Singleton pattern with combination of lazy loading and thread safety

Singleton pattern with combination of lazy loading and thread safety
http://stackoverflow.com/questions/15792186/singleton-pattern-with-combination-of-lazy-loading-and-thread-safety


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	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);
	}
	}

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