HashMap<String, int>
doesn't seem to work but HashMap<String, Integer>
does work.
Any ideas why?
views:
459answers:
2You can't use primitive types as generic arguments in Java. Use instead:
Map<String, Integer> myMap = new HashMap<String, Integer>();
With auto-boxing/unboxing there is little difference in the code. Auto-boxing means you can write:
myMap.put("foo", 3);
instead of:
myMap.put("foo", new Integer(3));
Auto-boxing means the first version is implicitly converted to the second. Auto-unboxing means you can write:
int i = myMap.get("foo");
instead of:
int i = myMap.get("foo").intValue();
The implicit call to intValue()
means if the key isn't found it will generate a NullPointerException
, for example:
int i = myMap.get("bar"); // NullPointerException
The reason is type erasure. Unlike, say, in C# generic types aren't retained at runtime. They are just "syntactic sugar" for explicit casting to save you doing this:
Integer i = (Integer)myMap.get("foo");
To give you an example, this code is perfectly legal:
Map<String, Integer> myMap = new HashMap<String, Integer>();
Map<Integer, String> map2 = (Map<Integer, String>)myMap;
map2.put(3, "foo");
GNU Trove support this but not using generics. http://trove4j.sourceforge.net/javadocs/gnu/trove/TObjectIntHashMap.html