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Module "junit-commons" of JUnit 5.

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/*
 * Copyright 2015-2016 the original author or authors.
 *
 * All rights reserved. This program and the accompanying materials are
 * made available under the terms of the Eclipse Public License v1.0 which
 * accompanies this distribution and is available at
 *
 * http://www.eclipse.org/legal/epl-v10.html
 */

package org.junit.gen5.commons.util;

import static org.junit.gen5.commons.meta.API.Usage.Internal;

import java.util.Collection;

import org.junit.gen5.commons.meta.API;

/**
 * Collection of utilities for working with {@link Collection Collections}.
 *
 * DISCLAIMER
 *
 * These utilities are intended solely for usage within the JUnit framework
 * itself. Any usage by external parties is not supported.
 * Use at your own risk!
 *
 * @since 5.0
 */
@API(Internal)
public final class CollectionUtils {

	private CollectionUtils() {
		/* no-op */
	}

	/**
	 * Read the only element of a collection of size 1.
	 *
	 * @param collection the collection to get the element from
	 * @return the only element of the collection
	 * @throws PreconditionViolationException if the collection is {@code null}
	 * or does not contain exactly one element
	 */
	public static  T getOnlyElement(Collection collection) {
		Preconditions.notNull(collection, "collection must not be null");
		Preconditions.condition(collection.size() == 1,
			() -> "collection must contain exactly one element: " + collection);
		return collection.iterator().next();
	}

}