001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import com.google.common.annotations.Beta;
020import com.google.common.annotations.GwtCompatible;
021
022/**
023 * A constraint that an element must satisfy in order to be added to a
024 * collection. For example, {@link Constraints#notNull()}, which prevents a
025 * collection from including any null elements, could be implemented like this:
026 * <pre>   {@code
027 *
028 *   public Object checkElement(Object element) {
029 *     if (element == null) {
030 *       throw new NullPointerException();
031 *     }
032 *     return element;
033 *   }}</pre>
034 *
035 * In order to be effective, constraints should be deterministic; that is,
036 * they should not depend on state that can change (such as external state,
037 * random variables, and time) and should only depend on the value of the
038 * passed-in element. A non-deterministic constraint cannot reliably enforce
039 * that all the collection's elements meet the constraint, since the constraint
040 * is only enforced when elements are added.
041 *
042 * @see Constraints
043 * @see MapConstraint
044 * @author Mike Bostock
045 * @since 3.0
046 */
047@Beta
048@GwtCompatible
049public interface Constraint<E> {
050  /**
051   * Throws a suitable {@code RuntimeException} if the specified element is
052   * illegal. Typically this is either a {@link NullPointerException}, an
053   * {@link IllegalArgumentException}, or a {@link ClassCastException}, though
054   * an application-specific exception class may be used if appropriate.
055   *
056   * @param element the element to check
057   * @return the provided element
058   */
059  E checkElement(E element);
060
061  /**
062   * Returns a brief human readable description of this constraint, such as
063   * "Not null" or "Positive number".
064   */
065  @Override
066  String toString();
067}