When I try to declare a Dictionary as such:
private Dictionary<String, int> map;
The compiler gives me the following error:
Syntax error on token "int", Dimensions expected after this token
But it works fine with Integer. I'm vaguely aware that java treats int / Integer differently (I come from a .NET background), but I was hoping someone could give me a full explanation on I can't use primitives in a Dictionary<>
Java collections only allow references not primitives. You need to use the wrapper classes (in this case java.lang.Integer) to do what you are after:
private Dictionary<String, Integer> map;
they you can do things like:
int foo = map.get("hello");
and
map.put("world", 42);
and Java uses autoboxing/unboxing to deal with the details of the conversion for you.
Here is a little description on it.
In Java primitives aren't objects, so you can't use them in place of objects. However Java will automatically box/unbox primitives (aka autoboxing) into objects so you can do things like:
List<Integer> intList = new LinkedList<Integer>();
intList.add(1);
intList.add(new Integer(2));
...
Integer first = intList.get(0);
int second = intList.get(1);
But this is really just the compiler automatically converting types for you.
Because in Java the primitives are truely primitives. In Java int will pass by value, while Integer will pass a reference. In .NET int or Int32 etc. are just different names.
In .Net, "primitive" types are backed by objects. In Java, there's a hard distinction between primitive types and Objects. Java 5 introduced autoboxing, which can coerce between the two in certain situations. However, because the Java generics system uses type-erasure, there isn't enough information to autobox in this case.
To expand on TofuBeer's answer.
int is a primitive
Integer is an Object.
Generics does not support primitives.