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Thread: method always returns 0

  1. #1
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    Default method always returns 0

    public static int Count( ArrayList<Student> studentList )
    {
    int j = 0;
    for( Student s : studentList )
    {
    j = j++;
    }
    return j;
    }

    What am I doing wrong?


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: method always returns 0

    Why do you think the value of j is changed from 0? Add some println statements to see what is happening.
    What is in studentList? Is the code in the loop ever executed?
    What is the value of j in the loop?

    Please edit your post and wrap your code with
    [code=java]
    <YOUR CODE HERE>
    [/code]
    to get highlighting and preserve formatting.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: method always returns 0

    Quote Originally Posted by bdennin View Post
    ... wrong...
    Four common ways to increment the value of the variable j: in memory:
        j = j + 1;

        j += 1;

        j++;

        ++j;

    Your statement in the loop is none of these. It is:
        j = j++;

    This has the following effect:

    Since the ++ operator has higher precedence than the '=' operator, it evaluates the expression on the right of the '=' first, then it stores the value of the expression in memory:

    So, here's the drill:
    • Evaluates the expression j++. The value of the expression is the previous value of the variable j in memory

    • This particular expression has a side-effect: It increments the value of j in memory (after the expression has been evaluated) and, therefore, stores the incremented value in memory. At this point you have incremented the value of j

      However...

    • Then, applying the '=' operator, it stores the previously obtained value of the expression in memory, thus overwriting the incremented value that you just stored, with the old value.


    Bottom line: It stores the old value in memory each and every time.

    You can easily verify my assertion by running something like the following:
    public class Z {
        public static void main(String [] args) {
            int j = 0;
            for (int i = 0; i < 5; i++) {
                j = j++;
                System.out.printf("i = %d: j = %d\n", i, j);
            }
        }
    }

    Output::
    i = 0: j = 0
    i = 1: j = 0
    i = 2: j = 0
    i = 3: j = 0
    i = 4: j = 0



    Cheers!

    Z

  4. #4
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    Default Re: method always returns 0

    Hey nice questions and I am agree with Norm that you should edit your post and wrap it with
    <YOUR CODE HERE>
    to get highlighting and preserve formatting.
    Have a wonderful day!

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