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  * 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.
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 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * See the License for the specific language governing permissions and
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A constraint that an element must satisfy in order to be added to a collection. For example, Constraints.notNull(), which prevents a collection from including any null elements, could be implemented like this:
   public Object checkElement(Object element) {
     if (element == null) {
       throw new NullPointerException();
     return element;

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.

Mike Bostock
interface Constraint<E> {
Throws a suitable RuntimeException if the specified element is illegal. Typically this is either a java.lang.NullPointerException, an java.lang.IllegalArgumentException, or a java.lang.ClassCastException, though an application-specific exception class may be used if appropriate.

element the element to check
the provided element
  E checkElement(E element);

Returns a brief human readable description of this constraint, such as "Not null" or "Positive number".
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