Abstraction:

Abstraction:  In a class if u want to declare a method but wants the implementation of thr method to be done in child class u must declare it as abstarct
if a method is abstact the class must be abstract.
we cannot instantiate abstarct class but child class can extend it in order to access its methods
abstarction is basically hiding certain details and showing only essentials features of  an object
abstract is a keyword in java

public abstract class Employee
{
   private String name;
   private String address;
   private int number;
   public Employee(String name, String address, int number)
   {
      System.out.println("Constructing an Employee");
      this.name = name;
      this.address = address;
      this.number = number;
   }
   public double computePay()
   {
     System.out.println("Inside Employee computePay");
     return 0.0;

will get error if u

public class AbstractDemo
{
   public static void main(String [] args)
   {
      /* Following is not allowed and would raise error */
      Employee e = new Employee("George W.", "Houston, TX", 43);

      System.out.println("\n Call mailCheck using Employee reference--");
      e.mailCheck();

We can extend Employee class in normal way as follows:

/* File name : Salary.java */
public class Salary extends Employee
{
   private double salary; //Annual salary
   public Salary(String name, String address, int number, double
      salary)
   {
       super(name, address, number);
       setSalary(salary);
   }

http://www.tutorialspoint.com/java/java_abstraction.htm