C ++ - 就像Java中的友元类机制一样 [英] C++-like friend class mechanism in Java

问题描述

你知道我怎样才能在一个特殊的类中改变一个对象？
在这个例子中，我希望对象PrivateObject只能在 Box 类中进行可更改（可递增），而不是其他任何地方。有没有办法实现这个目标？

Do you know how can I make an object changeable only inside a special class? In this example I want the object PrivateObject to be only changable (incrementable) inside the Box class, nowhere else. Is there a way to achieve this?

public class Box { 

    private PrivateObject prv;

    public void setPrivateObject(PrivateObject p){
        prv = p;
    }

    public void changeValue(){
        prv.increment();

    }

}


public class PrivateObject {
    private value;
    public increment(){
        value++;
    }

}

PrivateObject priv = new PrivateObject ();
Box box = new Box();
box.setPPrivateObject(priv);
box.changevalue();
priv.increment(); // I don't want it to be changeable outside the Box class!

在C ++中，我会生成所有 PrivateObject 属性和方法private，并声明 Box 类作为 PrivateObject 类的朋友。

In C++, I would make all the PrivateObject properties and methods private, and the declare the Box class as a friend for the PrivateObject class.

推荐答案

如果 PrivateObject 与密切相关Box 为什么不把它作为 Box 中的内部类？

If PrivateObject is strongly related to Box why not make it an inner class inside Box?

class Box { 

    public static class PrivateObject {
       private value;

       private increment(){
         value++;
       }
    }

    private PrivateObject prv;

    public void setPrivateObject(PrivateObject p){
        prv = p;
    }

    public void changeValue(){
        prv.increment();
    }       
}

现在你不能调用增量来自外部 Box ：

Now you cannot call increment from outside Box:

public static void main(String args[]) {
    Box.PrivateObject obj = new Box.PrivateObject();
    Box b = new Box();
    b.setPrivateObject(obj);
    obj.increment(); // not visible     
}

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