Singleton.java

    
       /**
 * Singleton class implements singleton pattern. Only one object can be
 * instantiated.
 * 
 */
public class Singleton {

  /**
   * Holds reference to single instance.
   */
  private static Singleton INSTANCE;

  /**
   * Overrides public Constructor.
   */
  private Singleton() {
  }

  /**
   * Creates the instance if it does not yet exist(lazy instantiation).
   * 
   * @return a reference to the single instance.
   */
  public static Singleton getInstance() {
    if (INSTANCE == null) {
      INSTANCE = new Singleton();
    }
    return INSTANCE;
  }
}

    

SingletonTest.java

    
       import static org.junit.Assert.assertEquals;
       import static org.junit.Assert.assertNotNull;
       import org.junit.Test;

/**
 * Test implementation of the Singleton pattern.
 */
public class SingletonTest {

  @Test
  public void testSingleton() {

    // invokes Singleton.getInstance() for first time,
    // object will be created
    Singleton singleton = Singleton.getInstance();
    assertNotNull(singleton);

    // invokes Singleton.getInstance() for second time,
    // reference to the same object will be returned
    Singleton secondSingleton = Singleton.getInstance();
    assertEquals(singleton, secondSingleton);
  }
}

    

Available in the diary and class teacher will have to enable it against the class.