設計模式：創建型 - 單例模式

單例模式(Singleton):保證一個類僅有一個實例，並提供一個訪問它的全局訪問點

 

UML：

JAVA：

public class SingletonTest {
    public static void main(String[] args) {
        Singleton s1 = Singleton.getInstance();
        Singleton s2 = Singleton.getInstance();
        System.out.println(s1.equals(s2));
    }

}
class Singleton{
    private static Singleton instance;
    private Singleton(){}//private構造方法，堵死外界利用new創建實例對象

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

//多線程時的單例
class SingletonThread{
    private static SingletonThread instance;
    private SingletonThread(){}
    public static SingletonThread getInstance(){
        if (instance == null){
            synchronized (Singleton.class){
                if (instance == null) {
                    instance = new SingletonThread();
                }
            }
        }
        return instance;
    }
}


//靜態內部類單例
class Singleton2{
    private static class Inner{
        private static final Singleton2 singleton2 = new Singleton2();
    }
    private Singleton2() {}
    public static final Singleton2 getInstance(){
        return Inner.singleton2;
    }
}