[Java]Abstract Class

A class that is declared using the abstract keyword is known as abstract class. Abstraction is a process of hiding the data implementation details, and showing only functionality to the user. Abstraction lets you focus on what the does instead of how it does it.

Main things of abstract class

An abstract class may or may not contain abstract methods.There can be non abstract methods.

An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:

ex : abstract void moveTo(double deltaX, double deltaY);

If a class has at least one abstract method then that class must be abstract

Abstract classes may not be instantiated (You are not allowed to create of Abstract class)

To use an abstract class, you have to inherit it from another class. Provide implementations to all the abstract methods in it.

If you inherit an abstract class, you have to provide implementations to all the abstract methods in it.

Declare abstract class Specifying abstract keyword before the class during declaration makes it abstract. Have a look at the code below:

abstract class AbstractDemo{ }
Declare abstract method Specifying abstract keyword before the method during declaration makes it abstract. Have a look at the code below,

abstract void moveTo();//no body
Why we need to abstract classes

In an -oriented drawing application, you can draw circles, rectangles, lines, Bezier curves, and many other graphic s. These s all have certain states (for ex -: position, orientation, line color, fill color) and behaviors (for ex -: moveTo, rotate, resize, draw) in common. Some of these states and behaviors are the same for all graphic s (for ex : fill color, position, and moveTo). Others require different implementation(for ex: resize or draw). All graphic s must be able to draw or resize themselves, they just differ in how they do it.

This is a perfect situation for an abstract superclass. You can take advantage of the similarities, and declare all the graphic s to inherit from the same abstract parent (for ex : Graphic ) as shown in the following figure. enter image de ion here

First, you declare an abstract class, Graphic , to provide member variables and methods that are wholly shared by all subclasses, such as the current position and the moveTo method. Graphic also declared abstract methods, such as draw or resize, that need to be a implemented by all subclasses but must be implemented in different ways. The Graphic class can look something like this:

abstract class Graphic {

  void moveTo(int x, int y) {
    // Inside this method we have to change the position of the graphic 
    //   according to x,y     
    // This is the same in every Graphic . Then we can implement here. 
  }

  abstract void draw(); // But every Graphic  drawing case is 
                        // unique, not common. Then we have to create that 
                        // case inside each class. Then create these    
                        // methods as abstract 
  abstract void resize();
}
Usage of abstract method in sub classes Each non abstract subclasses of Graphic , such as Circle and Rectangle, must provide implementations for the draw and resize methods.

class Circle extends Graphic  {
  void draw() {
    //Add to some implementation here
  }
  void resize() {
    //Add to some implementation here   
  }
}
class Rectangle extends Graphic  {
  void draw() {
    //Add to some implementation here
  }
  void resize() {
    //Add to some implementation here
  }
}

Inside the main method you can call all methods like this:

public static void main(String args[]){
   Graphic  c = new Circle();
   c.draw();
   c.resize();
   c.moveTo(4,5);   
}

Ways to achieve abstraction in Java

There are two ways to achieve abstraction in java

Abstract class (0 to 100%)
Interface (100%)
Abstract class with constructors, data members, methods, etc

abstract class Graphic  {

  Graphic  (){
    System.out.println(\"Graphic   is created\");
  }
  void moveTo(int y, int x) {
       System.out.println(\"Change position according to \"+ x+ \" and \" + y);
  }
  abstract void draw();
}

class Circle extends Graphic  {
  void draw() {
    System.out.println(\"Draw the Circle\");
  }
}

class TestAbstract {  
 public static void main(String args[]){

   Graphic   grObj = new Circle ();
   grObj.draw();
   grObj.moveTo(4,6);
 }
}

Output:

Graphic   is created
Draw the Circle
Change position according to 6 and 4
Remember two rules:

If the class has few abstract methods and few concrete methods, declare it as an abstract class.

If the class has only abstract methods, declare it as an interface.

References:

TutorialsPoint - Java Abstraction
BeginnersBook - Java Abstract Class Method
Java Docs - Abstract Methods and Classes
JavaPoint - Abstract Class in Java