/*
* 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
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* 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
* limitations under the License.
*/
package com.google.common.collect;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
/**
* A constraint that an element must satisfy in order to be added to a
* collection. For example, {@link Constraints#notNull()}, which prevents a
* collection from including any null elements, could be implemented like this:
* <pre> {@code
*
* public Object checkElement(Object element) {
* if (element == null) {
* throw new NullPointerException();
* }
* return element;
* }}</pre>
*
* 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.
*
* @see Constraints
* @see MapConstraint
* @author Mike Bostock
* @since 3.0
*/
@Beta
@GwtCompatible
public interface Constraint<E> {
/**
* Throws a suitable {@code RuntimeException} if the specified element is
* illegal. Typically this is either a {@link NullPointerException}, an
* {@link IllegalArgumentException}, or a {@link ClassCastException}, though
* an application-specific exception class may be used if appropriate.
*
* @param element the element to check
* @return the provided element
*/
E checkElement(E element);
/**
* Returns a brief human readable description of this constraint, such as
* "Not null" or "Positive number".
*/
@Override
String toString();
}
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