public class Singleton {

    private static Singleton instance;
    private Singleton(){}
    
    public static Singleton getInstance_0(){
        synchronized (Singleton.class) {
            if(instance == null){
                instance = new Singleton();
            }
        }
        return instance;
    }
    
    public static Singleton getInstance_1(){
        if(instance == null){
            synchronized (Singleton.class) {
                if(instance == null){
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

區別在于第一次的時候instance都是null，多線程就會產生兩個實例。