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diff --git a/guava/src/com/google/common/collect/MapConstraint.java b/guava/src/com/google/common/collect/MapConstraint.java
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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
+ *
+ * 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;
+
+import javax.annotation.Nullable;
+
+/**
+ * A constraint on the keys and values that may be added to a {@code Map} or
+ * {@code Multimap}. For example, {@link MapConstraints#notNull()}, which
+ * prevents a map from including any null keys or values, could be implemented
+ * like this: <pre>   {@code
+ *
+ *   public void checkKeyValue(Object key, Object value) {
+ *     if (key == null || value == null) {
+ *       throw new NullPointerException();
+ *     }
+ *   }}</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 key
+ * and value. 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 Mike Bostock
+ * @see MapConstraints
+ * @see Constraint
+ * @since 3.0
+ */
+@GwtCompatible
+@Beta
+public interface MapConstraint<K, V> {
+  /**
+   * Throws a suitable {@code RuntimeException} if the specified key or value 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.
+   */
+  void checkKeyValue(@Nullable K key, @Nullable V value);
+
+  /**
+   * Returns a brief human readable description of this constraint, such as
+   * "Not null".
+   */
+  @Override
+  String toString();
+}