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.

Answer 1

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