functional programming - Why are there primitive functions like DoubleFunction in Java 8

Question

I just had a look at the the new Java 8 function package and wonder why there are interfaces like

• DoubleFunction
• IntFunction
• LongFunction
• ...

which do not extend Function. Doesn't that mean I will not be able to pass a Function<T,Int> where a IntFunction<T> is required and vice versa? The same applies for *Block, *Supplier and *UnaryOperator.

I can see the advantage that I will not have to check for null when a primitive is returned, but the list of disadvantages seem to be much longer

Answer 1

This issue is related to the fact that primitive types in Java are not unified to be substitutable for Object, and with generic type erasure.

Using Function<T, Integer> instead of IntFunction<T> when the last one suffices has 2 disadvantages:

• Every returned int is boxed - meaning a larger memory footprint;
• Every returned Integer gets an automatic runtime check (which can be optimized away, but yeah...);

Note that these kinds of issues with the collection framework in Java have led people to write a whole library, named Trove, that eschews the generic interfaces in favor of specialized collection types for every primitive type.