Topic 8

Topic 8 Classes, Objects and Methods 1

Topic 8

Class and Method Definitions Information Hiding and Encapsulation Objects and Reference

Classes, Objects, and Methods


OO revision Objects

» A capsule that contains 2 important parts– State (attributes or data)

What does the object know?– Behaviour (methods or functions)

What does the object know how to do?» E.g. Cars, employee records, etc.

Class» is the definition of a kind of object (template for creating objects).» E.g. a general description of what an automobile is and what it can do» Encapsulates values & operations on those values.» Also used to group related methods, like the Math class.

Instantiate» An object that satisfies the class definition is said to instantiate the class.


Class Definition

Class Name: Car

Data:

amount of fuel

speed

license plate

Methods (actions):

increase speed:

How: Press on accelerator pedal

stop:

How: Press on brake pedal


General class syntax:

modifier(s) class ClassIdentifier{classMembers

}

Modifier(s)» Used to alter the behaviour of the class» public, private, static

ClassMembers» Named constants, variable declarations and/or methods» Can also be declared as public, private or static


class Car

{

double fuelAmount = 0.0;

double speed = 0.0;

String licencePlate = “ “;

}


Instantiations of the Class Car

amount of fuel: 10 litres

speed: 55 km / hr

licence plate: “135 XJK”


speed: 0 km /hr

licence plate: “SUES CAR”

First InstantiationObject name: patsCar

Second InstantiationObject name: suesCar

Third InstantiationObject name: ronsCar


speed: 75 km / hr

licence plate: “351 WLF”


Car Example

Car Class» Says Car objects has 3 pieces of data (i.e. specifies what kind of data

they have.– (litres of fuel in tank; how fast car is moving; license plate)

» Class definition has no data individual objects have the data.

» Also specifies what actions (methods) the objects can take & how they accomplish those actions.– increaseSpeed – stop

» All objects of the Car class have the exact same methods.


Car Example continued…..

Car Object» Instantiates the Car class.» Each object has a name these are variables of type Car.

– patsCar, suesCar, ronsCar


Types of methods

Every method belongs to a class. Method definitions given inside the definition of the class.

Single value methods» Used to return Java values

Void-methods» Performs some action rather than return a value.» Used to produce Java statements.» E.g. print a message or read in a value from the keyboard) and do

not return a value


Single-value Method Definitions:

public Type_Returned Method_Name(parameters){ list of statements must contain a return statement}

public double getFuelAt(){

return fuelAmount;}


void Method Definitions:

public void Method_Name(parameters) HEADING{ list of statements BODY}

public void writeOutput(){

System.out.println(“License: “ + licence); System.out.println(“Fuel: “ + fuelAmount);

System.out.println(“Speed: “ + speed);return; Can be left out

}


PARAMETERS

Formal parameters» Used in the method definition

Actual parameters» Value of the variable that is passed to the

method when that method is called/invoked.


Method invocation / Call a method

Object_name.method_name();

Method defined in a class is usually invoked using an object of that class object calling a method


CONSTRUCTORS: Special type of method .

» Not void » Does not return a value

One or more found in every class.

Has the same name as the class & it executes automatically when an object of that class is created.

Used to guarantee that the instance variables of the class are initialised to specific values.

Types of constructors» With parameters» Without parameters (known as the default constructor)


Instantiating / Constructing Objects

To instantiate an object, use the keyword new followed by the class’s constructor method (creates a new instance of a class).

Car patsCar; // creates a new Car variable

c = new Car(); // invokes the constructor method

or

Car patsCar = new Car();


The Member Access Separator

Once you’ve constructed a car, you want to do something with it. To access the fields or methods of the car you use the . (dot) separator.

» Selects a specific member of a Car object by name.

Car patsCar = new Car();

patsCar.fuelAmount = 10.0;

patsCar.speed = 55.0;

patsCar.licencePlate = “135XJK “;

System.out.println(c.licencePlate +

“ is moving at “ + patsCar.speed + “ kms/hr”);


Class file REMINDERS

Each Java class definition should be a separate file Use the same name for the class and the file, except add ".java"

to the file name For now put all the classes you need to run a program in the

same directory


Standard methods for a class Constructor methods

» Methods which initialise the object when it is first created.

Reader / Get methods» Methods which allow outside codes to view or get copies of the object’s

state.

Writer / Set methods» Methods which allow outside codes to assign new values for the object’s

state.

Mutator methods» Methods which allow outside codes to modify the state of an object.

Query methods» Methods which test the state of an object.


class Car

{

double fuelAmount = 0.0;

double speed = 0.0;

String licencePlate = “ “;

void setMaxSpeed()

{

speed = 200.0;

}

}


SIMPLE CLASS EXERCISE!!! Consider a simple class of objects which represent Rectangles. A brief (&

shallow) analysis will reveal the state & behaviour of our simple Rectangle objects.

The state (knows its?)» length (integer)» width (integer)

The behaviour (knows how to?)» Initialise itself» Set its length» Set its width» Return its width» Return its length» Calculate its area» Calculate its perimeter


// File: Rectangle.java

// A class which specifies simple Rectangle objects

// Note that NONE of the members of Rectangle class

// are labelled 'static'

class Rectangle

{

// State (data variables)

private int length;

private int width;


// Behaviour (instance methods)

// Constructor

public Rectangle()

{

length = 0;

width = 0;

}


// Writer or Set methods

public void setLength( int len )

{

length = len;

}

public void setWidth( int wid )

{

width = wid;

}


// Reader or Accessor methods

public int getLength()

{

return length;

}

public int getWidth()

{

return width;

}


// Other services

public int area() {

int the_area = length * width; return the_area;

}

public int perimeter() {

int perim; perim = 2 * (length + width); return perim;

}


public boolean isLegal()

{

if( length >= 0 && width >= 0 )

return true;

else

return false;

}

}// end class


Constructor Methods Must have same name as the class. Does not have any return type

» Not even void

Creates a new instance of the class.» Initialises all the variables & does any work necessary to prepare the

class to be used.

If no constructor is exists, Java provides a generic one that takes no arguments.» Better to write your own constructor.


Can have several constructor methods.» Each constructor method must have a different header

– Parameter list» Also known as overloading methods.

// another constructor

public Rectangle(int len, int wid)

{

length = len;

width = wid;

}


Access Control

public» Variable or method is available to other programs.

private» To protect variable from external modification» No other code outside this class can directly access these variables

or methods.» i.e. If you want objects in the same class to be able to get or set the

value of a field or invoke a method.

protected» If you want access restricted to subclasses & members of the same

package.


Instance / Non-static Methods

None of the methods in the Rectangle class are labelled static.

These types of methods are designed to be accessed via objects.


NB!! NB!! NB!!

The Rectangle source file must be compiled.» Note that the Rectangle.java file is not a program (no

main method) – you can’t run it.

To use this Rectangle class, you have to write another program which does have a main method.» Driver program / Client code» In this program you can create Rectangle objects and call

methods in those objects.

The Rectangle program and its driver program must both be compiled in the same directory.


Testing Rectangles (Driver program)

class RectTest

{

public static final void main(String[] args)

{

Rectangle page = new Rectangle();

page.setLength(30);

page.setWidth(21);

System.out.println(“The rectangle has a length of”);

System.out.println(page.getLength());

System.out.println(“and a width of “);

System.out.Println(page.getWidth());


System.out.println(“It’s area is “);

System.out.println(page.area());

System.out.println(“It’s perimeter is “);

System.out.println(page.perimeter());

} //end main

} // end class


Encapsulation» All data & methods of Rectangles encapsulated.

– Hidden the data from outside use» Provides protection of the data from malicious access

Public Interface» Methods are all public a.k.a. the public interface to Rectangles.

» Only way to deal with Rectangle objects is through their (public) methods.

Date post:	07-Jan-2016
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