单例模式

public class Singleton {
    private static Singleton instance;
    private Singleton (){}

    public static synchronized void getSingleton() {
        if (instance == null) {
            instance = new Singleton();
        }
    }
}

public class Singleton {
    private static Singleton instance;
    private Singleton (){}

    public static void getSingleton() {
        if (instance == null) {
            instance = new Singleton();
        }
    }
}

public class Singleton {
    private static Singleton instance = new Singleton();
    private Singleton (){}

    public static void getSingleton() {
        return instance;
    }
}

public class Singleton {
    private volatile static Singleton singleton;
    private Singleton (){}

    public static Singleton getSingleton() {
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;  
    }
}