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Java example source code file (Constraint.java)

This example Java source code file (Constraint.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

canignorereturnvalue, constraint, gwtcompatible, override, string

The Constraint.java Java example source code

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * 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 com.google.common.collect;

import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;

/**
 * A constraint that an element must satisfy in order to be added to a
 * collection. For example, {@link Constraints#notNull()}, which prevents a
 * collection from including any null elements, could be implemented like this:
 * <pre>   {@code
 *
 *   public Object checkElement(Object element) {
 *     if (element == null) {
 *       throw new NullPointerException();
 *     }
 *     return element;
 *   }}</pre>
 *
 * <p>In order to be effective, constraints should be deterministic; that is,
 * they should not depend on state that can change (such as external state,
 * random variables, and time) and should only depend on the value of the
 * passed-in element. A non-deterministic constraint cannot reliably enforce
 * that all the collection's elements meet the constraint, since the constraint
 * is only enforced when elements are added.
 *
 * @author Mike Bostock
 */
@GwtCompatible
interface Constraint<E> {
  /**
   * Throws a suitable {@code RuntimeException} if the specified element is
   * illegal. Typically this is either a {@link NullPointerException}, an
   * {@link IllegalArgumentException}, or a {@link ClassCastException}, though
   * an application-specific exception class may be used if appropriate.
   *
   * @param element the element to check
   * @return the provided element
   */
  @CanIgnoreReturnValue
  E checkElement(E element);

  /**
   * Returns a brief human readable description of this constraint, such as
   * "Not null" or "Positive number".
   */
  @Override
  String toString();
}

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