enum in Java

Enum was introduced in java with jdk5 release. It is one of the most overlooked feautures by java developers. So lets try to understand what is an enum and what are the features and advantage of using enum.Enum in java is a set of named constants. Enum is declared with the keyword enum.

enum Days {

      SUNDAY,MONDAY,TUESDAY,WEDNESDAY,THURSDAY,FRIDAY,SATURDAY;

}

The above enum is a set of named constants representing the days of  a week. Enums are internally represented as a class. So the above enum will roughly translated to below code:-

Class Days {

      public static final SUNDAY = new Days();

      public static final MONDAY = new Days();

      public static final TUESDAY = new Days();

public static final WEDNESDAY = new Days();

public static final THURSDAY = new Days();

public static final FRIDAY = new Days();

public static final SATURDAY = new Days();

}

So every named constant in an enum is an object of the same enum type and are implicitly public, static and final. Enum in java are more powerful as it can contain variable, methods and consturctors unlike the enum in other programming language. Some of the features of enum are as below:

1.All named constants in an enum are implictly public, static and final are represent an instance of same enum type.

2.enum in java can not extend any other class as every enum in java implicitly extends java.lang.Enum class.

3.enums can not be extended by any other class as enums in java are implicitly final and so can not be extened.

4.enums can implement any number of interfaces.

5.enum constants are final but it can also contain other variables apart from named constants.

6.enums can be used in swithch statement.

7.enum constructors are always private.

8. we cant create instance of enum using new operator.

9. We can declare abstract methods in java enum, then all the enum fields must implement the abstract method

Lets try to demonstrate all this through code example:

Lets declare an enum ThreadStatus:

public enum ThreadStatus {

      START, RUNNING, WAITING, DEAD;

}

Lets also add priority instance variable for the above enum and assign some value for each enum constant.lets also declare a constructor.

public enum ThreadStatus {

      START(1), RUNNING(2), WAITING(3), DEAD(4);

     

      private int priority;

     

      private ThreadStatus(int priority){

            this.priority = priority;

      }

}

Lets also add an abstract method called getDetail(). As the method is abstract all enum constants should implement this abstract method. Also add getters and setters for priority instance variable. The final enum code will look like this:-

public enum ThreadStatus {

      START(1) {

            @Override

            public String getDetail() {

                  return "Starting";

            }

      }, RUNNING(2) {

            @Override

            public String getDetail() {

                  return "Running";

            }

      }, WAITING(3) {

            @Override

            public String getDetail() {

                  return "Waiting";

            }

      }, DEAD(4) {

            @Override

            public String getDetail() {

                  return "Dead";

            }

      };

     

      private int priority;

     

      private ThreadStatus(int priority){

            this.priority = priority;

      }

      public int getPriority() {

            return priority;

      }

      public void setPriority(int priority) {

            this.priority = priority;

      }

     

      public abstract String getDetail();

}

Lets demonstrate the use of enum through example:

public class ThreadStatusEnumDemo {

      /**

       * @param args

       */

      public static void main(String[] args) {

            ThreadStatus[] status = ThreadStatus.values();

           

            for(ThreadStatus s : status){

                  System.out.println("Thread Priority:" +s.getPriority());

                  System.out.println("Thread Details:" +s.getDetail());

            }

            //change the priority of thread statuses

            ThreadStatus.STARTING.setPriority(5);

            ThreadStatus.RUNNING.setPriority(10);

            ThreadStatus.WAITING.setPriority(15);

            ThreadStatus.DEAD.setPriority(20);

           

            System.out.println("***New priority of threads****");

           

            for(ThreadStatus s : status){

                  System.out.println("Thread Priority:" +s.getPriority());

            }

           

            //demonstrate the use of switch

           

            System.out.println("****switch demo*****");

           

            for(ThreadStatus s : status){

                  switch(s){

                  case STARTING:

                        System.out.println("THREAD IS STARTING");

                        break;

                       

                  case RUNNING:

                        System.out.println("THREAD IS RUNNING");

                        break;

                       

                  case WAITING:

                        System.out.println("THREAD IS WAITING");

                        break;

                       

                  case DEAD:

                        System.out.println("THREAD IS DEAD");

                        break;

                         

                        default:

                              System.out.println("invalid thread status");

                       

                  }

            }

           

      }

}