Java Streams: Extend Built-In Collector

Question

I would like to know if it is possible to extend a built-in Java Stream collector from, the java.util.stream.Collectors class, as opposed to building a custom collector, from scratch, and therefore duplicating code already implemented in that class.

For example: Let's say I have Stream<Map.Entry<String, Long>> mapEntryStreamand I want to collect that to a map of type Map<String, Long>.

Of course I could do:

mapEntryStream.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

But let's say I would like keys and entries inferred like so:

//Not a real Java Collectors method
mapEntryStream.collect(Collectors.toMap());

So how do I make a collector, like the one above, that takes no arguments but invokes Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)?

Please note: This is not a question about whether such a collector ought to be made - only if it can be made.

Slaw · Accepted Answer · 2020-01-20 17:43:35Z

2

You can't add a method to the Collectors class. However, you could create your own utility method that returns what you want:

import java.util.Map;
import java.util.Map.Entry;
import java.util.stream.Collector;
import java.util.stream.Collectors;

public class MoreCollectors {

  public static <K, V> Collector<Entry<K, V>, ?, Map<K, V>> entriesToMap() {
    return Collectors.toMap(Entry::getKey, Entry::getValue);
  }
}

edited Jan 20, 2020 at 17:43

answered Jan 17, 2020 at 20:42

Slaw

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6 Comments

David Woodworth Over a year ago

Oh that's what I meant by extend = add my own. But yeah, this works. Thank you!

jaco0646 Over a year ago

What makes this a factory method?

Slaw Over a year ago

@jaco0646 Could be the wrong term. What would you call it? Utility method?

jaco0646 Over a year ago

Yeah, I think utility method is accurate here.

Slaw Over a year ago

@jaco0646 Edited my answer. Though I still think it has at least some characteristics of a factory method.

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