// 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

*/