02.单例-懒汉式

public class Dog {

    private Dog() {
    }

    private static Dog instance;

    //懒汉式加载
    public static Dog getInstance() {
        //通过判断对象是否被初始化,来选择是否创建对象
        if (instance == null) {
            instance = new Dog();
        }
        return instance;
    }

}

static void method2() {
    Dog d1 = Dog.getInstance();
    Dog d2 = Dog.getInstance();
    System.out.println(d1.equals(d2));
}
//测试看似安全 但是如果放在多线程里呢