Java Classes Methods Objects Classes Classes We have

  • Slides: 34
Download presentation
Java Classes Methods Objects

Java Classes Methods Objects

Classes

Classes

Classes �We have been using classes ever since we started programming in Java �Whenever

Classes �We have been using classes ever since we started programming in Java �Whenever we use the keyword class we would be declaring a class �Every program must have a main class in order to work, it is the class that the JVM uses to execute the program

Why are classes used? �Classes program are used to started off a �Classes in

Why are classes used? �Classes program are used to started off a �Classes in Java are also used to create objects (we will talk about objects later on) �A class specifies 1. The data it contains 2. The code that uses the data

Classes as Building Blocks �A classes are the building blocks to programs �For example

Classes as Building Blocks �A classes are the building blocks to programs �For example and architect draws a plan of the house before it is build, the class is the plan

Advantage of Classes �Classes provide reusability in programs �This means that a class can

Advantage of Classes �Classes provide reusability in programs �This means that a class can be used over and over again �For example we have a ready made class called the ‘Keyboard. class’, we use this over and over again when we want the program to read an input from the keyboad

Activity �Create a program and name the class my. Adding. Class �The code within

Activity �Create a program and name the class my. Adding. Class �The code within the class should be able to add two numbers �The output should be the result of the addiion

Methods

Methods

What is a method? �Programs are used to solve complex problems and are MUCH

What is a method? �Programs are used to solve complex problems and are MUCH longer than the ones we create at school �These programs are split up into a number of sections to make it easier to code and to read �These sections are called methods

… �A method holds its own specific task which is then joined to other

… �A method holds its own specific task which is then joined to other methods to form a complete process �A method contains the actions that the program will actually perform �So first we create a class and within the class we have a number of methods

Example �Lets say we have a program to hold student records �The program could

Example �Lets say we have a program to hold student records �The program could have the following methods 1. A method to calculate if a mark is a pass or fail 2. A method to calculate the average marks 3. A method to draw up a graph of the grades

Why do we use Methods? �Splitting a problem into different methods makes it much

Why do we use Methods? �Splitting a problem into different methods makes it much easier to program �Methods avoids having to have double code as if we need to use a method twice you would just need to call it

Our First Program class First. Progrsam{ public static void main (String args[]) { System.

Our First Program class First. Progrsam{ public static void main (String args[]) { System. out. println(“hello World”); } } �As we can see we have one class called First. Program �We have one methods which is the main method

How do we Identify Methods? �We know that a block of code is a

How do we Identify Methods? �We know that a block of code is a method as we would see the following syntax method name (parameter list) { // body of the method } �The parameter list would be what the method will be using or a statement of when the method should start

Calling Methods �In Java when we say we are calling a method we mean

Calling Methods �In Java when we say we are calling a method we mean that we are asking the program to perform the tasks of a method �This is done to save duplicate codes in our program �This makes it easier to write and understand codes

//using a methods //calling methods Class calling. Methods{ //method find. Area static int find.

//using a methods //calling methods Class calling. Methods{ //method find. Area static int find. Area (int lnegth, int width) { return length * width; } public static void main (String args[]){ //calling method find. Area System. out, println(“Area = “ +find. Area(5, 3); System. out, println(“Area = “ +find. Area(5, 4); System. out, println(“Area = “ +find. Area(5, 5); } }

Static Methods �The method in the previous program was a static method �They do

Static Methods �The method in the previous program was a static method �They do not make use of instance variables �Static methods take the parameters and compute/calculate something

Public Methods �A public method is a method that could be used by other

Public Methods �A public method is a method that could be used by other classes �In other words another class within the program would be able to use the public method by simply calling them

Void Methods �A void method is basically a method that contains the keyword ‘void’

Void Methods �A void method is basically a method that contains the keyword ‘void’ �Void methods perform an actions but do not output any sort of result

Instance Methods �This is the default type of method �Instance methods use instance variables

Instance Methods �This is the default type of method �Instance methods use instance variables (a set variable) �These methods are associated with objects of the program

Objects

Objects

What is an Object? �In life we have objects all around us such as

What is an Object? �In life we have objects all around us such as tables, chairs, desks ect… �Every objects has its attributes such as a table has four legs and a straight table top. All tables have this in common – common attributes �Objects also have a behaviour for examples dogs bark, eat and breath. All dogs have these behaviours in common

Object Oriented Programming �Java is an object oriented programming language, programs in java also

Object Oriented Programming �Java is an object oriented programming language, programs in java also take into account different objects with attributes and behaviours �For example if we want to draw a triangle on Java we would give it the attributes of the objects – size and colour �We could also give it a behaviour and

Object characteristics �Objects have two main characteristics; 1. Attributes = how the object looks

Object characteristics �Objects have two main characteristics; 1. Attributes = how the object looks and what it is - set as an instance variable in Java 2. Behaviour = what action it would do – set as a method in Java

Creating a Program � We will be using an architects plan to find out

Creating a Program � We will be using an architects plan to find out the following certain attributes of different rooms � So a common object we have is a room, so we would need to create an object room using a class.

//create a type room by listing the attributes of a room class Room{ double

//create a type room by listing the attributes of a room class Room{ double width; double height; double length; } class Home. Demo 1{ public static void main (String args[]){ //creating an object of type room //object is a room with the name bedroom 1 Room bedroom 1 = new Room(); double volume; //give bedroom 1 its attributes bedroom 1. width = 4; bedroom 1. height = 3; bedroom 1. length = 5; //find the volume of the room volume = bedroom 1. width * bedroom 1. height *bedroom 1. length; System. out. println("Volume of Bedroom 1 = " + volume); }

Outputs �What is the output of the program? Volume of Bedroom 1 = 60.

Outputs �What is the output of the program? Volume of Bedroom 1 = 60. 0 �Which is the class that is creating the data type for the room? Class Room { double width; double height; double length }

Creating two Objects �In the next program we will be creating two objects �The

Creating two Objects �In the next program we will be creating two objects �The two objects in this program will be; 1. Bedroom 1 (width 5, height 3, length 5) 2. A Kitchen (width 3, height 3, length 4) �Create a NEW program to output the volume of both bedroom 1 and the kitchen

We need a Method �As we can see in out second program the following

We need a Method �As we can see in out second program the following code is repeated twice; volume = bedroom 1. width * bedroom 1. height * bedroom 1. length; �When we have repeated code we realise that we need to create a method, this would eliminate the repeated code

Our new Method STEP ONE �Our new method would need to work out the

Our new Method STEP ONE �Our new method would need to work out the volume �We will name this method ‘get. Volume’ to understand what the method will be doing �In the class Room write the following method //method to calculate the volume double get. Volume(){ volume = width * height * length; return volume; }

Eliminating Code STEP TWO �Remove the following lines of code from your program; 1

Eliminating Code STEP TWO �Remove the following lines of code from your program; 1 Double volume; volume = bedroom 1. width * bedroom 1. height * bedroom 1. length; 2 3 volume = kitchen. width * kitchen. height * kitchen. length;

Replacing Code �Replace STEP THR EE the following code; System. out. println("Volume of Bedroom

Replacing Code �Replace STEP THR EE the following code; System. out. println("Volume of Bedroom 1 = " + volume); System. out. println("Volume of kitchen = " + volume); �With; System. out. println("Volume of Bedroom 1 = " + bedroom 1. get. Volume()); System. out. println("Volume of Kitchen = " + kitchen. get. Volume());

Add a Living Room �Add a living room to your program with the following

Add a Living Room �Add a living room to your program with the following attributes; Width = 5 2. Height = 3 3. Length = 6 1.

Area �Create �This a method called get. Area method should return the area Area

Area �Create �This a method called get. Area method should return the area Area = ((width * height) * 2) + ((length * height * 2) + (width * length)); �So now your program should 1. Hold three objects 2. Two methods 3. Output the Volume of all objects 4. Output the Area of all objects