單例模式：就是一個類最多有一個實例對象

public class Singleton {
    // volatile 避免指令重排
    private volatile Singleton instance;
    private Singleton() {}

    public Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class){
                if (instance == null) {
                    instance = new Singleton();
                    return instance;
                }
            }

        }
        return instance;
    }
}

// 線程不安全版本
// 參考鏈接
//public class Singleton {
//    private Singleton instance;
//    private Singleton() {}
//
//    public Singleton getInstance() {
//     if (instance == null) {
//
//         instance = new Singleton();
//         return instance;
//     }
//     return instance;
//    }
//}