method ref from lambda

Question

In one of the method we used lambda expression (below) nesting streams.

return req.gUs().stream()
.map((**cfg) -> {
.filter(Optional::isPresent)
.map(Optional::get);

I want to move part of the code using some helper methods and method reference (below).

(cfg) -> {
return rsp.getUs().stream()
.filter(result.getId())    
.map((result) -> gEvent(index, Builder, cfg));

Any suggestions are greatly appreciated.

Besides the nested lambdas (your cfg -> {} method should really be moved to its own method), the method looks fine to me. — Joe C
– Joe C, Commented Mar 8, 2017 at 7:04
There is no sense in using .map(function_returning_stream).flatMap(stream -> stream); you can just use .flatMap(function_returning_stream) in the first place. Further, there is no need to write (cfg) -> { return expression; }, you can just write cfg -> expression. Now, also remove all obsolete brackets and you have halved the code… — Holger
– Holger, Commented Mar 8, 2017 at 17:50

mrs · Accepted Answer · 2017-06-02 11:04:27Z

1

You can make a method, that returns a Function:

return req.getUs().stream()
.map(myFunction(rsp, index, headerBuilder))
.flatMap(stream -> stream)
.filter(Optional::isPresent)
.map(Optional::get);

private Function<CFGType, GenerateReturnType> myFunction(RspType rsp, IndexType index, HeaderType header){
  return (cfg) -> {
    return rsp.getPerUs().stream()
    .filter((result) -> cfg.getId() == result.getId())
    .filter((result) -> result.getCode() == ResultCode.SUCCESS)
    .map((result) -> generateEvent(index, headerBuilder, cfg));
  }
}

Or you could use a method reference if the rsp, index and header are fields:

return req.getUs().stream()
.map(this::myFunction)
.flatMap(stream -> stream)
.filter(Optional::isPresent)
.map(Optional::get);

private GenerateType myFunction(CFGType cfg) {
  return rsp.getUs().stream()
  .filter((result) -> cfg.getUsChId() == result.getChId())
  .filter((result) -> result.getResultCode() == ResultCode.SUCCESS)
  .map((result) -> generateEvent(index, headerBuilder, cfg));
}

edited Jun 2, 2017 at 11:04

mrs

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answered Mar 8, 2017 at 7:59

Matthias Danetzky

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3 Comments

Matthias Danetzky Over a year ago

Ok, I made a typo in the myFunction signature. Should be myFunction(CFGType cfg).

Matthias Danetzky Over a year ago

And I assumed, that rsp is a field in the class.

Matthias Danetzky Over a year ago

The CFGType, RspType and GenerateRemoveEventReturnType just the placeholders for the types which are written somewhere in the rest of your class.

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