Java ConcurrentModificationException on List.addAll method without any looping

Question

I have the ConcurrentModifcationException raised, on the following snippet, which does not make sense based on what I read on what causes this error (modifying something you have an open iterator over):

    List<Field> kibanaFields = Arrays.asList(KibanaRow.class.getSuperclass().getDeclaredFields());
    kibanaFields.addAll(Arrays.asList(KibanaRow.class.getDeclaredFields()));

Second line, is where it happens. I don't understand this.. even if the first line had some iterator implemented inside, it should have been closed by the time it returns and second method runs.

The purpose here, is to get a list of field names, and later on add the value of @JsonPropertyannotation, on each of the KibanaRow's fields.

Stephen C · Accepted Answer · 2020-04-10 03:35:43Z

1

I don't understand the particular reason for a CME. If you showed us the stacktrace we may be able to work it out ....

However what you are trying to do will not work anyway:

List<Field> kibanaFields 
        Arrays.asList(KibanaRow.class.getSuperclass().getDeclaredFields());

This creates a list backed by an array. The list's size is fixed. Elements cannot be added or removed.

kibanaFields.addAll(Arrays.asList(KibanaRow.class.getDeclaredFields()));

If KibanaRow.class.getDeclaredFields() returns a non-empty array, then this will attempt to add more elements to kibanaFields. It can't. An exception should ensue.

One solution to the problem I have identified is to copy the asList result's contents into a new ArrayList; e.g.

List<Field> kibanaFields = new ArrayList<>(Arrays.asList(....));

edited Apr 10, 2020 at 3:35

answered Apr 10, 2020 at 2:46

Stephen C

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1 Comment

Carmageddon Over a year ago

Thanks, this solved the CME and obviously, avoided the other error. Perhaps the CME is the exception for adding to a fixed array? I don't know..

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