Enum Types in Java

• Enum type enables a variable to be set with predefined constants. Examples include days of week, directions etc.
• In Java Enum type is defined using the keyword enum.
• In Java Enum types are treated as class types and they are more powerful than other languages.
• Enum types can have constructor, instance variables and methods.
• One primary difference compared to classes is Enum types are not created using new() unlike class types.
• Each enumeration constant is treated as an object.
• Constructor would be called for each Enum constant.

Java Enum Type Example

public enum DefectStates {
 NEW(10), ASSIGNED(20), OPEN(30), RESOLVED(40), CLOSED(50);
 
 private int val;
 DefectStates(int val) {
  this.val = val;
  System.out.println("Constructor: " + val);
 }
 public int getVal() { return val; }
 public static void main(String[] args) {
  DefectStates ds = DefectStates.New;
 }
}

Output:-

Constructor: 10
Constructor: 20
Constructor: 30
Constructor: 40
Constructor: 50

Iterate Enum type values

Enum has static values method that returns an array containing all of the values of the enum in the order they are declared. This method can be used to iterate over the values of an enum type.

public class JavaEnumTest {
   public static void main(String[] args) {
     for ( DefectStates state : DefectStates.values() ) {
       System.out.println(state)
     }
   }
}

Output:-

NEW
ASSIGNED
OPEN
RESOLVED
CLOSED

Enum approach for creating singleton objects

The most simplest approach to implementing Singleton's is the enum approach. Leverages the guarantee from Java that an enum value is initialized only once and is thread safe. The example below shows a sample implementation with Enum approach.

public enum EnumTest {
 INSTANCE;
 
 // Test code
 public static void main(String[] args) {
  
  // First thread
  Thread t1 = new Thread(new Runnable() {
   @Override
   public void run() {
    EnumTest obj = EnumTest.INSTANCE;
   }   
  });
  t1.start();
  
  // Second thread
  Thread t2 = new Thread(new Runnable() {
   @Override
   public void run() {
    EnumTest obj = EnumTest.INSTANCE;
   }   
  });
  t2.start();
 }
}