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

Abaric

I want to check if anything in Map, HashMap, ArrayList, List or Collections is empty or empty?

I have this, but it doesn't work when I pass the map:

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

  return c.isEmpty();
 }

fail:

  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));

Guttman

Yours isCollectionMapNullOrEmptycompiles and works, but not the way you want.

<T extends Collection, Map>

You have declared two type variables, Tand Map, which Tmust be Collection. Type variables Maphave nothing to do with interfaces Mapand are actually not even used. (Also, you're using a raw interface Collectionhere .) What you're passing aListis Collection, so it compiles. However, it's aMapnot Collection, so compilation doesn't pass Map.

It looks like you want to Tbe 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.

(By the way, you can say one thing and the other: T extends Collection & Map, or no primitive types T extends Collection<?> & Map<?, ?>, but I don't know of any classes that implement both interfaces.)

You can have two overloaded methods, one for a Collectionand one for a Map, which will perform the same function. Here, I take advantage of short-circuiting to combine the following 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 since there is no super interface declaration isEmpty, this looks as good as it gets.

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