Changing MultivalueMap keys throws ConcurrentModificationException

Question

I have MultivalueMap and a TreeMap, both have key-value of type String.

I wish to change any key in my MultivalueMap to keys found in my TreeMap.

This is my code, why do I get ConcurrentModificationException and how to solve it?

public RestRequest(MultivaluedMap<String, Object> myHeaders,TreeMap<String, String> testParameters) {
    this.testParameters = testParameters;
    buildUrl();     
    Client client = ClientBuilder.newClient();

    Set<String> keys = myHeaders.keySet();
    for (String key : keys) {
        myHeaders.remove(key);
        myHeaders.add(key, testParameters.get(key));
    }
    this.myHeaders = myHeaders;
    myResource = client.target(URL);
    entity=Entity.entity(replaceRequestBodyWithParams(requestBody), MediaType.APPLICATION_JSON);
}  

Answer 1

You can't modify the underlying collection that you're iterating over. You are looping over the keySet of a map and in the loop try to change that map, which would also alter the keySet.

A better way to do achieve what you're trying to do in this method, would be to construct a new data structure like this:

public RestRequest(MultivaluedMap<String, Object> myHeaders,TreeMap<String, String> testParameters) {
    this.testParameters = testParameters;
    buildUrl();     
    Client client = ClientBuilder.newClient();

    Set<String> keys = myHeaders.keySet();
    final Map<String, String> testParamsByHeader = new HashMap<>();

    for (String key : keys)
        testParamsByHeader.add(key, testParameters.get(key));
    }
    this.myHeaders = testParamsByHeader;
    myResource = client.target(URL);
    entity=Entity.entity(replaceRequestBodyWithParams(requestBody), MediaType.APPLICATION_JSON);
}