OOPS PRINCIPLES IN JAVA
2) OBJECT
3) ENCAPSULATION
4) INHERITANCE
5) POLYMORPHISM
CLASS
- A class consists of variables and methods.
- The combination of variables andmethods are known as members of the class.
class classname
{
type instancevar1;
……
type instancevarN;
type methodname1()
{
// body
}
……
type methodnameN()
{
//body
}
}
NOTE:-
- In the class, the variable is known as instance variable because when we create object for a class a copy of variable is stored in the object and in java object is also known as instance.
OBJECT
- In java, we can create object for a class by using new operator dynamically at run time.
classname objectname = new classname();
or
classname objectname;//here objectname is variable and null is stored
objectname = new classname();
ENCAPSULATION
- It binds together code and data from outside misuse.
- In a java program we use private keyword for a variable to achieve encapsulation.
Definition 1: Inheriting all the properties of one class to another class is known as inheritance.
Definition 2: Inheriting all the properties of super class by sub class by using the keyword extends is known as inheritance.
NOTE:-
- The class which is inherited is known as superclass.
- The class that does inheriting is known as subclass.
1) Single level Inheritance
2) Multilevel Inheritance
3) Multiple level Inheritance
4) Hierarchical Inheritance.
- In single level inheritance there will be only two classes.They are one super class and one sub class.
A (Super class)
B (Sub class)
- In a multi-level inheritance, a sub class can be a super class for another sub class.
A (Super class)
B (Sub class)
(Super class)
C (Sub class)
(Super class)
D (Sub class)
- In multiple level inheritance, a sub class can inherit two or more super classes at a time.
A B C …….(Super class)
D (Sub class)
- Hierarchical Inheritance is a combination of multi and multiple level inheritance.
- Multiple level inheritance is not supported by Java.
- To achieve multiple level inheritance in Java we have to use interface.
- Interface consists of methods declaration.
- The methods declared in the interface must be public abstract void methods.
- The methods declared in the interface can be implemented in a class as public void methods.
- The methods declared in the interface must compulsorily be implemented in a class.
- Interfaces are implemented by a class by a using the keyword implements.
- One interface can inherit another interface by using the keyword extends.
- Interface resembles to polymorphism i.e., one interface and multiple methods.
- By default the methods declared in the interface are abstract methods.
STEPS TO WRITE JAVA PROGRAM
- There are six steps to write a Java Program. They are:
1) DOCUMENTATION SECTION
2) PACKAGE STATEMENT
3) IMPORT STATEMENT
4) INTERFACE STATEMENT
5) CLASS DEFINITION
6) MAIN METHOD
1) DOCUMENTATION SECTION
- Here we have to write comment.
- Non executable code in the program is known as comment.
1. Single line comment:
- In single line comment, the comment is written in single line.
Symbol: //
2. Multi line comment:
- In multi-line comment, multiple lines are written.
/*
………………
……………….
………………
*/
3. Documentation comment
- This comment is used to prepare the document.
/**
* ………………
*……………….
* ………………
*/
2) PACKAGE STATMENT
- Package is a group of classes, sub classes and interfaces.
- Package resembles to directory structure.
package packagename;
3) IMPORT STATEMENT
- By using this import keyword we can import a package into a file.
import packagename.*;
import packagename.class name;
import packagename.interfacename;
6) MAIN METHOD
Syntax:-
public static void main(String args[])
EXPLANATION
public - keyword
static - keyword
void - keyword
main - method
String - built in class present in java.lang package
args - arguments
[] - array