1 /* 2 * Copyright (C) 2007 The Guava Authors 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package com.google.common.collect; 18 19 import com.google.common.annotations.GwtCompatible; 20 21 /** 22 * A constraint that an element must satisfy in order to be added to a 23 * collection. For example, {@link Constraints#notNull()}, which prevents a 24 * collection from including any null elements, could be implemented like this: 25 * <pre> {@code 26 * 27 * public Object checkElement(Object element) { 28 * if (element == null) { 29 * throw new NullPointerException(); 30 * } 31 * return element; 32 * }}</pre> 33 * 34 * <p>In order to be effective, constraints should be deterministic; that is, 35 * they should not depend on state that can change (such as external state, 36 * random variables, and time) and should only depend on the value of the 37 * passed-in element. A non-deterministic constraint cannot reliably enforce 38 * that all the collection's elements meet the constraint, since the constraint 39 * is only enforced when elements are added. 40 * 41 * @author Mike Bostock 42 */ 43 @GwtCompatible 44 interface Constraint<E> { 45 /** 46 * Throws a suitable {@code RuntimeException} if the specified element is 47 * illegal. Typically this is either a {@link NullPointerException}, an 48 * {@link IllegalArgumentException}, or a {@link ClassCastException}, though 49 * an application-specific exception class may be used if appropriate. 50 * 51 * @param element the element to check 52 * @return the provided element 53 */ 54 E checkElement(E element); 55 56 /** 57 * Returns a brief human readable description of this constraint, such as 58 * "Not null" or "Positive number". 59 */ 60 @Override 61 String toString(); 62 } 63
