Java ArrayList contains method doesn't work

Question

So when I run this program it shows that the ArrayList "test" doesn't contain the array [5,6] inside the variable "position".When I checked the output, it is clearly in there and i see that "test" does contain that element.

Output:

[5, 6] [5, 6] false

Code:

package arraylisttest;

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

public class ArrayListTest {
    public static void main(String[] args) {
        int[] position = { 5, 6 };
        ArrayList<int[]> test = new ArrayList<>();

        test.add(new int[] {50, 2});
        test.add(new int[] {0, 7});
        test.add(new int[] {5, 6});
        test.add(new int[] {2, 1});

        System.out.println(Arrays.toString(position));
        System.out.println(Arrays.toString(test.get(2)));
        System.out.println(test.contains(position));
    }
}

ArryList#contains(obejct) will check whethet object is in the list.In your case, position is different the object created by 'new int[] {50, 2}' — user6796845
– user6796845, Commented May 25, 2017 at 2:19

Tim Biegeleisen · Accepted Answer · 2017-05-25 01:52:01Z

3

I believe that List.contains() will use the equals() method to determine if the list contains a given object (q.v. the source code for ArrayList#contains()). It will not compare the two points in each 2D array to see if they be the same. So even though the point {5, 6} logically appears in the list, it is a different object than position which you are using for the comparison, and hence the comparison fails.

Note that the following code would have behaved as you expected:

int[] position = { 5, 6 };
ArrayList<int[]> test = new ArrayList<>();
test.add(new int[] {50, 2});
test.add(new int[] {0, 7});
test.add(position);
test.add(new int[] {2, 1});

System.out.println(Arrays.toString(position));
System.out.println(Arrays.toString(test.get(2)));

System.out.println(test.contains(position));

edited May 25, 2017 at 1:52

answered May 25, 2017 at 1:50

Tim Biegeleisen

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2 Comments

Logan Over a year ago

Yeah, contains(), in this case, would look to see if the array references are the same using ==.

StackOverflowAccount Over a year ago

Haha, yea I was assuming that Arrays.equals() would have been called during contains() since it is being treated as an object. Meh, I guess it's not implemented that way.

Ousmane D. · Accepted Answer · 2017-05-25 02:16:11Z

3

The problem here is that arrays don't override Object#equals method hence the output you're receiving; rather you can create ArrayList of ArrayLists instead e.g. the code below will output true.

ArrayList<Integer> position = new ArrayList<>(Arrays.asList(5, 6));
ArrayList<ArrayList<Integer>> test = new ArrayList<>();
ArrayList<Integer> y = new ArrayList<>(Arrays.asList(5,6));
test.add(y);
System.out.println(test.contains(position));

another option would be to leave your current solution as it is, but use a stream to perform a comparison against the other arrays within the list:

System.out.println(test.stream().anyMatch(e -> Arrays.equals(e,position)));

edited May 25, 2017 at 2:16

answered May 25, 2017 at 1:56

Ousmane D.

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Comments

Fady Saad · Accepted Answer · 2017-05-25 02:01:46Z

0

Arrays can only be compared with Arrays.equals().

You should use ArrayList instead:

ArrayList<Integer> position = new ArrayList<Integer>(Arrays.asList(5, 6));

ArrayList<ArrayList<Integer>> test = new ArrayList<>();
test.add(new ArrayList<>(Arrays.asList(50, 2)))
test.add(new ArrayList<>(Arrays.asList(0, 7)));
test.add(new ArrayList<>(Arrays.asList(5, 6)));
test.add(new ArrayList<>(Arrays.asList(2, 1)));


System.out.println(test.contains(position));

answered May 25, 2017 at 2:01

Fady Saad

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