explicitly do so. For example,– ObjectName.DataField1 = 0; // OR– ObjectName.SetDataField1(0);

In many cases, it makes sense if this initialisation can be carried out by default without the users explicitly initializing them.– For example, if you create an object of the class called

“Counter”, it is natural to assume that the counter record-keeping field is initialized to zero unless otherwise specified differently.

class Counter{

int CounterIndex;…

}Counter counter1 = new Counter();

– What is the value of “counter1.CounterIndex” ? In Java, this can be achieved though a mechanism called

constructors.

Class & Constructor

What is a Constructor?A special method automatically called when an

object is created by new() Java provide a default one that takes no

arguments and perform no special initialization – Initialization is guaranteed– All fields set to default values: primitive types to 0

and false, reference to null

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Class & Constructor

What is a Constructor? Constructor in java is a special type of method that is used to

initialize the object.

Java constructor is invoked at the time of object creation.

It constructs the values i.e. provides data for the object that is why it is known as constructor.

The constructors are responsible for ensuring that an object is set up properly when it is first created.

The construction process is also known as initialization.

If a constructor is not defined in the class, Java will create a default constructor that will invoke automatically when object is create.

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Class

What is a Constructor? Constructor is called at the time of object creation.

Once object has been created, the constructor plays no further role in that object life and can not be called on it.

One of the distinguished feature of constructor is that it has the same name as the class in which they are defined.

In the constructor the fields are initialized with default values or external information passed into object at the time of creation.

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Class

Constructor Rules and Example Constructor name must be same as class name. Constructor must have no explicit return type.

public class Dog {

private String name;private int age;

public Dog(){name=NULL;age=0;

}

rest of class }

In the constructor the fields are initialized with default values or external information passed into object at the time of creation.

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Class

Types of Constructor There are two types of Constructor

1. Default Constructor (No arguments required)A constructor that have no parameter (as in previous

slide)2. Parameterized Constructor

A constructor that have parameter. Example public Dog(String name, int age){

this.name=name;this.age=age;

}

Class Activity Can you guess what types some of the Book class’s fields

might be, from the parameters in its constructor? Can you assume anything about the names of its fields?

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Class & Constructor

Constructor Example

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class Circle{double r; public static void main(String[] args){

Circle c2 = new Circle(); // OK, default constructor

Circle c = new Circle(2.0); //error!!}

}

Class & Constructor

Constructor Example

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class Circle{double r; public Circle (double r) {

this.r = r; //same name!} public static void main(String[] args){

Circle c = new Circle(2.0); //OKCircle c2 = new Circle(); //error!!, no more default

}}

Circle.java:8: cannot resolve symbolsymbol : constructor Circle ()location: class Circle

Circle c2 = new Circle(); //error!! ^

1 error

Class & Constructor

Constructor Example

28Multiple Constructors Now. What is this?

class Circle{double r; public Circle(){

r = 1.0; //default radius value;}public Circle (double r) {

this.r = r; //same name!} public static void main(String[] args){

Circle c = new Circle(2.0); //OKCircle c2 = new Circle(); // OK now!

}}

Class

Exercise To what class the following constructor belong?

Public Student (String name)

Suppose that the class Pet has a field called name that is of type String. Write an assignment statement in the body of the following constructor so that the name field will be initialized with the value of the constructor’s parameter.

public Pet(String petsName){

} Write the constructor header for the following constructor

call. Date(“September”,08,2015)

Try to give meaningful name to the parameters29

Class & Constructors

Constructor Overloading One context in which you will regularly need to use

overloading is when you write constructors for your Class.

Constructors are methods that can be overloaded, just like any other method in a class.

In most situations, you will want to generate objects of a class from different sets of initial defining data

Overloading constructors provides a way to create objects with or without initial arguments, as needed

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Constructor Overloading Example 1public class Employee { public Employee (String n, double a) { name = n; salary = a; } public Employee ( ) { name = “ “; salary = 0; }} 31


Constructor Overloading Example 2public class MyClass{ int x; MyClass() { System.out.println("Inside MyClass() constructor."); x=0; } MyClass(int i) { System.out.println("Inside MyClass(int)

constructor."); x=i; } 32


Constructor Overloading Example 2 MyClass(double d) { System.out.println("Inside MyClass(double)

constructor."); x=(int)d; } void getXvalue() { System.out.println("The value of the instance

variable \nof the object " +this+" is "+x+"."); }}

This prints the address/reference of object 33


Constructor Overloading Example 2

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public class MyClassTest{ public static void main(String[] args) { MyClass first=new MyClass(); MyClass second=new MyClass(52); MyClass third=new MyClass(13.6); first.getXvalue(); second.getXvalue(); third.getXvalue(); }}



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OUT PUTInside MyClass() constructor.Inside MyClass(int) constructor.Inside MyClass(double) constructor.The value of the instance variable of the object TimoSoft.MyClass@19821f is 0.The value of the instance variable of the object TimoSoft.MyClass@addbf1 is 52.The value of the instance variable of the object TimoSoft.MyClass@42e816 is 13.


Constructor Overloading One common reason that constructors are

overloaded is to allow one object to initialize another.

The need to produce an identical copy of an object occurs often:

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public class MyClass{ int x, y; MyClass() { System.out.println("Inside MyClass() constructor."); x=0; y=0; } MyClass(int i, int j) { System.out.println("Inside MyClass(int) constructor."); x=i; y=j; }



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MyClass(MyClass obj) { System.out.println("Inside MyClass(MyClass) constructor."); x=obj.x; y=obj.y; } void getXYvalues() { System.out.println("The value of the instance variables \nof the object " +this+" are "+x+" and "+y+"."); }}



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public class MyClassTest{ public static void main(String[] args) { MyClass first=new MyClass(); MyClass second=new MyClass(52, 18); MyClass third=new MyClass(second); first.getXYvalues(); second.getXYvalues(); third.getXYvalues(); }}



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OutputInside MyClass() constructor.Inside MyClass(int) constructor.Inside MyClass(MyClass) constructor.The value of the instance variables of the object TimoSoft.MyClass@19821f are 0 and 0.The value of the instance variables of the object TimoSoft.MyClass@addbf1 are 52 and 18.The value of the instance variables of the object TimoSoft.MyClass@42e816 are 52 and 18.

Program Structure

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• A program consists of one or more Class

• Typically, each class is in a separate .java file

File File

File

FileClassVariable

sConstructors

Methods

Variables

Variables

Statements

Statements

Program

Home Assignment/ Self Review Exercise

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1. Write out a class Person. Write out a constructor that should take two parameters. The first is of type String and is called myName. The second is of type int and is called myAge. The first parameter should be used to set the value of a

field called name, and the second should set a field called age. You don’t have to include the definitions for the fields, just the text of the constructor.

2. Write an accessor method called getName that returns the value of a field called name, whose type is String.

3. Write a mutator method called setAge that takes a single parameter of type int and sets the value of a field called age.

4. Write a method called printDetails for a class that has a field of type String called name. The printDetails method should print out a string of the form “The name of this person is”, followed by the value of the name field. For instance, if the value of the name field is “Ali”, then printDetails would print:

The name of this person is Ali

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