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;
021import com.google.common.base.Preconditions;
022
023/**
024 * A constraint that an element must satisfy in order to be added to a
025 * collection. For example, {@link Constraints#notNull()}, which prevents a
026 * collection from including any null elements, could be implemented like this:
027 * <pre>   {@code
028 *
029 *   public Object checkElement(Object element) {
030 *     if (element == null) {
031 *       throw new NullPointerException();
032 *     }
033 *     return element;
034 *   }}</pre>
035 *
036 * <p>In order to be effective, constraints should be deterministic; that is,
037 * they should not depend on state that can change (such as external state,
038 * random variables, and time) and should only depend on the value of the
039 * passed-in element. A non-deterministic constraint cannot reliably enforce
040 * that all the collection's elements meet the constraint, since the constraint
041 * is only enforced when elements are added.
042 *
043 * @see Constraints
044 * @see MapConstraint
045 * @author Mike Bostock
046 * @since 3.0
047 * @deprecated Use {@link Preconditions} for basic checks. In place of
048 *     constrained collections, we encourage you to check your preconditions
049 *     explicitly instead of leaving that work to the collection implementation.
050 *     For the specific case of rejecting null, consider the immutable
051 *     collections.
052 *     This interface is scheduled for removal in Guava 16.0.
053 */
054@Beta
055@Deprecated
056@GwtCompatible
057public
058interface Constraint<E> {
059  /**
060   * Throws a suitable {@code RuntimeException} if the specified element is
061   * illegal. Typically this is either a {@link NullPointerException}, an
062   * {@link IllegalArgumentException}, or a {@link ClassCastException}, though
063   * an application-specific exception class may be used if appropriate.
064   *
065   * @param element the element to check
066   * @return the provided element
067   */
068  E checkElement(E element);
069
070  /**
071   * Returns a brief human readable description of this constraint, such as
072   * "Not null" or "Positive number".
073   */
074  @Override
075  String toString();
076}