I am writing a library call that will allow you to filter and map in the same iteration of the loop, this is async/callback style in Java. The problem of course is that for filtering we usually return true/false, but for mapping we need to return the value we want.

So I am thinking of something like this:

(val, v) -> {
   v.include();   // we want to keep this value
   return val * 2;  // the value to map to
}

so the include() method call is like returning true in a typical filter operation. We could add an exclude() method which is returning false.

My question is - is this a good methodology? Are there libraries that do something like this that have a well-understood way of filtering and mapping in the same iteration?

if it's not clear, this would discard the element in the array.

(val, v) -> {
   v.exclude();   
   return null; 
}

another solution would be for them to return a unique key if they want to filter the value out:

(val, v) -> {
   return DISCARD_KEY;  // return a unique key representing discard, like returning false in a typical filter 
}