Why doesn't this "for" loop remove values from an array list?

Question

I want the following method to remove the inputted value and return the value if successful, however the "for" loop never actually removes any values.

Integer remove(Integer value)
    {
        if (_set_array.size() == 0)
        {
            return null;
        }
        else if (value == null)
        {
            return null;
        }
        else
        {
            for (int i = _set_array.size() - 1; i >= 0; i--)
            {
                if (value == _set_array.get(i))
                {
                    _set_array.remove(i);
                    return value;
                }
            }
            return null;
        }
    }

try: value.equals(_set_array.get(i)) – slipperyseal Nov 28 '18 at 05:03 — slipperyseal, Nov 28 '18 at 05:03

Sandeepa · Answer 1 · 2018-11-28T05:11:05.607

0

It works fine for me. Can you check this code?

import java.util.ArrayList;
import java.util.Arrays;

class Main {
    public static void main(String[] args) {
        ArrayList _set_array = new ArrayList(Arrays.asList(10000, 20000, 30000, 40000, 50000));
        System.out.println(remove(30000, _set_array));
        System.out.println(_set_array);
    }

    static Integer remove(Integer value, ArrayList _set_array) {
        if (_set_array.size() == 0) {
            return null;
        } else if (value == null) {
            return null;
        } else {
            for (int i = _set_array.size() - 1; i >= 0; i--) {
                if (value == (int) _set_array.get(i)) { // cast Integer to int primitive type
                    _set_array.remove(i);
                    return value;
                }
            }
            return null;
        }
    }
}

edited Nov 28 '18 at 05:11

answered Nov 28 '18 at 04:51

Sandeepa

3,457
5
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because the Integers are low values and in the constant pool. larger values wont == – slipperyseal Nov 28 '18 at 04:55
i changed 1,2,3,4,5 to 1000,2000,3000,4000,5000 and remove returns null. suggest value.equals(_set_array.get(i)) – slipperyseal Nov 28 '18 at 05:01
@slipperyseal agreed. Since here he is comparing the object type instead of primitive type its not working. So he can cast the value from Interger to int primitive type. I think it would be better since he is comparing int values other than objects. Whats your thoughts? – Sandeepa Nov 28 '18 at 05:13
you could also change the method param to int and each of these options would cause autoboxing to unbox the int from the object. personally i think value.equals(_set_array.get(i)) is less obscure about how it works as it is using the regular equals method on the Integer type – slipperyseal Nov 28 '18 at 05:15
why not use `equals` – Scary Wombat Nov 28 '18 at 05:20

score 0 · Answer 2 · answered Nov 28 '18 at 05:09

0

use this for comparison:

value.intValue() == _set_array.get(i).intValue()

this would rather compare the int primitives represented by individual Integers.

answered Nov 28 '18 at 05:09

AppleCiderGuy

1,249
1
9
16

Saurav Sahu · Accepted Answer · 2018-11-30T18:28:01.633

-1

You are comparing references. Convert the references to primitive value using intValue method:

value.intValue() == _set_array.get(i).intValue()

Explained here : https://stackoverflow.com/a/53511942/1465553

Alternatively, as suggested in comments, you can use method equals to compare the two references.

edited Nov 30 '18 at 18:28

answered Nov 28 '18 at 04:47

Saurav Sahu

13,038
6
64
79

Won’t the code in question throw an error? One is not supposed to modify the collection while iterating it in a for loop, right? – mettleap Nov 28 '18 at 04:49
the loop goes from the top the bottom, so it doesnt cause any index misalignment when elements are removed – slipperyseal Nov 28 '18 at 04:50
I was definitely comparing references. After converting them to primitives everything ran flawlessly. I should brush up on masking for sure. Thank you! – Monolithic Nov 28 '18 at 05:23

Why doesn't this "for" loop remove values from an array list?

3 Answers3