• The static attribute instance contains the single instance of the class
• The constructor is defined as private so that other classes cannot create instances.
• The static method getSingletonInstance returns the single instance of the class. The first time this method is called, it creates the single instance.

// Singleton.java
// Demonstrates Singleton design pattern

public final class Singleton {

   // Singleton object to be returned by getSingletonInstance
   private static Singleton singleton = new Singleton();

   // private constructor prevents instantiation by other objects
   private Singleton()
   {
      System.err.println( "Singleton object created." );
   }

   // return existing Singleton object
   public static Singleton getSingletonInstance()
   {
      return singleton;
   }
}

// SingletonTest.java
// Attempt to create two Singleton objects

public class SingletonTest {
   
   // run SingletonExample
   public static void main( String args[] )
   {
      Singleton firstSingleton;
      Singleton secondSingleton;

      // create Singleton objects
      firstSingleton = Singleton.getSingletonInstance();
      secondSingleton = Singleton.getSingletonInstance();

      // the "two" Singletons should refer to same Singleton
      if ( firstSingleton == secondSingleton )
         System.err.println( "firstSingleton and secondSingleton " +
            "refer to the same Singleton object" );
   }
}