Java Classes Methods Objects. Classes Classes We have been using classes ever since we started...
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Transcript of Java Classes Methods Objects. Classes Classes We have been using classes ever since we started...
JavaClassesMethods Objects
Classes
ClassesWe 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 are used to started off a
program
Classes in Java are also used to create objects (we will talk about objects later on)
A class specifies1. The data it contains2. The code that uses the data
Classes as Building BlocksA 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 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
ActivityCreate a program and name the
class myAddingClass
The code within the class should be able to add two numbers
The output should be the result of the addiion
Methods
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 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 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 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
As we can see we have one class called FirstProgram
We have one methods which is the main method
class FirstProgrsam{ public static void main (String args[]) {
System.out.println(“hello World”);
}}
How do we Identify Methods?We know that a block of code is a
method as we would see the following syntax
The parameter list would be what the method will be using or a statement of when the method should start
method name (parameter list) { // body of the method}
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 methodsClass callingMethods{
//method findAreastatic int findArea (int lnegth, int width) {
return length * width;}
public static void main (String args[]){//calling method findArea
System.out,println(“Area = “ +findArea(5,3);
System.out,println(“Area = “ +findArea(5,4);
System.out,println(“Area = “ +findArea(5,5);
}}
Static MethodsThe 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 classes
In other words another class within the program would be able to use the public method by simply calling them
Void MethodsA 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 MethodsThis is the default type of method
Instance methods use instance variables (a set variable)
These methods are associated with objects of the program
Objects
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 ProgrammingJava 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 make it move around the screen
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 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 roomclass Room{
double width;double height;double length;
}class HomeDemo1{
public static void main (String args[]){//creating an object of type room//object is a room with the name bedroom1 Room bedroom1 = new Room();double volume;//give bedroom1 its attributesbedroom1.width = 4;bedroom1.height = 3;bedroom1.length = 5;
//find the volume of the roomvolume = bedroom1.width * bedroom1.height
*bedroom1.length;System.out.println("Volume of Bedroom1 = " +
volume);}
}
Outputs What is the output of the program?
Volume of Bedroom1 = 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 ObjectsIn the next program we will be creating
two objects
The two objects in this program will be;1. Bedroom1 (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 bedroom1 and the kitchen
We need a Method As we can see in out second
program the following code is repeated twice;
When we have repeated code we realise that we need to create a method, this would eliminate the repeated code
volume = bedroom1.width * bedroom1.height * bedroom1.length;
Our new MethodOur new method would need to
work out the volumeWe will name this method
‘getVolume’ to understand what the method will be doing
In the class Room write the following method
//method to calculate the volume double getVolume(){
volume = width * height * length;return volume;
}
STEP ONE
Eliminating CodeRemove the following lines of
code from your program;
STEP TWO
volume = bedroom1.width * bedroom1.height * bedroom1.length;
volume = kitchen.width * kitchen.height * kitchen.length;
Double volume; 1
2
3
Replacing CodeReplace the following code;
With;
STEP THREE
System.out.println("Volume of Bedroom1 = " + volume);
System.out.println("Volume of kitchen = " + volume);
System.out.println("Volume of Bedroom1 = " + bedroom1.getVolume());
System.out.println("Volume of Kitchen = " + kitchen.getVolume());
Add a Living RoomAdd a living room to your
program with the following attributes;
1. Width = 52. Height = 33. Length = 6
AreaCreate a method called getArea
This method should return the area
So now your program should 1. Hold three objects2. Two methods 3. Output the Volume of all objects4. Output the Area of all objects
Area = ((width * height) * 2) + ((length * height * 2) + (width * length));