How do I fix "The expression of type List needs unchecked conversion...' ?

Question

In the Java snippet:

SyndFeedInput fr = new SyndFeedInput();
SyndFeed sf = fr.build(new XmlReader(myInputStream));
List<SyndEntry> entries = sf.getEntries();

the last line generates the warning

"The expression of type List needs unchecked conversion to conform to List<SyndEntry>"

What's an appropriate way to fix this?

Answer 1

Did you wirte SyndFeed?

Does sf.getEntries return List or List<SyndEntry>? My guess is it returns List and changing it to return List<SyndEntry> will fix the problem.

If SyndFeed is part of a library, I don't think you can remove the warning without adding the @SuppressWarning("unchecked") annotation to your method.

Answer 2

If you look at the javadoc for the class SyndFeed (I guess you are the class com.sun.syndication.feed.synd.SyndFeed), the method getEntries() doesn't return java.util.List<SyndEntry>, but returns just java.util.List.

So you need an explicit cast for this.

Answer 3

It looks like SyndFeed is not using generics.

You could either have an unsafe cast and a warning suppression:

@SuppressWarnings("unchecked")
List<SyndEntry> entries = (List<SyndEntry>) sf.getEntries();

or call Collections.checkedList - although you'll still need to suppress the warning:

@SuppressWarnings("unchecked")
List<SyndEntry> entries = collections.checkedList(sf.getEntries(), SyndEntry.class);

Answer 4

If you don't want to put @SuppressWarning("unchecked") on each sf.getEntries() call, you can always make a wrapper that will return List.

See this other question

Answer 5

This is a common problem when dealing with pre-Java 5 APIs. To automate the solution from erickson, you can create the following generic method:

public static <T> List<T> castList(Class<? extends T> clazz, Collection<?> c) {
    List<T> r = new ArrayList<T>(c.size());
    for(Object o: c)
      r.add(clazz.cast(o));
    return r;
}

This allows you to do:

List<SyndEntry> entries = castList(SyndEntry.class, sf.getEntries());

Because this solution checks that the elements indeed have the correct element type by means of a cast, it is safe, and does not require SuppressWarnings.