Java List removing items without any action

Question

I'm facing a weird behavior in my Java code using List.

The code is very simple, I have a List of Object called AccessRequest which comes from a database and I'm using this first List to create a new one but with a filter to select only a few objects.

Here is the code :

private void updateCommentIfNeeded() {
    List accessRequestList = getAllRequest();
    List commentsList = getCommentsListProcessedManually(accessRequestList);
}

public List getCommentsListProcessedManually(List accessRequests) {

    accessRequests.removeIf(ar -> !ar.getComment().equals("To be processed manually"));

    if (accessRequests.size() != 0) {
        SQLServerConnection sqlServerConnection = new SQLServerConnection(sqlServerUrl);
        accessRequests.removeIf(ar -> !sqlServerConnection.emailExists(ar.getEmail()));
    }
    return accessRequests;
}

I'm supposed to get a second List only containing the objects that has their comments to To be processed manually, which I do. But the weird part is that the first List also takes the value of the second as if I wrote accessRequestList = commentsList but there is no such thing and I'm using local variable.

Ex :

I have 3 objects in my first List, but only one containing the required comment
Both list ends with containing the only objects containing the comment

I'm kind of lost here if anyone has an idea !

user_8819761 · Answer

Your method getCommentsListProcessedManually modifies the list you're passing. I believe you're operating under the assumption that passing the list as a parameter somehow creates a copy of the list, whereas what is actually happening is that a reference to the list is passed by value.

There are several ways to solve this, but the easiest is to simply create a copy of your input list at the start of your method:

public List getCommentsListProcessedManually(List input) {
    List accessRequests = new ArrayList<>(input);

    accessRequests.removeIf(ar -> !ar.getComment().equals("To be processed manually"));

    if (accessRequests.size() != 0) {
        SQLServerConnection sqlServerConnection = new SQLServerConnection(sqlServerUrl);
        accessRequests.removeIf(ar -> !sqlServerConnection.emailExists(ar.getEmail()));
    }
    return accessRequests;
}

You could also use the Stream API for this (using the filter operation), but that's quite a bit trickier in this situation.

user_1108098 · Answer

You are passing a reference of the list to the method getCommentsListProcessedManually.

So accessRequestList and the one passed as a parameter are the same, hence any operation done to the list is done to the same list.

You can create a copy of the list before passing it as a parameter:

List newList = new ArrayList(accessRequestList);

Java List removing items without any action

2 Answers