// patterns/SingletonPattern.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

interface Resource {
 int getValue();
 void setValue(int x);
}

// Since this isn't inherited from a Cloneable
// base class and cloneability isn't added,
// making it final prevents cloneability from
// being added through inheritance. This also
// implements thread-safe lazy initialization:

final class Singleton {
 private static final class
 ResourceImpl implements Resource {
 private int i;
 private ResourceImpl(int i) {
 this.i = i;
 }
 public synchronized int getValue() {
 return i;
 }
 public synchronized void setValue(int x) {
 i = x;
 }
 }
 private static class ResourceHolder {
 private static Resource resource =
 new ResourceImpl(47);
 }
 public static Resource getResource() {
 return ResourceHolder.resource;
 }
}

public class SingletonPattern {
 public static void main(String[] args) {
 Resource r = Singleton.getResource();
 System.out.println(r.getValue());
 Resource s2 = Singleton.getResource();
 s2.setValue(9);
 System.out.println(r.getValue());
 try {
 // Can't do this: compile-time error.
 // Singleton s3 = (Singleton)s2.clone();
 } catch(Exception e) {
 throw new RuntimeException(e);
 }
 }
}
/* Output:
47
9
*/