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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.
  * You may obtain a copy of the License at
 * 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
 * limitations under the License.
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.

Use for basic checks. In place of constrained collections, we encourage you to check your preconditions explicitly instead of leaving that work to the collection implementation. For the specific case of rejecting null, consider the immutable collections. This interface is scheduled for removal in Guava 16.0.
Mike Bostock
See also:
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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