[java] Best way to create an empty map in Java

I need to create an empty map.

if (fileParameters == null)
    fileParameters = (HashMap<String, String>) Collections.EMPTY_MAP;

The problem is that the above code produces this warning: Type safety: Unchecked cast from Map to HashMap

What is the best way to create this empty map?

Since in many cases an empty map is used for null-safe design, you can utilize the nullToEmpty utility method:

class MapUtils {

  static <K,V> Map<K,V> nullToEmpty(Map<K,V> map) {
    if (map != null) {
      return map;
    } else {
       return Collections.<K,V>emptyMap(); // or guava ImmutableMap.of()
    }
  }

}  

Similarly for sets:

class SetUtils {

  static <T> Set<T> nullToEmpty(Set<T> set) {
    if (set != null) {
      return set;
    } else {
      return Collections.<T>emptySet();
    }
  }

}

and lists:

class ListUtils {

  static <T> List<T> nullToEmpty(List<T> list) {
    if (list != null) {
      return list;
    } else {
      return Collections.<T>emptyList();
    }
  }

}

If you need an instance of HashMap, the best way is:

fileParameters = new HashMap<String,String>();

Since Map is an interface, you need to pick some class that instantiates it if you want to create an empty instance. HashMap seems as good as any other - so just use that.

Collections.emptyMap()

What about :

Map<String, String> s = Collections.emptyMap();

The emptyMap method of the Collections class.

Either Collections.emptyMap(), or if type inference doesn't work in your case,
Collections.<String, String>emptyMap()