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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.

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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.commons.collections4;

/**
 * Marker interface for collections, maps and iterators that are unmodifiable.
 * 
 * This interface enables testing such as:
 * 
 * if (coll instanceof Unmodifiable) {
 *   coll = new ArrayList(coll);
 * }
 * // now we know coll is modifiable
 * 
 * Of course all this only works if you use the Unmodifiable classes defined
 * in this library. If you use the JDK unmodifiable class via {@code java.util Collections}
 * then the interface won't be there.
 *
 * @since 3.0
 */
public interface Unmodifiable {
    // marker interface - no methods to implement
}