Abstract Class

Abstract Class

A class which contains the abstract keyword in its declaration is known as abstract class.

• Abstract classes may or may not contain abstract methods ie., methods with out body ( public void get(); )
• But, if a class have at least one abstract method, then the class mustbe declared abstract.
• If a class is declared abstract it cannot be instantiated.
• To use an abstract class you have to inherit it from another class, provide implementations to the abstract methods in it.
• If you inherit an abstract class you have to provide implementations to all the abstract methods in it.

Example

This section provides you an example of the abstract class to create an abstract class just use the abstract keyword before the class keyword, in the class declaration .

/* File name : Employee.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;
   }
   public void mailCheck()
   {
      System.out.println("Mailing a check to " + this.name
       + " " + this.address);
   }
   public String toString()
   {
      return name + " " + address + " " + number;
   }
   public String getName()
   {
      return name;
   }
   public String getAddress()
   {
      return address;
   }
   public void setAddress(String newAddress)
   {
      address = newAddress;
   }
   public int getNumber()
   {
     return number;
   }
}

You can observe that except abstract methods the Employee class is same as normal class in Java. The class is now abstract, but it still has three fields, seven methods, and one constructor.