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Apache Commons Lang, a package of Java utility classes for the classes that are in java.lang's hierarchy, or are considered to be so standard as to justify existence in java.lang.

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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.lang3.tuple;

/**
 * An immutable pair consisting of two {@code Object} elements.
 *
 * Although the implementation is immutable, there is no restriction on the objects
 * that may be stored. If mutable objects are stored in the pair, then the pair
 * itself effectively becomes mutable. The class is also {@code final}, so a subclass
 * can not add undesirable behaviour.
 *
 * #ThreadSafe# if both paired objects are thread-safe
 *
 * @param  the left element type
 * @param  the right element type
 *
 * @since 3.0
 */
public final class ImmutablePair extends Pair {

    /**
     * An immutable pair of nulls.
     */
    // This is not defined with generics to avoid warnings in call sites.
    @SuppressWarnings("rawtypes")
    private static final ImmutablePair NULL = of(null, null);

    /** Serialization version */
    private static final long serialVersionUID = 4954918890077093841L;

    /**
     * Returns an immutable pair of nulls.
     *
     * @param  the left element of this pair. Value is {@code null}.
     * @param  the right element of this pair. Value is {@code null}.
     * @return an immutable pair of nulls.
     * @since 3.6
     */
    @SuppressWarnings("unchecked")
    public static  ImmutablePair nullPair() {
        return NULL;
    }

    /** Left object */
    public final L left;
    /** Right object */
    public final R right;

    /**
     * Obtains an immutable pair of two objects inferring the generic types.
     *
     * This factory allows the pair to be created using inference to
     * obtain the generic types.
     *
     * @param  the left element type
     * @param  the right element type
     * @param left  the left element, may be null
     * @param right  the right element, may be null
     * @return a pair formed from the two parameters, not null
     */
    public static  ImmutablePair of(final L left, final R right) {
        return new ImmutablePair<>(left, right);
    }

    /**
     * Create a new pair instance.
     *
     * @param left  the left value, may be null
     * @param right  the right value, may be null
     */
    public ImmutablePair(final L left, final R right) {
        super();
        this.left = left;
        this.right = right;
    }

    //-----------------------------------------------------------------------
    /**
     * {@inheritDoc}
     */
    @Override
    public L getLeft() {
        return left;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public R getRight() {
        return right;
    }

    /**
     * Throws {@code UnsupportedOperationException}.
     *
     * This pair is immutable, so this operation is not supported.
     *
     * @param value  the value to set
     * @return never
     * @throws UnsupportedOperationException as this operation is not supported
     */
    @Override
    public R setValue(final R value) {
        throw new UnsupportedOperationException();
    }

}