Use of contains in Java ArrayList<String>

Question

If I have an ArrayList of String forming part of a class in Java like so:

private ArrayList<String> rssFeedURLs;

If I want to use a method in the class containing the above ArrayList, using ArrayList contains to check if a String is contained in this ArrayList, I believe I should be able to do so as follows:

if (this.rssFeedURLs.contains(rssFeedURL)) {

Where rssFeedURL is a String.

Am I right or wrong?

Answer 1

You are right. ArrayList.contains() tests equals(), not object identity:

returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e))

If you got a NullPointerException, verify that you initialized your list, either in a constructor or the declaration. For example:

private List<String> rssFeedURLs = new ArrayList<String>();

Answer 2

Yes, that should work for Strings, but if you are worried about duplicates use a Set. This collection prevents duplicates without you having to do anything. A HashSet is OK to use, but it is unordered so if you want to preserve insertion order you use a LinkedHashSet.

Answer 3

You are right that it should work; perhaps you forgot to instantiate something. Does your code look something like this?

String rssFeedURL = "http://stackoverflow.com";
this.rssFeedURLS = new ArrayList<String>();
this.rssFeedURLS.add(rssFeedURL);
if(this.rssFeedURLs.contains(rssFeedURL)) {
// this code will execute
}

For reference, note that the following conditional will also execute if you append this code to the above:

String copyURL = new String(rssFeedURL);
if(this.rssFeedURLs.contains(copyURL)) {
// code will still execute because contains() checks equals()
}

Even though (rssFeedURL == copyURL) is false, rssFeedURL.equals(copyURL) is true. The contains method cares about the equals method.