Why is `Stream.collect` type-safe and `Stream.toArray(IntFunction)` is not?

Question

Consider the following code fragment

String strings[] = {\"test\"};
final List collect = java.util.Arrays.stream(strings).collect(java.util.str         


        

Answer 1

The signature of the method Stream::toArray looks as follows. Please note that the type parameters T and A are completely unrelated.

public interface Stream<T> {
    <A> A[] toArray(IntFunction<A[]> generator);
}

In the source of ReferencePipeline.java, you can find the following comment:

Since A has no relation to U (not possible to declare that A is an upper bound of U) there will be no static type checking. Therefore use a raw type and assume A == U rather than propagating the separation of A and U throughout the code-base. The runtime type of U is never checked for equality with the component type of the runtime type of A[]. Runtime checking will be performed when an element is stored in A[], thus if A is not a super type of U an ArrayStoreException will be thrown.