How would you make a java.util.stream.Collector that collects to com.google.common.collect.ImmutableSet?

问题

That seems too tricky for me since ImmutableSet instances are only built with ImmutableSet.Builder instances, which don't implement Collection so you can't just use Collectors.toCollection(ImmutableSet::new) or Collectors.toCollection(ImmutableSet.Builder::new).

回答1:

This is built into guava now,

ImmutableSet#toImmutableSet

Use like,

something.stream().collect(ImmutableSet.toImmutableSet())

回答2:

In fact, 3 months after :-), instead of defining a whole class for this, you can use Collector.ofand wrap it in a simple utility method:

public static <T> Collector<T, Builder<T>, ImmutableSet<T>> immutableSetCollector() {
    return Collector.of(Builder<T>::new, Builder<T>::add, (s, r) -> s.addAll(r.build()), Builder<T>::build);
}

and then:

import com.google.common.collect.ImmutableSet;
import com.google.common.collect.ImmutableSet.Builder;
....

ImmutableSet<Point2D> set = 
    Stream.of(new Point2D(1, 2), ...).collect(immutableSetCollector());

回答3:

We can write custom collector by extending the Collector interface.

Here is a solid reference that talks about it good detail: http://java.dzone.com/articles/introduction-writing-custom

回答4:

This is the closest that I found:

Set<String> set = list.stream().collect(ImmutableSet.Builder<String>::new, ImmutableSet.Builder<String>::add, (builder1, builder2) -> builder1.addAll(builder2.build())).build();

There is no method that takes append the elements of one builder into another builder, only an Iterable or an Iterator, so you cannot use a method reference for this one.

回答5:

So it appears that writing a custom collector in this case is not such a difficult task as I imagined it to be:

package org.tendiwa.collections;

import com.google.common.collect.ImmutableSet;

import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;

public class ImmutableSetCollector<T> implements Collector<T, ImmutableSet.Builder<T>, ImmutableSet<T>> {

    @Override
    public Supplier<ImmutableSet.Builder<T>> supplier() {
        return ImmutableSet.Builder::new;
    }

    @Override
    public BiConsumer<ImmutableSet.Builder<T>, T> accumulator() {
        return (builder, element) -> builder.add(element);
    }

    @Override
    public BinaryOperator<ImmutableSet.Builder<T>> combiner() {
        return (b1, b2) -> b1.addAll(b2.build());
    }

    @Override
    public Function<ImmutableSet.Builder<T>, ImmutableSet<T>> finisher() {
        return ImmutableSet.Builder::build;
    }

    @Override
    public Set<Characteristics> characteristics() {
        return ImmutableSet.of();
    }
}

Usage:

public static void main(String[] args) {
    ImmutableSet<Point2D> set = Arrays.asList(
        new Point2D(1, 2),
        new Point2D(2, 4),
        new Point2D(3, 5),
        new Point2D(4, 4),
        new Point2D(5, 6),
        new Point2D(8, 6)
    ).stream().collect(new ImmutableSetCollector<>());
    System.out.println(set);
}

Output:

[{1.0:2.0}, {2.0:4.0}, {3.0:5.0}, {4.0:4.0}, {5.0:6.0}, {8.0:6.0}]

回答6:

This functionality is available out-of-the-box in the guava-jdk8 library (which I authored).

来源：https://stackoverflow.com/questions/27612165/how-would-you-make-a-java-util-stream-collector-that-collects-to-com-google-comm

标签

java

java-8

Guava

collectors