Operators

[Alpana Singh]

public class Q7
{
public static void main(String[] args)
{

int i=3;
System.out.println(ii*=2 + i++);
}
}

I came across this question.When I ran this o/p is 15.How can o/p is 15.

Please tell me the logic.

[Steve Morrow]

I'm assuming the 'ii' is a typo (else it won't compile), and that the line actually reads as follows:

System.out.println(i *= 2 + i++);

If so, the logic is as follows:

i = 3 * (2 + 3); // i++ evaluates to the original value of i
i = 3 * 5;
i = 15;

Does that help? I can probably guess the next question, though. It's, "where did my incremented value go?", right? Good question!

It helps to understand the difference between variables and an operand stack. I'll write up the steps from a bytecode point of view...

i = 3;          // iconst_3:    variables { },  stack {3}
                // istore_1:    variables {3},  stack { }
i *= 2 + i++;   // iload_1:     variables {3},  stack {3}
                // iconst_2:    variables {3},  stack {3, 2}
                // iload_1:     variables {3},  stack {3, 2, 3}
                // iinc 1, 1:   variables {4},  stack {3, 2, 3}
                // iadd:        variables {4},  stack {3, 5}
                // imul:        variables {4},  stack {15}
                // istore_1:    variables {15}, stack { }

Hope this helps...
[ October 31, 2005: Message edited by: Steve Morrow ]

[Alpana Singh]

Now I got it.I was confused ny i++.

Thanks for detailed explanation.