在Java中访问私有字段

Question

I don't understand why I can access the private int i outside of the class while it's private.

public class Fish {

    private int i = 1;

    public static void main(String[] args) {
        Fish k = new Fish();
        k.i = 2; // it IS possible
    }

}

Answer 1

Your main method is part of the Fish class isn't it?

The following doesn't work:

public class Fish {

    private int i = 1;

}

class Reptile{
  public static void main(String[] args) {
         Fish k = new Fish();

         k.i = 2; // Compiler error.
     }
}

Answer 2

Access modifiers work on class level not on object level .

You are allowed to access ki since the code lies within the same class as in which the member i is declared.

The rationale is (afaik) the following: You encapsulate the data (partly) in order to ease future maintenance and refactorings. When you refactor code, you refactor classes, not objects.

Answer 3

Private means that only the defining class can access the variable.

Your main method is a method of the class Fish, and so it is allowed to access the variable

Answer 4

This is not outside the class. You're in the same class.

You're instantiating a new instance of the same class and therefore you can access the private fields of this class.

Answer 5

如果这是SomeOtherClass中的主要方法，则同一行将失败。