Unexpected output for int type

Question

Learning JAVA, i was trying to test the upper limit of while loop which goes on incrementing an int.Please see the program below :

 public class Test {

   public static int a(){
     int a = 10;
      while(a > 9 )
          ++a;
      return a; 
   }

   public static void main(String[] argc) {

         Test t = new Test();
         int k = t.a();
         System.out.println("k = "+(1 * k));    
    }
}

I am aware that 32 bits range is from -2,147,483,648 to 2,147,483,647, so on the basis of that, i was expecting output as 2,147,483,647 but instead i am getting :

k = -2147483648

I even tried

 System.out.println("k = "+(1 * k/2)); 

but still output is :

k = -1073741824

Question :

Why is solution negative when it should be positive?

Answer 1

You are incrementing your a int by 1 until it reaches 1 + Integer.MAX_VALUE, which shifts its value to -2147483648 == Integer.MIN_VALUE.

Here's your loop commented:

// "infinite" loop as a is assigned value 10
while(a > 9)
    // when a reaches Integer.MAX_VALUE, it is still incremented by 1
    ++a;
// loop condition now false, as value for a has shifted to -2147483648
return a; 

Answer 2

What is happening is called integer overflow.

Maximum 32-bit integer value in binary is:

0111 1111 1111 1111 1111 1111 1111 1111

When you add 1 to this number you get:

1000 0000 0000 0000 0000 0000 0000 0000

This is the twos compliment, or -2,147,483,648. Since any negative number is less than 9, the while loop exits.

Answer 3

You increment the value until it reaches the positive limit and it becomes all bits but the sign bit becomes 1.

0x7FFFFFFF = 01111111 11111111 11111111 11111111

This is binary representation of 2147483647, which is INT_MAX. When you increment it by one once again, it becomes

0x80000000 = 10000000 00000000 00000000 00000000

which is equal to INT_MIN, -2147483648.

Now,

2147483647 is greater than 9 so your loop continues. One more increment and oops, suddenly it is -2147483648 which is smaller than 9. This is the point where your loop condition fails.