Java 實現 設計模式 單例模式

public class SingletonTest {

}

//懶漢式
class Bank {
    private Bank() {
    }

    private static Bank instance = null;

    public static Bank getInstance() {

        if (instance == null) {
            instance = new Bank();
        }
        return instance;
    }
}

//餓漢式
class Bank2 {
    private Bank2() {
    }

    private static Bank2 instance = new Bank2();

    public static Bank2 getInstance() {
        return instance;
    }
}

//懶漢式 線程安全
class Bank3 {
    private Bank3() {
    }

    private static Bank3 instance = null;

    public static synchronized Bank3 getInstance() {
        if (instance == null) {
            instance = new Bank3();
        }
        return instance;
    }
}

//雙重鎖
class Bank4 {
    private Bank4() {
    }

    private static Bank4 instance = null;

    public static Bank4 getInstance() {
        if (instance == null) {
            synchronized (Bank4.class) {
                if (instance == null) {
                    instance = new Bank4();
                }
            }
        }
        return instance;
    }
}