Example Java 8 collectors for Guava types without method references or lambda expressions. https://gist.github.com/JakeWharton/9734167

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

import static java.util.stream.Collector.Characteristics.UNORDERED;

/** 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() {
    return Collector.of(new Supplier<ImmutableList.Builder<T>>() {
      @Override public ImmutableList.Builder<T> get() {
        return new ImmutableList.Builder<>();
      }
    }, new BiConsumer<ImmutableList.Builder<T>, T>() {
      @Override public void accept(ImmutableList.Builder<T> builder, T item) {
        builder.add(item);
      }
    }, new BinaryOperator<ImmutableList.Builder<T>>() {
      @Override public ImmutableList.Builder<T> apply(ImmutableList.Builder<T> builder1,
          ImmutableList.Builder<T> builder2) {
        return builder1.addAll(builder2.build());
      }
    }, new Function<ImmutableList.Builder<T>, ImmutableList<T>>() {
      @Override public ImmutableList<T> apply(ImmutableList.Builder<T> builder) {
        return builder.build();
      }
    });
  }

  /** Collect a stream of elements into an {@link ImmutableSet}. */
  public static <T> Collector<T, ImmutableSet.Builder<T>, ImmutableSet<T>> immutableSet() {
    return Collector.of(new Supplier<ImmutableSet.Builder<T>>() {
      @Override public ImmutableSet.Builder<T> get() {
        return new ImmutableSet.Builder<>();
      }
    }, new BiConsumer<ImmutableSet.Builder<T>, T>() {
      @Override public void accept(ImmutableSet.Builder<T> builder, T item) {
        builder.add(item);
      }
    }, new BinaryOperator<ImmutableSet.Builder<T>>() {
      @Override public ImmutableSet.Builder<T> apply(ImmutableSet.Builder<T> builder1,
          ImmutableSet.Builder<T> builder2) {
        return builder1.addAll(builder2.build());
      }
    }, new Function<ImmutableSet.Builder<T>, ImmutableSet<T>>() {
      @Override public ImmutableSet<T> apply(ImmutableSet.Builder<T> builder) {
        return builder.build();
      }
    }, UNORDERED);
  }

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