Basic Java issue: equivalent integer arithmetic producing different results

Question

Can anybody explain the difference between the following two programs? They look equivalent to me but they generate different outputs. What's the reason?

Program 1: Outputs incorrect value; i=1

public class Test1{
    public static void main(String[] args) {
        int[] values = new int[] { 2, 3, 5, 1 };

        int i = 0;
        for (Integer integer : values) {
            i =+ integer.intValue();
        } // for loop ends
        System.out.println("i=" + i);
    }
}

Program 2: Outputs expected value; i=11"

public class Test2{
    public static void main(String[] args) {
        int[] values = new int[] { 2, 3, 5, 1 };

        int i = 0;
        for (Integer integer : values) {
            i = i + integer.intValue();
        } // for loop ends
        System.out.println("i=" + i);
    }
}

You could have debugged this with a simple System.out.println or some time with a debugger or javap -c to really see what's going on. — jason
– jason, Commented Jul 5, 2013 at 7:05
Whoever approved this minor edit? Dammit people, it's too minor and introduced same fake edits to even get through the minimum edit length — Tobias Kienzler
– Tobias Kienzler, Commented Jul 5, 2013 at 7:13

sasha.sochka · Accepted Answer · 2013-08-12 19:22:43Z

17

Obviously, the major difference is within this line:

i =+ integer.intValue();

Perhaps it was intended to use i += instead of i =+ in the first program. In your version the first program just assigns a value every iteration so it's final result it the last value in array (which is 1). Your second program does what it's supposed to do - it adds all the elements in the array and it's result is 11.

edited Aug 12, 2013 at 19:22

answered Jul 5, 2013 at 6:54

sasha.sochka

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Comments

Buhake Sindi · Accepted Answer · 2013-07-05 06:57:59Z

10

You try to add and assign but you use the wrong operator.

 for (Integer integer : values) {
     i =+ integer.intValue();
 } // for loop ends

This translates to i = (+ integer.intvalue())

Correct would be:

for (Integer integer : values) {
     i += integer.intValue();
 } // for loop ends

edited Jul 5, 2013 at 6:57

Buhake Sindi

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answered Jul 5, 2013 at 6:55

ssindelar

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Comments

William Morrison · Accepted Answer · 2013-08-13 00:52:31Z

5

You are using =+ instead of +=

You are probably only getting the last value in your array printed out in one, and the sum in the other right?

That's because you're just overwritting the value repeatedly in one, and actually summing the values correctly in the other.

edited Aug 13, 2013 at 0:52

answered Jul 5, 2013 at 6:55

William Morrison

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Comments

Makoto · Accepted Answer · 2013-07-05 06:55:33Z

4

i =+ integer.intValue(); is a reassignment of the unary operation +integer.intValue(). You're overwriting i with every value in the array. The last one is the final result.

answered Jul 5, 2013 at 6:55

Makoto

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Comments

Community · Accepted Answer · 2017-05-23 12:21:03Z

4

Because you are using unary operator '+', which operates on the sign of integer and does not do addition.

So basically what happens is that the variable i will contain the last value int the values array.

EDIT:

Use of Unary plus operator in Java

When the array value is negative, suppose -1 In this case due to the good old maths rule for sign

(+)(+) = (+)
(+)(-) = (-)
(-)(+) = (-)
(-)(-) = (+)

So when you do i = (+ (-1)) = -1. The answer remains -1.

So if you keep last value of the array -1 or some other negative value, you will get the same value.

edited May 23, 2017 at 12:21

CommunityBot

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answered Jul 5, 2013 at 6:57

Narendra Pathai

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3 Comments

MadProgrammer Over a year ago

+1 Finally an answer which explains why (in a manner I can understand)

jason Over a year ago

Actually, unary + doesn't do anything (and it most certainly doesn't change the sign of anything).

Narendra Pathai Over a year ago

I have added the explanation for negative values.

Bharat Gaikwad · Accepted Answer · 2013-07-05 08:30:51Z

3

Always write operator (+,-,| etc...) before = if LHS is to be involved without writing LHS explicitly like

+= ,-=, |=

or as even comparison operators <=, >= ....

answered Jul 5, 2013 at 8:30

Bharat Gaikwad

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Comments

witrus · Accepted Answer · 2013-07-05 06:55:34Z

3

is there anything wrong in class test1 code.It seems it should be i + =integer.intValue() instead of =+

answered Jul 5, 2013 at 6:55

witrus

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Comments

Ruchira Gayan Ranaweera · Accepted Answer · 2013-07-05 08:10:12Z

0

    int[] values = new int[] { 2, 3, 5, 1 };
    int i = 0;
    for (Integer integer : values) {
        i =+ integer.intValue();// here i's value  2,3,5 and finally 1. then your out put is 1.
    } // for loop ends
    System.out.println("i=" + i);

But your next case you are adding current value of i with int values in array. So obviously you will get the some of int value in array for i.

    int[] values = new int[] { 2, 3, 5, 1 };
    int i = 0;
    for (Integer integer : values) {
        i = i + integer.intValue();
    } // for loop ends
    System.out.println("i=" + i);

edited Jul 5, 2013 at 8:10

answered Jul 5, 2013 at 7:06

Ruchira Gayan Ranaweera

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1 Comment

Marco Forberg Over a year ago

well a comment with "Try with debug, you will learn more by your self." would have been sufficient and your "answer" could be narrowed down to this

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