Why protected and private attributes are accessible by same class rather than by the same object ?

Question

For example, we have the class Man

If Man.age is protected, then I don't see why chuckNorris (instance of class Man) can change the protected/private attribute age of the object jackBauer (another instance of class Man). He shouldn't be able to do that (IMO).

In my mind, the value of a protected/private attribute is supposed to belong only to the object itself, not the class...

I need some explanation I think, I'm confused.

Answer 1

A private attribute is accessible only by method in the class. A protected attribute is accessibe in descendant class only. Therefore an object jackbauer can't modify anything private or protected in an object chuckNorris of class Man. Hope this would help

Answer 2

Consider this Java class:

public class Base {
  private int a
  protected int b;

  public Base(int a,int b) {
    this.a = a;
    this.b = b;
  }

  public int getA() {
     return a;
  }

  public int getB() {
    return b;
  } 
}

...
 Base foo = new Base(1,2);
 Base bar = new Base(3,4);

There is no way(maybe except via dirty reflection) the foo instance can change the protected or private variable in bar

You might allow it to if you want,

public class Base {
  private int a
  protected int b;

  public Base(int a,int b) {
    this.a = a;
    this.b = b;
  }

  public int getA() {
     return a;
  }

  public int getB() {
    return b;
  } 
 public void changeB(int newB,Base other) {
   other.b = newB;
 }
}
... 
Base foo = new Base(1,2);
Base bar = new Base(3,4);
foo.changeB(5,bar);

You can't protect the changeB method from altering stuff inside the other object [*], you just have to be careful about what your program does. With some languages you could have marked the other argument as unchangable , but not in Java - I don't find it a big deal.

[*} You could, by marking all the fields of Base as final, though then not even the instance itself could change the members after the object has been constructed.