Java tutorial
/* * 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.text; import java.util.Enumeration; import java.util.Map; import java.util.Properties; /** * Lookup a String key to a String value. * <p> * 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. * <p> * This class comes complete with various factory methods. * If these do not suffice, you can subclass and implement your own matcher. * <p> * 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 2.2 * @version $Id$ */ public abstract class StrLookup<V> { /** * Lookup that always returns null. */ private static final StrLookup<String> NONE_LOOKUP = new MapStrLookup<String>(null); //----------------------------------------------------------------------- /** * Returns a lookup which always returns null. * * @return a lookup that always returns null, not null */ public static StrLookup<?> noneLookup() { return NONE_LOOKUP; } /** * Creates a copy of the given properties instance. * * @param input the Properties instance to copy. * @return a copy of {@code input}. */ private static Properties copyProperties(Properties input) { if (input == null) { return null; } Properties output = new Properties(); @SuppressWarnings("unchecked") // Property names are Strings. Enumeration<String> propertyNames = (Enumeration<String>) input.propertyNames(); while (propertyNames.hasMoreElements()) { String propertyName = propertyNames.nextElement(); output.setProperty(propertyName, input.getProperty(propertyName)); } return output; } /** * Returns a new lookup which uses a copy of the current * {@link System#getProperties() System properties}. * <p> * If a security manager blocked access to system properties, then null will * be returned from every lookup. * <p> * If a null key is used, this lookup will throw a NullPointerException. * * @return a lookup using system properties, not null */ public static StrLookup<String> systemPropertiesLookup() { Properties systemProperties = null; try { systemProperties = System.getProperties(); } catch (final SecurityException ex) { // Squelched. All lookup(String) will return null. } Properties properties = copyProperties(systemProperties); @SuppressWarnings("unchecked") // System property keys and values are always Strings final Map<String, String> propertiesMap = (Map) properties; return new MapStrLookup<String>(propertiesMap); } /** * Returns a lookup which looks up values using a map. * <p> * If the map is null, then null will be returned from every lookup. * The map result object is converted to a string using toString(). * * @param <V> the type of the values supported by the lookup * @param map the map of keys to values, may be null * @return a lookup using the map, not null */ public static <V> StrLookup<V> mapLookup(final Map<String, V> map) { return new MapStrLookup<V>(map); } //----------------------------------------------------------------------- /** * Constructor. */ protected StrLookup() { super(); } /** * Looks up a String key to a String value. * <p> * 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. * <p> * 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. * <p> * The {@link #lookup(String)} method always returns a String, regardless of * the underlying data, by converting it as necessary. For example: * <pre> * Map<String, Object> map = new HashMap<String, Object>(); * map.put("number", Integer.valueOf(2)); * assertEquals("2", StrLookup.mapLookup(map).lookup("number")); * </pre> * @param key the key to be looked up, may be null * @return the matching value, null if no match */ public abstract String lookup(String key); //----------------------------------------------------------------------- /** * Lookup implementation that uses a Map. */ static class MapStrLookup<V> extends StrLookup<V> { /** Map keys are variable names and value. */ private final Map<String, V> map; /** * Creates a new instance backed by a Map. * * @param map the map of keys to values, may be null */ MapStrLookup(final Map<String, V> map) { this.map = map; } /** * Looks up a String key to a String value using the map. * <p> * If the map is null, then null is returned. * The map result object is converted to a string using toString(). * * @param key the key to be looked up, may be null * @return the matching value, null if no match */ @Override public String lookup(final String key) { if (map == null) { return null; } final Object obj = map.get(key); if (obj == null) { return null; } return obj.toString(); } } }