Singleton

A class which can create only one object is called singleton class.

Constructors are made private and object of the singleton class is created in static method of that class.

In singleton class, getInstance() method is used.

class Singleton {
    private static Singleton instance = new Singleton();

    private Singleton() {}

    public static Singleton getInstance() {
        return instance;
    }
}

Because the reference is static:

it’s reachable for the entire program lifetime
the object cannot be garbage collected
heap memory is held for entire JVM runtime

Lazy Singleton

class Singleton {
    private static Singleton instance;

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

Difference:

object creation is delayed
lifetime is still entire program runtime after creation

Lazy does not mean short-lived. It just means procrastination.

Example

class CoffeeMachine {
    private float coffeeQ;
    private float waterQ;

    static private CoffeeMachine machine = null;

    static CoffeeMachine getInstance() {
        // This insures that object is created only once.
        if (machine == null)
            machine = new CoffeeMachine();
        return machine;
    }
}

class Main {
    public static void main(String[] args) {
        CoffeeMachine m1 = CoffeeMachine.getInstance();
        CoffeeMachine m2 = CoffeeMachine.getInstance();
        CoffeeMachine m3 = CoffeeMachine.getInstance();

        // All three references point to the same object.

        if (m1 == m2 && m1 == m3)
            System.out.println("Same");
    }
}

Output:

Same

Last modified: 08 February 2026