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}