language features - Java Private Field Visibility

Question

So I was making a class the other day and used Eclipse's method to create the equals method when I realized that it generated the following working code:

class Test {
  private int privateInt;
  [...]
  public boolean equals(Object obj) {
    [...]
    Test t = (Test) obj;
    if ( t.privateInt == privateInt ) {
    [...]
  }
}

t.privateInt..???? It's suppose to be private! So I guess there is one more field visibility other than private, protected, package protected and public.

So what is happening here? How is this called? Where would somebody use this? Doesn't this break encapsulation? What if the class didn't have a mutator and I changed this? Does this happen to C++ as well? Is this an OO idiom? If not, then why did Java do it?

Generally, where can I find information about this?

Thank you.

Answer 1

It's accessible from different instances of the same class.

According to this page (bolding mine):

At the member level, you can also use the public modifier or no modifier (package-private) just as with top-level classes, and with the same meaning. For members, there are two additional access modifiers: private and protected. The private modifier specifies that the member can only be accessed in its own class.

For clarity I'll rewrite this line:

if ( t.privateInt == this.privateInt )

We can agree that "this.privateInt" should be allowed: you are accessing it from within the instance of class Test that the message "equals" has been sent to.

It's less clear that "t.privateInt" should be visible, because t is a separate instance of class Test and we are not executing inside its equals method. However java allows this since both objects (t and this) are of the same class Test and can see each others private members.