OptionalInt vs Optional<Integer>

When scrolling through the documentation for the java.util package, I was surpised to find that Optional<T> and OptionalInt have no relationship to each other. This seems very hard to belive, as it suggests that they are unrelated classes.

1. Why don't they have a common interface, class, are sub-types, or something to reveal the relationship they have? (They're very similar classes when you look at their uses.)
2. Also, why the need for an additional OptionalInt class? Why can't you just use Optional<Integer>? I thought it was due to the fact that int is primitive, but there is no OptionalChar so that would be an inconsistent design choice.

Java 8 introduced a whole lot dedicated to primitives. The reason is most likely that boxing primitives can create a lot of waste "boxes".

For example this

OptionalInt optionalFirst = IntStream
    .range(0, 100)
    .filter(i -> i % 23 > 7)
    .findFirst();

Here, an Optional<Integer> as result would be inconsistent. Also methods like ifPresent(IntConsumer consumer) then allow to stay withing the IntStream world. Optional<Integer> would force you to convert (which you can do easily if you want)

There is no need for special support for char or short or byte because all of them can be represented as int. The missing one is boolean but there is not much you can do in a stream with them since there are only 2 values.

There needs to be an OptionalInt class for Java 8's streams to be consistent. If you take a look at the Stream class, you'll see that many of the methods return Optional<T>. However, dealing with a Stream<Integer>, Stream<Long> or any other streams of primitives is exhausting, so there is an IntStream class and a LongStream class which replace the object with its unboxed value. For instance, finding the sum of the elements of a Stream<Integer> is not trivial, whereas for an IntStream, you just call IntStream#sum

In these classes, the JDK helpfully replaces Optional<T> with OptionalInt, OptionalLong, and so forth.

OptionalInt is a container holding value of primitive type int. Primitive types cannot be used as type parameters for generic classes, so there is no way it could be a subclass of Optional<T>.