LazySingleton

LazySingleton.java

import junit.framework.*;

final class LazySingleton {
	private int i;
    private static class LazySingletonLoader {
        private static final LazySingleton INSTANCE = new LazySingleton();
    }
    private LazySingleton() {
        if (LazySingletonLoader.INSTANCE != null) {
            throw new IllegalStateException("Already instantiated");
        }
    }
    public static LazySingleton getReference() {
        return LazySingletonLoader.INSTANCE;
    }
  public int getValue() { return i; }
  public void setValue(int x) { i = x; }	
}

public class LazySingletonPattern extends TestCase {
  public void test() {
    LazySingleton s = LazySingleton.getReference();
    String result = "" + s.getValue();
    System.out.println(result);
    assertEquals(result, "0");
    LazySingleton s2 = LazySingleton.getReference();
    s2.setValue(9);
    result = "" + s.getValue();
    System.out.println(result);
    assertEquals(result, "9");
    try {
      // Can't do this: compile-time error.
      // LazySingleton s3 = (LazySingleton)s2.clone();
    } catch(Exception e) {
      throw new RuntimeException(e);
    }
  }
  public static void main(String[] args) {
    junit.textui.TestRunner.run(LazySingletonPattern.class);
  }
}