Abstraction in java

How to achieve Abstraction in java?

• Abstract class are used to achieve abstraction on java.
• Abstraction is the concept of hiding the actual implementation from user.
• Usually abstract method forces subclass to implement the methods.
• By using abstract method we can have the consistency in method signature in all sub class.

Abstract Class in Java:

• Abstract class consists of complete method or concrete method and abstract method.
• The method with have method head and method signature is called concrete method or complete method.

Abstract Method:

• The method without method body is called as abstract method.
• This method also known as incomplete method because it’s contains only method headings.

Syntax:  abstract void test ();
Here test () is an abstract method.

• Abstract method must ends with semicolon (;).
• Abstract method should be declared with a keyword ‘abstract’ otherwise compiler throws error.
• A class which has at least one abstract method is known as abstract class.
• If a class has abstract methods, then it’s mandatory to declare that class as abstract, otherwise compiler through error.
• For abstract class we can’t create object.
• An abstract class can contains only concrete methods or only abstract methods or both concrete and abstract methods.

When to make a class as abstract?

• When you have a requirement where your base class should provide default implementation of certain methods whereas other methods should be open to being overridden by child classes use abstract classes
• The purpose of an abstract class is to provide a common definition of a base class that multiple derived classes can share.

 Sample Program :

public abstract class SampleProgram {

             abstract void test1 ();

             abstract void test2 ();

}

class SampleImplement extends SampleProgram{

   

void test1() {

 System.out.println("test1 in SampleImplement class");

}

void test2() {

 System.out.println("test2 in SampleImplement class");

 }

}

class Run{ 

 public static void main(String[] args) {

  SampleImplement sampleImplement = new SampleImplement();

  sampleImplement.test1();

  sampleImplement.test2();

  }

}

Can an abstract class have a constructor?

 Answer:

• Yes, when we define a class to be an Abstract Class it cannot be instantiated but that does not mean an Abstract class cannot have a constructor. 
• Each abstract class must have a concrete subclass which will implement the abstract methods of that abstract class. 
• If we are not creating any constructor for abstract class, the java by default will create a constructor like other java class.

If we are in one of these below situation, we need to create constructor in an abstract class:

• You have defined final fields in the abstract class but you did not initialize them in the declaration itself; in this case, you MUST have a constructor to initialize those fields

Sample Program:

package com.sample;

abstract class Motor { 

    int somenewProduct;

    public Motor( int somenewProduct ) {

        this.somenewProduct = somenewProduct;

    }

    public int MotorIncrese(int val) {

       return somenewProduct * val;

    }

}

class Honda extends Motor {

    public Honda() {

        super(2);

    }

}

class Cycle extends Motor {

    public Cycle(int cycleName) {

        super(cycleName);

    }

}