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

There is a newer version: 4.5.0-M2

/*
 * 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.set;

import java.util.Collection;
import java.util.Iterator;
import java.util.Set;

import org.apache.commons.collections4.Unmodifiable;
import org.apache.commons.collections4.iterators.UnmodifiableIterator;

/**
 * Decorates another Set to ensure it can't be altered.
 * 
 * This class is Serializable from Commons Collections 3.1.
 * 
 * Attempts to modify it will result in an UnsupportedOperationException.
 *
 * @param  the type of the elements in this set
 * @since 3.0
 */
public final class UnmodifiableSet
        extends AbstractSerializableSetDecorator
        implements Unmodifiable {

    /** Serialization version */
    private static final long serialVersionUID = 6499119872185240161L;

    /**
     * Factory method to create an unmodifiable set.
     *
     * @param  the element type
     * @param set  the set to decorate, must not be null
     * @return a new unmodifiable set
     * @throws NullPointerException if set is null
     * @since 4.0
     */
    public static  Set unmodifiableSet(final Set set) {
        if (set instanceof Unmodifiable) {
            @SuppressWarnings("unchecked") // safe to upcast
            final Set tmpSet = (Set) set;
            return tmpSet;
        }
        return new UnmodifiableSet<>(set);
    }

    //-----------------------------------------------------------------------
    /**
     * Constructor that wraps (not copies).
     *
     * @param set  the set to decorate, must not be null
     * @throws NullPointerException if set is null
     */
    @SuppressWarnings("unchecked") // safe to upcast
    private UnmodifiableSet(final Set set) {
        super((Set) set);
    }

    //-----------------------------------------------------------------------
    @Override
    public Iterator iterator() {
        return UnmodifiableIterator.unmodifiableIterator(decorated().iterator());
    }

    @Override
    public boolean add(final E object) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean addAll(final Collection coll) {
        throw new UnsupportedOperationException();
    }

    @Override
    public void clear() {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean remove(final Object object) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean removeAll(final Collection coll) {
        throw new UnsupportedOperationException();
    }

    @Override
    public boolean retainAll(final Collection coll) {
        throw new UnsupportedOperationException();
    }

}