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 /*
  * Copyright (C) 2007 The Guava Authors
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  * 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
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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,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
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package com.google.common.collect;
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.

Author(s):
Mike Bostock
Since:
3.0
See also:
Constraints
MapConstraint
public 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.

Parameters:
element the element to check
Returns:
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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