Why can private variables be accessed by other instances?

Question

From an OO design perspective, I'm sure this makes sense, but perhaps somebody could articulate it for me so that I can understand the rationale.
I can see that one way of looking at encapsulation is that the object itself should be encapsulated and if you have 2 Person objects, why should one Person be allowed access to another Person's private data?
Why is this not how Java decides to implement access modifiers (i.e. not a per-instance basis)?...I have no idea how other languages do it because I'm new to all this, but perhaps you can explain for me.

Answer 1

I don't know why it was designed that way, but
consider this:public class Example {
private int value = 42;
public void setAnotherValue(Example ex, int
nt newValue) {
ex.value = newValue;
Thanks for your example. Looking at it, it makes me realize that this per-class access is specially meant for access to other instances, because otherwise the method would just look like this, correct?
public void setAnotherValue(int newValue) {
   this.value = newValue;
}How would the runtime check you mentioned look? I couldn't figure out which of the following two you had in mind and are appropriate:
public void setAnotherValue(Example ex, int newValue) {
   if (ex == this)
     ex.value = newValue;
  else
     throw new RuntimeException();
}or this...
Example e1 = new Example();
Example e2 = new Example();
if (e2 != e1) throw new RuntimeException();
e2.setAnotherValue(e1, 57);