Design Pattern-單例模式(Singleton Pattern)

Lazy initialization type

Lazy initialization type 指的是只有在需要的時候才創建物件，當呼叫getInstance()的方法時，只有在instance為空值時才會創建物件，否則直接return Object。

class SingletonPattern

public class SingletonPattern {

    //TODO SingletonPattern 重點
        //TODO 宣告 SingletonPattern Object, Modifier 定義為 private and static
        private static SingletonPattern instance;

        //TODO 定義 建構子為 private
        private SingletonPattern(){}

        public static SingletonPattern getInstance(){
                //TODO 第一次被呼叫的時候才建立物件
                if(instance == null){
                        instance = new SingletonPattern();
                        System.out.println("Singleton instance 已被實例化");
                }
                return instance;
        }

}

class Main

SingletonPattern Object透過 SingletonPattern.getInstance() instance Object

public class Main {
    public static void main(String[] args) {

        SingletonPattern singletonPattern = SingletonPattern.getInstance();
        SingletonPattern singletonPattern2 = SingletonPattern.getInstance();

        System.out.println("Object1 : "+singletonPattern.hashCode() +"\n"+ "Object2 : " + singletonPattern2.hashCode());

    }
}

Test Result

分別創建兩個物件，來自SingletonPattern Class，可以看到”Singleton instance 已被實例化”只出現一次，而這兩個物件的記憶體位子來自同一個。

Singleton instance 已被實例化
Object1 : Singleton.SingletonPattern@27d6c5e0
Object2 : Singleton.SingletonPattern@27d6c5e0