Question

        class Bar{
        int barNum=28;
    }

    class Foo{
        Bar myBar = new Bar();
        void changeIt(Bar myBar){   //Here is object's reference is passed or the object is passed?
            myBar.barNum = 99;
            System.out.println("myBar.barNum in changeIt is " + myBar.barNum);
            myBar = new Bar();  //Is the old myBar object destroyed now and new myBar is referring to something new now?
            myBar.barNum = 420;
            System.out.println("myBar.barNum in changeIt is now "+myBar.barNum);
        }
        public static void main(String args[]){
            Foo f = new Foo();
            System.out.println("f.myBar.barNum is "+f.myBar.barNum);    //Explain f.myBar.barNum!
            f.changeIt(f.myBar);    //f.myBar refers to the object right? but we need to pass reference of the type Bar in changeIt!?
            System.out.println("f.myBar.barNum after changeIt is "+ f.myBar.barNum);    //How do you decide f will call which myBar.barNum?
        }
    }

请解释评论中提到的问题代码的输出是 f.myBar.barNum是28岁 changeIt中的myBar.barNum为99 changeIt中的myBar.barNum现在为420

f.myBar.barNum after changeIt is 99

Answer 1

Java传递对方法中对象的引用。如果需要，可以将其称为指针。假设你在类foo中有对象myBar，并将它传递给change it方法。它不传递对象本身它传递对该对象的引用（如果您愿意，则为指针）。

当你做myBar.barNum = 99;这里myBar指向类foo中的实际对象myBar并更改其属性。

当你做myBar = new Bar（）;这里的myBar（我们看到的是指向对象的指针）开始指向一个不同于类foo的myBar的新Bar对象。您将其更改为400，并且在该对象所在的上下文中（方法）它为400.当您离开该方法时，您将返回到foo类，其中是原始对象。由指针改为99.

我希望我能正确解释。如果一切都没有被称为mybar，那将更容易理解。因为在行myBar = new Bar（）中你实际上使用的是本地mybar（在方法中）而不是全局mybar。例如：

Bar myBar = new Bar();
    void changeIt(Bar aBar){ 
        aBar.barNum = 99; //Here aBar is a pointer to the myBar above and changes it
        aBar = new Bar();  //Here aBar points to a new myBar instance
        a.barNum = 420; //This will be lost when you leave the method with the new instance created
    }

解释评论中提到的问题

1 个答案: