Java interview: for loop, object/class reference and bound limit

Question

This question came up in one of my interview questions for an internship position written in Java. Notice that the boolean function isSame actually declares 2 parameters as Integer class - not int, so I thought a and b are objects, correct?

public class ForLoop{

 public static boolean isSame(Integer a, Integer b) {
     return a == b;
 }

 public static void main(String []args){

    int i = 0;
    for (int j=0; j<500; ++j) {
        if (isSame(i,j)) {
            System.out.println("Same i = "+i);
            System.out.println("Same j = "+j);
            ++i;
            continue;
        } else {
            System.out.println("Different i = "+i);
            System.out.println("Different j = "+j);
            ++i;
            break;
        }
    }
    System.out.println("Final i = " + i);

 }
}

My first thought is that the for loop would terminate in the 1st run with the result Final i = 1, but to my surprise the final output is i = 129. The loop terminated when both i & j = 128.

  Same i = 126
  Same j = 126
  Same i = 127
  Same j = 127
  Different i = 128
  Different j = 128
  Final i = 129

Can someone please explain?

[The answer here](http://stackoverflow.com/questions/20877086/confusion-in-method-integer-valueofstring) will answer your question — Baby, Oct 24 '14 at 02:23

score 5 · Accepted Answer · answered Oct 24 '14 at 02:16

When testing Object types for equality use .equals(), as you have found -128 to 127 (inclusive) are pooled. The JLS-5.1.7 says in part,

If the value p being boxed is true, false, a byte, or a char in the range \u0000 to \u007f, or an int or short number between -128 and 127 (inclusive), then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2.

Java interview: for loop, object/class reference and bound limit

1 Answers1