001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *     http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.configuration2.interpol;
018
019/**
020 * <p>
021 * A simple dummy {@code Lookup} implementation.
022 * </p>
023 * <p>
024 * This implementation always returns <b>null</b> for a passed in variable
025 * indicating that it cannot resolve that variable. This is useful for instance
026 * as an application of the <em>null object</em> pattern.
027 * </p>
028 * <p>
029 * This class does not define any state, therefore a single instance can be
030 * shared. To enforce usage of only a single instance this class is actually an
031 * enumeration.
032 * </p>
033 *
034 * @since 2.0
035 */
036public enum DummyLookup implements Lookup
037{
038    /** The single instance of this class. */
039    INSTANCE;
040
041    /**
042     * {@inheritDoc} This implementation always returns <b>null</b>.
043     */
044    @Override
045    public Object lookup(final String variable)
046    {
047        return null;
048    }
049}