Best way to create an empty map in Java

138
votes

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?

7
javadictionarycollectionshashmap
What is your declared type for fileParameters? – jjnguy
You'll probably get a ClassCastException too. – Tom Hawtin - tackline
fileParameters should be a Map and not a HashMap. – Steve Kuo

7 Answers

276
votes

1) If the Map can be immutable:

Collections.emptyMap()

// or, in some cases:
Collections.<String, String>emptyMap()

You'll have to use the latter sometimes when the compiler cannot automatically figure out what kind of Map is needed (this is called type inference). For example, consider a method declared like this: 

public void foobar(Map<String, String> map){ ... }

When passing the empty Map directly to it, you have to be explicit about the type:

foobar(Collections.emptyMap());                 // doesn't compile
foobar(Collections.<String, String>emptyMap()); // works fine

2) If you need to be able to modify the Map, then for example:

new HashMap<String, String>()

(as tehblanx pointed out)

Addendum: If your project uses Guava, you have the following alternatives:

1) Immutable map:

ImmutableMap.of()
// or:
ImmutableMap.<String, String>of()

Granted, no big benefits here compared to Collections.emptyMap(). From the Javadoc:

This map behaves and performs comparably to Collections.emptyMap(), and is preferable mainly for consistency and maintainability of your code.

2) Map that you can modify: 

Maps.newHashMap()
// or:
Maps.<String, String>newHashMap()

Maps contains similar factory methods for instantiating other types of maps as well, such as TreeMap or LinkedHashMap.

Update (2018): On Java 9 or newer, the shortest code for creating an immutable empty map is:

Map.of()

...using the new convenience factory methods from JEP 269. 😎

10
votes

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.

7
votes

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

2
votes

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();
    }
  }

}