Equality comparison of `boolean` and `Object` allowed?

Question

The following code

public class TestComparison {
    public static void main(String[] args) throws Exception {
        boolean b = true;
        Object o = n         


        

Answer 1

Concerning your 'Note' that the code compiles and works when o is changed to Boolean:

This code:

public class Tester{
  public static void main(String args[]){
    Boolean one = new Boolean(true);
    Object two = new Boolean(true);
    boolean three = true;
    System.out.println("SAME 1:2 " + (one == two) + " 1:3 " + (one == three) + " 2:3 " + (two == three));
    System.out.println("EQUAL 1:2 " + (one.equals(two)) + " 1:3 " + (one.equals(three)) + " 2:3 " + (two.equals(three)));
  }
}

produces this result:

SAME 1:2 false 1:3 true 2:3 false
EQUAL 1:2 true 1:3 true 2:3 true

To see why this is, we need to consider the compile-time types of the various expressions:

• one == two compares a Boolean with an Object - these are both reference types, so the test is reference equality (Java Language Specification, Java SE 7 edition, §15.21.3)
• one == three compares a Boolean with a boolean - this is treated as a comparison of primitive boolean values (§15.21.2); one is unboxed and compared with three.
• two == three compares an Object with a boolean - in this case the boolean is converted to Object by a casting conversion (§5.5, in this case boxing boolean to Boolean and then widening Boolean to Object) and the two are then compared for reference equality.

The EQUAL line is much simpler - all three cases are calls to Boolean.equals(Object other), using boxing when the argument is three.