What needs to be changed in this code to have a correct thread-safe implementation of the Singleton pattern? public class Singleton { private static final Singleton instance; public Singleton() { } public Singleton getInstance() { if (instance != null) { synchronized (instance) { if (instance == null) { instance = new Singleton(); } } } return instance; } }

The provided code contains several errors:

1. The instance field is declared as final, but it is not initialized immediately — this is impossible.
2. In the getInstance() method, the check and synchronization are done incorrectly.

A correct thread-safe implementation with lazy initialization and double-checked locking looks like this:

public class Singleton {
    private static volatile Singleton instance;

    private Singleton() {
        // private constructor
    }

    public static Singleton getInstance() {
        if (instance == null) { // first check without synchronization
            synchronized (Singleton.class) {
                if (instance == null) { // second check inside synchronized
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

Key points:

• instance should be static volatile for correct visibility of changes across threads.
• Synchronization occurs on the class, not on the instance.
• The constructor is private to prevent creating instances from outside.