7. Sample JAVA program - Abstraction - Interfaces

/* ABSTRACTION - INTERFACE
- Interface looks like a class but is NOT a class.
- An interface can have methods and variables like class but methods in interface are "abstract" by default.i.e. only signature , NO body.
- Variables declared in an interface are public, static and final by default.
- Methods in an interface do not have a body. They have to be implemented by the class to access them.
- The class must implement all the methods of that interface.
- You can implement multiple interfaces in your class although JAVA doesnt allow you to extend more than one class.
*/

//My interface is an interface with two methods DoubleMyNumber and DoubleMyString which are both abstract by default.
//The methods in the interface are implemented by DoubleMyINput Class.
interface MyInterface{
// Although you dont see abstract keyword for the following methods.
//They are treated as abstract methods by the compiler since they are part of an interface
public void DoubleMyNumber(int num);
public void DoubleMyString(String Str);
}

class DoubleMyInput implements MyInterface{
public void DoubleMyNumber(int num){
System.out.println("Double of the number: "+num+" is : "+2*num);
}
public void DoubleMyString(String Str){
System.out.println("Printing the String twice: "+Str+" "+Str);
}

}

public class DriverClass{
public static void main(String args[]){
DoubleMyInput obj = new DoubleMyInput();
obj.DoubleMyNumber(9);
obj.DoubleMyString("MyInterface");
}

}