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?
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. 😎