單例模式

懶漢式（雙重鎖定）

public class Singleton {

    private volatile static Singleton singleton;//第一步檢查

    public static Singleton getInstance() {
        if (singleton == null) {//第二步檢查
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
}

餓漢式（先創建出實例）

class Singleton{
    //靜態實例
    private static Singleton singleton = new Singleton();

    //構造函數
    private Singleton(){};

    public Singleton getInstance(){
        return singleton;
    }
}

枚舉

enum Singleton{
    INSTANCE;  //枚舉成員

    private Singleton singleton;

    private Singleton{
        singleton = new Singleton();
    }

    public Singleton getInstance(){
        return singleton;
    }
}

//獲得實例：Singleton.INSTANCE.getInstance();

靜態內部類

class Singleton{
    //靜態內部類
    private static calss SingletonHolder{
        private static final Singleton singleton = new Singleton();
    }

    //構造函數
    private Singleton(){};

    public static final Singleton getInstance(){
        return SingletonHolder.singleton;
    }
}

靜態代碼塊

class Singleton{
    //靜態變量
    private static Singleton singleton = null;

    //構造函數
    private Singleton(){};

    //靜態代碼塊
    static{
        singleton = new Singleton();
    }

    public Singleton getInstance(){
        return singleton;
    }
}