com.palantir.common.streams.MoreCollectors.java Source code

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Here is the source code for com.palantir.common.streams.MoreCollectors.java

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/*
 * Copyright 2016 Palantir Technologies, Inc. All rights reserved.
 *
 * 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.
 */
package com.palantir.common.streams;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;

import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collector;

/**
 * Java 8 {@link Collector} for immutable collections.
 */
public class MoreCollectors {

    /*
     * This collector has similar semantics to {@link Collectors::toSet} except that the resulting set will be
     * immutable.
     */
    public static <T> Collector<T, ?, Set<T>> toImmutableSet() {
        return Collector.of(ImmutableSet::<T>builder, ImmutableSet.Builder::<T>add,
                (left, right) -> left.addAll(right.build()), ImmutableSet.Builder::build,
                Collector.Characteristics.UNORDERED);
    }

    /*
     * This collector has similar semantics to {@link Collectors::toList} except that the resulting list will be
     * immutable.
     */
    public static <T> Collector<T, ?, List<T>> toImmutableList() {
        return Collector.of(ImmutableList::<T>builder, ImmutableList.Builder::<T>add,
                (left, right) -> left.addAll(right.build()), ImmutableList.Builder::build);
    }

    /*
     * This collector has similar semantics to {@link Collectors::toMap} except that the resulting map will be
     * immutable. Duplicate keys will result in an error.
     */
    public static <T, K, V> Collector<T, ?, Map<K, V>> toImmutableMap(Function<T, K> keyFunction,
            Function<T, V> valueFunction) {
        return Collector.of(ImmutableMap::<K, V>builder,
                (builder, value) -> builder.put(keyFunction.apply(value), valueFunction.apply(value)),
                (left, right) -> left.putAll(right.build()), ImmutableMap.Builder::build);
    }

    private MoreCollectors() {
    }
}