GroupingByCollector.java

/*
 * Copyright 2020 Collect Utils Authors
 *
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements. See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership. The ASF licenses this file
 * to you 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.
 */
package net.morimekta.collect.collectors;

import net.morimekta.collect.UnmodifiableList;
import net.morimekta.collect.UnmodifiableMap;
import net.morimekta.collect.UnmodifiableSet;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
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;

import static java.util.Objects.requireNonNull;

public class GroupingByCollector<E, K, V> implements Collector<E, Map<K, List<V>>, UnmodifiableMap<K, List<V>>> {
    private final Function<E, K> toKey;
    private final Function<E, V> toValue;

    public GroupingByCollector(Function<E, K> toKey, Function<E, V> toValue) {
        this.toKey = requireNonNull(toKey);
        this.toValue = requireNonNull(toValue);
    }

    @Override
    public Supplier<Map<K, List<V>>> supplier() {
        return HashMap::new;
    }

    @Override
    public BiConsumer<Map<K, List<V>>, E> accumulator() {
        return (b, e) -> b
                .computeIfAbsent(toKey.apply(e), k -> new ArrayList<>())
                .add(toValue.apply(e));
    }

    @Override
    public BinaryOperator<Map<K, List<V>>> combiner() {
        return (b1, b2) -> {
            for (Map.Entry<K, List<V>> entry : b2.entrySet()) {
                b1.computeIfAbsent(entry.getKey(), k -> new ArrayList<>(entry.getValue().size()))
                  .addAll(entry.getValue());
            }
            return b1;
        };
    }

    @Override
    public Function<Map<K, List<V>>, UnmodifiableMap<K, List<V>>> finisher() {
        return map -> {
            UnmodifiableMap.Builder<K, List<V>> builder = UnmodifiableMap.newBuilder();
            for (Map.Entry<K, List<V>> entry : map.entrySet()) {
                builder.put(entry.getKey(), UnmodifiableList.asList(entry.getValue()));
            }
            return builder.build();
        };
    }

    @Override
    public Set<Characteristics> characteristics() {
        return UnmodifiableSet.setOf();
    }
}