DGM: more Optional and Stream extension methods

def opt = Optional.ofNullable(...)

opt.filter(Type) // DGM that is shorthand for opt.filter(it -> it instanceof Type).map(it -> (Type) it) // result is Optional<Type>

// methods available on Stream, but missing for Optional:
opt.mapToInt(...) // DGM that accepts ToIntFunction and returns OptionalInt
opt.mapToLong(...) // DGM that accepts ToLongFunction and returns OptionalLong
opt.mapToDouble(...) // DGM that accepts ToDoubleFunction and returns OptionalDouble

opt.stream() // DGM that is shorthand for opt.map(Stream::of).orElseGet(Stream::empty)
// useful for Stream.concat() or generally interfacing with stream-based APIs

// add get() to OptionalInt, OptionalLong and OptionalDouble as shorthand for getAsInt(), etc.

OptionalInt optInt = ...
optInt.filter(i -> ...) // accepts IntPredicate and returns OptionalInt.  Not sure why this is missing from OptionalInt, but this allows testing the value against some criteria.
// similarly for OptionalLong and OptionalDouble

def one = Stream.of(...)
def two = Stream.of(...)
def str = one + two // plus() overload for stream as shorthand for Stream.concat()
str = str + obj // plus() overload for Stream.concat(str, obj.stream())

str << obj // leftShift() overload; may make sense, may not

def str2 = Stream.ofNullable(obj) // DGSM to return Stream.empty() for null and Stream.of(obj) for non-null -- exists in Java 9+

// or maybe a general stream(Object) so you could do this instead:
def str3 = obj.stream() // Stream.empty() for null and Stream.of(obj) for non-null