    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
    017    package com.google.common.collect;
    018
    019    import com.google.common.annotations.Beta;
    020    import 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
    049    public 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    }