Singleton Pattern not returning same object in my Java Code

Question

Is there any wrong in my Code. Because the variable not changing its value while applying Singleton Pattern.

My Code :

class Singleton{

private final static Supplier<Singleton> INSTANCE = Singleton::new;
private int variable = 33;


public static synchronized Supplier<Singleton> getInstance() { 
    return INSTANCE;
}

void print(){
    System.out.println(variable);
}

void change(int variable){
    this.variable = variable;
}

}

public class Design {

public static void main(String[] args) {
    Singleton.getInstance().get().print(); // print 33
    Singleton.getInstance().get().change(99);
    Singleton.getInstance().get().print(); // need to print 99 but it prints 33

}

}

Please correct me Guys, if I'm wrong in implement Singleton Pattern.

You have only one **Supplier**. And each time you call the get() method on the unique Supplier, it creates a **new** instance of the SIngleton class. I sugges you google for "how to implement a singleton in Java". You'll get plenty of relevant answers, and I doubt any of them will look like the one you posted. The easiest, simplest way is to simply create an enum with a single constant. — JB Nizet, Nov 24 '19 at 10:19
https://stackoverflow.com/questions/70689/what-is-an-efficient-way-to-implement-a-singleton-pattern-in-java — George Z., Nov 24 '19 at 10:22
The code seems to be incomplete, what does `get()` method do ? — Shayan Tabatabaee, Nov 24 '19 at 11:25
@ShayanTabatabaee get() calls the Singleton constructor, since the Supplier is initialized to `Singleton::new`. — JB Nizet, Nov 24 '19 at 12:02
Where did you come up with the idea of using a Supplier for singleton implementation? — ACV, Nov 24 '19 at 12:13
@JBNizet: Yes it seems that this interface is implemented in java 8, you are right every time the `get` method called a new instance of Singleton class returned. — Shayan Tabatabaee, Nov 24 '19 at 13:24

score 1 · Answer 1 · edited Nov 24 '19 at 17:05

Your Supplier is a Singleton but creates a new instance of your supposed singleton with every call to Supplier.get() as each call invokes Singleton::new returning a new not-singleton instance.

Trying to implement singletons is hard and usually (if not part of a bad homework) sign of a bad design. There is a reason, the 'S' in S.T.U.P.I.D. code stands for Singleton. There only a very few cases where a Singleton is needed and those cases should usually handled by framework code.

score 0 · Answer 2 · answered Nov 24 '19 at 13:43

It is because every call to get method will instantiate a new object and the Singleton design pattern is not implemented well, So there is a tons of tutorials that implemented Singleton design pattern but if you are insist of using Supplier interface I changed your Singleton class and it seems to be work :

import java.util.function.Supplier;

class Singleton implements Supplier<Singleton> {

    private static final Singleton INSTANCE = new Singleton();
    private int variable = 33;

    private Singleton() {

    }

    public static synchronized Singleton getInstance() {
        return INSTANCE;
    }

    void print() {
        System.out.println(variable);
    }

    void change(int variable) {
        this.variable = variable;
    }

    @Override
    public Singleton get() {
        return this;
    }
}

Singleton Pattern not returning same object in my Java Code

2 Answers2