org.apache.aurora.common.base.Consumers.java Source code

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/**
 * 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 org.apache.aurora.common.base;

import java.util.List;
import java.util.function.Consumer;

import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.common.collect.Iterables;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

/**
 * Utilities for dealing with Consumers.
 *
 * @author John Sirois
 */
public final class Consumers {

    private static final Consumer<?> NOOP = item -> {
        // noop
    };

    private Consumers() {
        // utility
    }

    /**
     * Combines multiple consumers into a single consumer, whose calls are replicated sequentially
     * in the order that they were provided.
     * If an exception is encountered from a consumer it propagates to the top-level consumer and the
     * remaining consumer are not executed.
     *
     * @param consumers Consumers to combine.
     * @param <T> Type accepted by the consumers.
     * @return A single consumer that will fan out all calls to {@link Consumer#accept(Object)} to
     *    the wrapped consumers.
     */
    public static <T> Consumer<T> combine(List<Consumer<T>> consumers) {
        checkNotNull(consumers);
        checkArgument(Iterables.all(consumers, Predicates.notNull()));

        return consumers.stream().reduce(noop(), Consumer::andThen);
    }

    /**
     * Applies a filter to a consumer, such that the consumer will only be called when the filter is
     * satisfied (returns {@code true}}.
     *
     * @param filter Filter to determine when {@code consumer} is called.
     * @param consumer Consumer to filter.
     * @param <T> Type handled by the filter and the consumer.
     * @return A filtered consumer.
     */
    public static <T> Consumer<T> filter(final Predicate<T> filter, final Consumer<T> consumer) {
        checkNotNull(filter);
        checkNotNull(consumer);

        return item -> {
            if (filter.apply(item)) {
                consumer.accept(item);
            }
        };
    }

    /**
     * Returns a consumer that will do nothing.
     *
     * @param <T> The consumer argument type.
     * @return A consumer that does nothing.
     */
    @SuppressWarnings("unchecked")
    public static <T> Consumer<T> noop() {
        return (Consumer<T>) NOOP;
    }
}