How to make method accept List which contains objects of any datatype

Question

My goal is to check the type of messages and then accordingly convert them to Strings and add them. How can I achieve this?

public void addMessages(List<?> messages) {
    if (messages != null) {
        if (messages instanceof String) {
            for (String message : messages) {
                this.messages.add(message);
            }
        }
    }

    else {
        for (Object message:messages) {
            this.messages.add(String.valueOf(message));
        }
    }
}

Answer 1

You can just pass in a List of Objects - you don't even need the if/else since you can just always call "toString()" or "String.valueOf" on the message object:

public void addMessages(List<Object> messages) {
    if (!CollectionUtils.isEmpty(messages)) {
        for (Object message : messages) {
            this.messages.add(String.valueOf(message));
        }
    }
}

On a side note: Potential problems could arise from having null elements in the messages list - so you might want to check for that in your loop. Other potential pitfalls are:

• this.messages is not initialised and adding messages throws a NullPointerException
• if this is a method of a singleton (e.g. a spring service) working with instance variables should be avoided