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Apache Commons Text is a library focused on algorithms working on strings.

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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.text.lookup;

/**
 * Lookups a String key for a String value.
 * 
 * This class represents the simplest form of a string to string map. It has a benefit over a map in that it can create
 * the result on demand based on the key.
 * 
 * 
 * For example, it would be possible to implement a lookup that used the key as a primary key, and looked up the value
 * on demand from the database.
 * 
 *
 * @since 1.3
 */
@FunctionalInterface
public interface StringLookup {

    /**
     * Looks up a String key to provide a String value.
     * 
     * The internal implementation may use any mechanism to return the value. The simplest implementation is to use a
     * Map. However, virtually any implementation is possible.
     * 
     * 
     * For example, it would be possible to implement a lookup that used the key as a primary key, and looked up the
     * value on demand from the database Or, a numeric based implementation could be created that treats the key as an
     * integer, increments the value and return the result as a string - converting 1 to 2, 15 to 16 etc.
     * 
     * 
     * This method always returns a String, regardless of the underlying data, by converting it as necessary. For
     * example:
     * 
     *
     *      * Map<String, Object> map = new HashMap<String, Object>();
     * map.put("number", new Integer(2));
     * assertEquals("2", StringLookupFactory.mapStringLookup(map).lookup("number"));
     * 
     *
     * @param key the key to look up, may be null.
     * @return The matching value, null if no match.
     */
    String lookup(String key);
}