A method to check if a Collection or Map is empty or null?

Question

I want to check if a Map, HashMap, ArrayList, List or anything from Collections is Empty or null?

I have this, but its not working when I pass a Map:

protected static <T extends Collection, Map> boolean isCollectionMapNullOrEmpty(final T c) {
  if (c == null) {
      return true;
  }

  return c.isEmpty();
 }

Failure:

  List<String> aList = Arrays.asList("a1", "a2", "a4");
  Map<String, Object> aMap = new HashMap<String, Object>();
  aMap.put("a2", "foo");
  aMap.put("a1", "foo");
  aMap.put("a3", "foo");
  aMap.put("a4", "foo");
  System.out.println(isCollectionMapNullOrEmpty(aList));  // works

  // fails with The method isCollectionMapNullOrEmpty(T) in the type LearnHashMap is not applicable for the arguments (Map<String,Object>)
  System.out.println(isCollectionMapNullOrEmpty(aMap));

Answer 1

Your isCollectionMapNullOrEmpty compiles and works, but not the way you intended.

<T extends Collection, Map>

You've declared two type variables, T and Map, where T must be a Collection. The type variable Map has nothing to do with the interface Map, and is in fact not even used. (Also, you used the raw Collection interface here.) You are passing aList which is a Collection, so it compiles. However, aMap is not a Collection, so the compilation fails passing in a Map.

It looks like you wanted T to be either a Collection or a Map. But Java's generics don't work that way; you can't declare a type parameter to be one thing or another thing.

(As an aside, you can say one thing and another thing: T extends Collection & Map, or without raw types, T extends Collection<?> & Map<?, ?>, but I'm not aware of any classes that implement both interfaces.)

You can have two overloaded methods, one for a Collection and one for a Map, that will perform the same functionality. Here, I've taken advantage of short-circuiting to combine the statements:

protected static boolean isCollectionMapNullOrEmpty(final Collection<?> c) {
    return c == null || c.isEmpty();
}
protected static boolean isCollectionMapNullOrEmpty(final Map<?, ?> m) {
    return m == null || m.isEmpty();
}

Both methods have the same exact code, but because there is no super-interface declaring isEmpty, this appears to be as good as it gets.