Why can't I initialize a Map<int, String>?

Question

I want to store a set of int/String values, but the ints are not necessarily incremental, meaning the data can be:

<1, "first">, <3, "second">, <9, "third">. 

So I'm trying to create the c# equivalent of a Dictionary<int, string> but it just fails to compile, saying "Syntax error on token "int", Dimensions expected after this token" on the line:

private Map<int, String> courses;

Can anyone please tell me why this is? And a good alternative to creating an object as a placeholder for the int and String, then using an array to store them?

Solution

You cannot use primitive types as generic type arguments.

Use

private Map<Integer, String> courses;

See more restrictions on Generics here.

Dev's contribution: the JLS specifies that only reference types (and wildcards) can be used as generic type arguments.

Comments

[Dev]

As for the why, The [Java Language Specification] (docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.‌​5.1) specifies that only ReferenceType may be used as a Type Argument.

[Sotirios Delimanolis]

@Dev Thanks, gave up looking for it :p

[Ted Hopp]

If you don't want the overhead of boxing key values as Integer objects, then you can use a third-party solution. The Trove library has TIntObjectHashMap and Android has SparseArray (the latter only useful for Android projects, of course). However, these third-party maps don't implement the Map interface from the Java collections framework, since the keys are not objects.