Singleton is not thread safe

[Cililing]

Hello. In your readme there is:

We'll implement the thread safe one here.

But your Singleton-class is not thread safe.

[Cililing]

To be more precisely - if you try access your singleton from more than one thread you can create more than one instance of your singleton:

Let's try this slightly modified code:

`
public class SingletonXD {

private static SingletonXD instance;

private SingletonXD() {
}

public static SingletonXD getInstance() {
    if (instance == null) {
        System.out.println("Creating new instance");
        instance = new SingletonXD();
        System.out.println("New instance created");
    }
    return instance;
}

public static void main(String[] args) {

    Runnable singletonAccessRunnable = () -> {
        SingletonXD xd = SingletonXD.getInstance();
        System.out.println(xd);
    };

    for (int i = 0; i < 10; i++) {
        Thread thread = new Thread(singletonAccessRunnable);
        thread.start();
    }

}

}
`

For most of the time my output is OK: First there is a message "Creating new instance", then "New instance created", and all objects have the same hashcode.

But, sometimes, output is like:
'
Creating new instance
New instance created
SingletonXD@2e00fa9
SingletonXD@2e00fa9
SingletonXD@2e00fa9
SingletonXD@2e00fa9
SingletonXD@2e00fa9
SingletonXD@2e00fa9
SingletonXD@2e00fa9
SingletonXD@2e00fa9
Creating new instance
New instance created
SingletonXD@7b111699
SingletonXD@2e00fa9
'

As you can see, one of the threads created his own instance - this can lead to very unexcpected behaviours.

[mohamedebrahim96]

yes you are right
but ..
how can we avoid it ?
ammm maybe using Dagger2