The problem with primitive types (at least until Java 5) is that they didn't extend from the base Object class. All of the collections need to specify a class for all the methods they are using - and they specify Object, since Object is the base of all the classes.

As of Java 5, you will find that Java will implicitly switch between a primitive and it's corresponding wrapper class when you need it. This means you can add an int, or a double, etc. to a Collection. The VM will automatically wrap the primitive in a wrapper class for you and place the wrapper in the Collection.