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Error collection in Java method, empty ArrayList and LinkedList

It seems I sometimes create these two types of methods:

// return null on errors, and append errors to 2nd param, otherwise return result
String fetchSomething(String parameter, List<String> errorMessagesOut);

// return empty list or null on no errors, otherwise list of errors
List<String> verifySomething(String parameter);

And then the code that calls them joins the error list with the appropriate separator (for example, a comma, a new line, HTML tags ...), usually using the Apache Commons method Stringutils.join. And in the normal case there is no error, and the list will be empty.

So, I began to wonder about these two questions:

  • Do you see a problem returning the error message line in a list? If so, which alternative is better? (Not an exception that will cause code that calls these methods when required.)

  • Is it new LinkedList()or new ArrayList(0)or new ArrayList()better for a list that is expected to remain empty and that usually should only have sequential access to the iterator when it is not empty?

EDIT: Usage Example:

List<String> verifyParameters(JSONObject params) {
    List<String> ret = new ArrayList<String>(0);

    if (!verifyKey(params.get("key"))) 
        ret.add("Invalid key: " + key);

    if (!verifyAccess(params.get("user"), params.get("pass"))) 
        ret.add("Authentication error");

   return ret;
}

...

List<String> errors = verifyParameters(params);
if (!errors.isEmpty()) {

    connection.sendErrorListMessage(errors);
    logger.warn(StringUtils.join(errors, ", "));
    controlPanel.show("Errors: \n- " + StringUtils.join(errors, "\n- ") + '\n');
    throw new AbortException("invalid params); // or maybe return false/null;
}
// proceed with valid params

Usually error list processing will not have all of them, it just tries to illustrate the point at which the error list is a list of messages intended for people, regardless of how it will be shown, and also not related to / useful for handling different errors differently.

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4 answers

, . Result fetchSomthing , errorMessagesOut - , :

Result result = fetchSomething(String parameter);
if (result.hasErrors()) {
    List<String> errors = result.getErrors();
} else {
    String fetched = result.getValue();
}

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String errorMessage = result.getErrorString();

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Each instance of ArrayList has a capacity. Capacity is the size of an array used to store items in a list. This is always less than the size of the list. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not indicated beyond the fact that adding the item is a constant depreciation expense of time.

However, you will save some memory with new ArrayList(0)if the list is most often empty, since the size of the list is initialized to 10if the initial capacity is not specified.

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