JAVA_单例模式

枚举

public enum  Demo03 {

    AAAA; //初始化
    private A instance;
    Demo03(){
        instance = new A();
    }
    public A getInstance(){
        return instance;
    }

}
class A{

}
class Test{
    public static void main(String[] args) {
      //  Demo03 demo03 = new Demo03();
        Demo03 aaaa = Demo03.AAAA;
        A instance = aaaa.getInstance();


        //  A c = Demo03.BBB.getInstance();
       // A a = Demo03.INSTANCE.getInstance();
        System.out.println(instance);
     //   System.out.println(c);

    }
}
2.懒汉

//懒汉式
public class Demo02 {
    private volatile static Demo02 demo02;
    private boolean flag = true;
    private Demo02(){
       if (flag){
           flag = false;
       }
           else{
               throw new RuntimeException("拒绝使用反射获取对象");

       }
    }
//双重检测锁模式
    public static Demo02 getInstance(){
           if (demo02 == null){
               synchronized (Demo02.class) {
                    if (demo02 == null) {
                       demo02 = new Demo02();
                   }
                }
           }
        return demo02;
    }




}