單例模式

懶漢模式：

線程不安全懶漢模式：

public class Singleton {  
    private static Singleton instance;  
    private Singleton (){}  
  
    public static Singleton getInstance() {  
    if (instance == null) {  
        instance = new Singleton();  
    }  
    return instance;  
    }  
}

線程安全懶漢模式：

public class Singleton {  
    private static Singleton instance;  
    private Singleton (){}  
    // 就是加了synchronized關鍵字
    public static synchronized Singleton getInstance() {  
    if (instance == null) {  
        instance = new Singleton();  
    }  
    return instance;  
    }  
}

餓漢模式：

餓漢就是上來就得到實例對象。 

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

雙重檢驗模式（重要）：

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;  
    }  
}

https://www.runoob.com/design-pattern/singleton-pattern.html