Abstract classes in Java

A class that contains the abstract keyword in its declaration is called an abstract class.

• Abstract classes may or may not contain abstract methods, that is, methods without a body (public void get(); )
• However, if a class has at least one abstract method, Then the class must be declared abstract.
• If a class is declared abstract, it cannot be instantiated.
• To use an abstract class, you must inherit it from another class and provide implementations of the abstract methods in it.
• If you inherit an abstract class, you have to provide implementations for all the abstract methods in it.

Examples

This section provides you with examples of abstract classes. 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 for the abstract methods, the Employee class is the same as a normal class in Java. The class is now abstract, but it still has three fields, seven methods, and a constructor.

Now you can try to instantiate the Employee class in the following way -

/* File name : AbstractDemo.java */
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();
    }
 }

When you compile the above class, it will give the following error -

Employee.java:46: Employee is abstract; cannot be instantiated
Employee e = new Employee("George W.", "Houston, TX", 43); ^ 1 error  

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