Java custom function on stream

Question

Say I have something like

something.stream()
         .filter(filterMethod) // Same
         .map(mapMethod)       //
         .map(somethingElse)
         .filter(filterMethod) // Same
         .map(mapMethod)       //
         .filter(otherFilter)
         .filter(filterMethod) // Same
         .map(mapMethod)       //

Could I create custom function on Stream and convert .filter().map() to one method? Implementing own Stream seems to overkill. It would be nice to have some short lambda function or method like

Stream<T> fooFiterMap(Stream<T> stream){
    return stream.filter(filterMethod).map(mapMethod);
}

and then reduce my something stream into

something.stream()
         .fooFilterMap()     // New
         .map(somethingElse)
         .fooFilterMap()     // New
         .filter(otherFilter)
         .fooFilterMap()     // New

You mean like something.stream().myFilterAndMap().myFilterAndMap().myFilterAndMap()? — D. Kovács
– D. Kovács, Commented Oct 10, 2017 at 18:00
That seems pretty hard to do. My guess is you would have to write your own Stream interface (extends Stream) to begin with. — Paul Lemarchand
– Paul Lemarchand, Commented Oct 10, 2017 at 18:15
@PaulLemarchand: if that is the question, then indeed it is, and then it's a duplicate of stackoverflow.com/questions/30685623/… :) — D. Kovács
– D. Kovács, Commented Oct 10, 2017 at 18:21
yeah... you can't do that obviously without extending Stream in the first place (StreamEx library has done it for example); but looks like a big overkill here — Eugene
– Eugene, Commented Oct 10, 2017 at 19:53

Eugene · Accepted Answer · 2017-10-10 19:39:33Z

2

You can obviously write your own one:

<T> Stream<T> fooFiterMap(Stream<T> stream, Predicate<T> predicate, UnaryOperator<T> function) {
    return stream.filter(predicate).map(function);
}

But the real question is why? It's too verbose? If so, than I'll argue - I like the chaining of filter and map more - bit it's subjective I guess. If you think about multiple objects created, than just think about the fact that these are probably stateless lambdas used on the same call-site, thus a single instance of Predicate and Function.

answered Oct 10, 2017 at 19:39

Eugene

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4 Comments

Holger Over a year ago

The OP doesn’t want a method for combining an arbitrary Predicate and an arbitrary Function/UnaryOperator. The OP wants to repeatedly apply the same filter and map function, so that not specifying the functions again would truly shorten the code, though, I admittedly can’t imagine real life scenarios for that…

Post Self Over a year ago

@Holger I do: your own snippet of filter(T.class::isInstance).map(T.class::cast)

Eugene Over a year ago

@PostSelf how would T.class work? I don't get your example here

Post Self Over a year ago

Probably not, I don't get this whole Java syntax, but you should get the meaning (filtering and downcasting).

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