Thread-safe lazy initialization using holder class Bill Pugh Singleton implementation

public class Singleton {
    private static class InstanceHolder {
        static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return InstanceHolder.INSTANCE;
    }

    private Singleton() {}
}

This initializes the INSTANCE variable on the first call to Singleton.getInstance(), taking advantage of the language’s thread safety guarantees for static initialization without requiring additional synchronization.

This implementation is also known as Bill Pugh singleton pattern.[Wiki]