Say, I have a method:
public static <T> Collection<T> addToCollection(T element, Collection<T> collection) {
collection.add(element);
return collection;
}
And then when trying to compile this code:
Integer i = 42;
Collection<Integer> result = addToCollection(i, Collections.emptyList());
I get an error Type mismatch: cannot convert from Collection<Object> to Collection<Integer>. Could anyone explain why the type system cannot infer that Collections.emptyList() should be of type Collection<Integer>?
The example above is obviously quite artificial, but I stumble upon that limitation all the time and it's really annoying. After having read Effective Java I have found out that you can simply do Collections.<Integer>emptyList() (must say, that was quite a revelation for me at the time) and have everything compiling smoothly, but when you have some complicated type then it really is a nuisance.
I'm just wondering if this is some sort of bug, or are there any valid reasons for it to work that way?
The type inference system has been improved in Java 8, with the introduction of target typing, in order to give more expressivity to streams and lambdas. As a consequence your code compiles with Java 8.
More about it in the updated tutorial, with a very similar example towards the very bottom of the page.
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