Singleton in JAVA

First the code

public class SingletonDemo {

    private static SingletonDemo singleton = null;


    private SingletonDemo() {
        System.out.println("singleton created");
    }

    public static SingletonDemo getInstance() {
        if (singleton == null) {
            singleton = new SingletonDemo();
        }
        return singleton;
    }
}

The code above is an example of a lazy singleton.

When we should use singleton?
A singleton should be used when managing access to a resource that is shared by the entire application, and it would be destructive to potentially have multiple instances of the same class. Making sure that access to shared resources thread-safe is one very good example of where this kind of pattern can be vital.