Update #0 - First Release

sources/main/java/com/google/common/collect/Constraint.java

@@ -0,0 +1,66 @@
+/*
+ * 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.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>
+ *
+ * <p>
+ * 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 Mike Bostock
+ */
+@GwtCompatible
+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();
+}