class invariant ,internal invariant

[roja potti]

hi all!!!
Presently I was working through some questions on assertions.I came across the words class invariant and internal invariant.What does they mean???
please give me the answer.

class E {
  private boolean b1, b2, b3;
  public void setB1(boolean b) {b1 = b;}
  public void setB2(boolean b) {b2 = b;}
  public void setB3(boolean b) {b3 = b;}
  public void m1 (int i) {
    b2 = i % 2 == 0;
    if (!b3 & !b2 & !b1) {System.out.print("A");
    } else if (!b3 & !b2 & b1) {System.out.print("B");
    } else if (!b3 & b2 & !b1) {System.out.print("C");
    } else { // Only b3 is true.
      assert b3 & !b2 & !b1;
    }
    System.out.print(b1 + "," + b2 + "," + b3);
    b1 = b2 = b3 = false;
  }
  public static void main (String[] args) {
    E e = new E(); e.setB1(true); e.m1(2);
  }
}

this is a question regarding the assertions.Given below are the answers.I was pretty sure that the answers a & b are right.I did not understand answers c and d.Please explain.
a.)With assertions enabled it prints an error message.
b.)With assertions disabled it prints: true,true,false
c.)The combination of the if/else statements and the assert statement indicate that the programmer expects no more than one boolean, b1, b2 or b3, to be true.
d.)The assert statement is being used to check an internal invariant
(code tags added - Barry)
[ May 05, 2004: Message edited by: Barry Gaunt ]

[Anil Hulikal]

Programming With Assertions
Regards,
Anil