Java ArrayList: contains() method returns false when arraylist contains the given object

Question

I have a problem with the contains() method which returns false even though the ArrayList contains the given Object. My code is following:

String [] transaction = dbConnection.getPendingTransaction(username);
if (!onlineConnection.getSentTransactionRequests().contains(transaction)) {
    onlineConnection.getSentTransactionRequests().add(transaction);

    String packet = "RTR" + "_" + transaction[0] + "_" + transaction[2] + 
                        "_" + transaction[3] + "_" + transaction[4];
    onlineConnection.send(packet);
}

I have tried Thread.sleep() between iterations, so the ArrayList wasn't load as eagerly without success.

Please, please, please model your transaction as a proper object, and not as an array of strings. Then see @haylem's comment above. — Perception, Nov 13 '12 at 12:50

score 8 · Accepted Answer · answered Nov 13 '12 at 12:46

8

The hashCode() and equals() of arrays are a bit broken when it comes to this (it is a long different discussion why).

A possible work around is to use ArrayList<ArrayList<String>> instead of ArrayList<String[]>, the equals() method for ArrayList will be as you expect it to.

For example:

    ArrayList<String> l1 = new ArrayList<>();
    ArrayList<String> l2 = new ArrayList<>();
    l1.add("asdf");
    l2.add("asdf");
    ArrayList<ArrayList<String>> coll = new ArrayList<>();
    coll.add(l1);
    System.out.println(coll.contains(l2));

Will yield true, as expected

answered Nov 13 '12 at 12:46

amit

175,853
27
231
333

If this is indeed the issue, the "long different discussion why" would be worth it. – Don Roby Nov 13 '12 at 12:48
@DonRoby: I believe the main reason it is still this way in java 7 is backward compability, to be honest. I believe that if the designers could change it without change the backward compability guideline of java - they would... But I am not an architect in Oracle, so I could be wrong. I also believe it is off topic. – amit Nov 13 '12 at 12:50
This is indeed the issue, and the reason is in the related question at http://stackoverflow.com/questions/8777257/equals-vs-arrays-equals-in-java. – Don Roby Nov 13 '12 at 12:56

score 1 · Answer 2 · answered Nov 13 '12 at 12:43

Look at getPendingTransaction and getSentTransactionRequests if it should return SAME array and not create NEW one. Second, try to debug and look for arrays object id. If this is not same, but it should be (and contains same elements), try to workaround of this (for instance create comparator or List or something like).

score 1 · Answer 3 · answered Nov 13 '12 at 12:53

The problem is that equals for arrays is defined to be reference equality. In other words, two different arrays that contain the same elements are NOT equal ... according to the equals method.

If you want equals for the "transaction" objects to be based on the equality of the strings, you need to create a custom class to hold the String[], and override the equals method and the hashcode method.

By the way, array equals(Object) method is not "a bit broken". The defined semantics of equals are simply reflecting the fact that any array object is inherently mutable.

Aleksandr M · Answer 4 · 2012-11-13T12:44:54.760

0

It is because String [] transaction is object. And when you are calling contains list compares new object to some other one using equals method. Imagine it like doing

new Object().equals(new Object())

edited Nov 13 '12 at 12:44

answered Nov 13 '12 at 12:39

Aleksandr M

24,264
12
69
143

1

Not entirely correct, note that if the collection contains "asdf" and you check for `list.contains(new String("asdf"))` you will get true. – amit Nov 13 '12 at 12:42

score 0 · Answer 5 · edited Aug 28 '18 at 06:02

If you must use List<String[]>, possibly you are out of luck of using equals() or hashCode() it seems......

Maybe you'd better create method to do compare your objects

Something like

public static boolean stringArrayListEquals(List<String[]> list, List<String[]> list2) {
    if (list.size() != list2.size()) return false;
    for (int i = 0; i < list.size(); ++i){
        if (!Arrays.equals(list.get(i), list2.get(i)) return false;
    }
    return true;
}

To test for contains(), you would need another loop...

Though you should seriously use List<List<String>> instead.

score 0 · Answer 6 · answered Apr 19 '21 at 11:05

I had faced the similar issue. I just wrote the below snippet:

assertContains(list, toLookFor);  // To call the below private method  

private void assertContains(ArrayList<String> list, String toLookFor) {
        String arrayListToString = rulesList.toString();
        
         if (arrayListToString.contains(toLookFor))
                System.out.println("The list contains "+ toLookFor);
            else
                System.out.println("The list does not contains "+ toLookFor);
        
    }

Java ArrayList: contains() method returns false when arraylist contains the given object

6 Answers6