GuavaCollectors.java

/*
 * Copyright (C) 2014 Square, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;

import java.util.function.BiConsumer;
import java.util.function.Supplier;
import java.util.stream.Collector;

/** Stream {@link Collector collectors} for Guava types. */
public final class GuavaCollectors {
    /** Collect a stream of elements into an {@link ImmutableList}. */
    public static <T> Collector<T, ImmutableList.Builder<T>, ImmutableList<T>> immutableList() {
        final Supplier<ImmutableList.Builder<T>> builder = ImmutableList.Builder::new;
        final BiConsumer<ImmutableList.Builder<T>, T> adder = ImmutableList.Builder::add;

        return Collector.of(builder, adder, (l, r) -> l.addAll(r.build()), ImmutableList.Builder<T>::build);
    }

    /** Collect a stream of elements into an {@link ImmutableSet}. */
    public static <T> Collector<T, ImmutableSet.Builder<T>, ImmutableSet<T>> immutableSet() {
        final Supplier<ImmutableSet.Builder<T>> builder = ImmutableSet.Builder::new;
        final BiConsumer<ImmutableSet.Builder<T>, T> adder = ImmutableSet.Builder::add;

        return Collector.of(builder, adder, (l, r) -> l.addAll(r.build()), ImmutableSet.Builder<T>::build);
    }

    private GuavaCollectors() {
        throw new AssertionError("No instances.");
    }
}

i also use this one...

    public static <T, K, V> Collector<T, com.google.common.collect.ImmutableMap.Builder<K, V>, ImmutableMap<K, V>> immutableMap(Function<T, K> keyFunc, Function<T, V> valFunc) {
        return Collector.of(com.google.common.collect.ImmutableMap.Builder::<init>, (builder, entry) -> {
            builder.put(keyFunc.apply(entry), valFunc.apply(entry));
        }, (l, r) -> {
            return l.putAll(r.build());
        }, com.google.common.collect.ImmutableMap.Builder::build, new Characteristics[0]);
    }