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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *      http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.apache.commons.lang3.text;
18  
19  import java.util.Map;
20  
21  /**
22   * Lookup a String key to a String value.
23   * <p>
24   * This class represents the simplest form of a string to string map.
25   * It has a benefit over a map in that it can create the result on
26   * demand based on the key.
27   * <p>
28   * This class comes complete with various factory methods.
29   * If these do not suffice, you can subclass and implement your own matcher.
30   * <p>
31   * For example, it would be possible to implement a lookup that used the
32   * key as a primary key, and looked up the value on demand from the database
33   *
34   * @since 2.2
35   * @version $Id: StrLookup.java 1583482 2014-03-31 22:54:57Z niallp $
36   */
37  public abstract class StrLookup<V> {
38  
39      /**
40       * Lookup that always returns null.
41       */
42      private static final StrLookup<String> NONE_LOOKUP;
43      /**
44       * Lookup that uses System properties.
45       */
46      private static final StrLookup<String> SYSTEM_PROPERTIES_LOOKUP;
47      static {
48          NONE_LOOKUP = new MapStrLookup<String>(null);
49          StrLookup<String> lookup = null;
50          try {
51              final Map<?, ?> propMap = System.getProperties();
52              @SuppressWarnings("unchecked") // System property keys and values are always Strings
53              final Map<String, String> properties = (Map<String, String>) propMap;
54              lookup = new MapStrLookup<String>(properties);
55          } catch (final SecurityException ex) {
56              lookup = NONE_LOOKUP;
57          }
58          SYSTEM_PROPERTIES_LOOKUP = lookup;
59      }
60  
61      //-----------------------------------------------------------------------
62      /**
63       * Returns a lookup which always returns null.
64       *
65       * @return a lookup that always returns null, not null
66       */
67      public static StrLookup<?> noneLookup() {
68          return NONE_LOOKUP;
69      }
70  
71      /**
72       * Returns a lookup which uses {@link System#getProperties() System properties}
73       * to lookup the key to value.
74       * <p>
75       * If a security manager blocked access to system properties, then null will
76       * be returned from every lookup.
77       * <p>
78       * If a null key is used, this lookup will throw a NullPointerException.
79       *
80       * @return a lookup using system properties, not null
81       */
82      public static StrLookup<String> systemPropertiesLookup() {
83          return SYSTEM_PROPERTIES_LOOKUP;
84      }
85  
86      /**
87       * Returns a lookup which looks up values using a map.
88       * <p>
89       * If the map is null, then null will be returned from every lookup.
90       * The map result object is converted to a string using toString().
91       *
92       * @param <V> the type of the values supported by the lookup
93       * @param map  the map of keys to values, may be null
94       * @return a lookup using the map, not null
95       */
96      public static <V> StrLookup<V> mapLookup(final Map<String, V> map) {
97          return new MapStrLookup<V>(map);
98      }
99  
100     //-----------------------------------------------------------------------
101     /**
102      * Constructor.
103      */
104     protected StrLookup() {
105         super();
106     }
107 
108     /**
109      * Looks up a String key to a String value.
110      * <p>
111      * The internal implementation may use any mechanism to return the value.
112      * The simplest implementation is to use a Map. However, virtually any
113      * implementation is possible.
114      * <p>
115      * For example, it would be possible to implement a lookup that used the
116      * key as a primary key, and looked up the value on demand from the database
117      * Or, a numeric based implementation could be created that treats the key
118      * as an integer, increments the value and return the result as a string -
119      * converting 1 to 2, 15 to 16 etc.
120      * <p>
121      * The {@link #lookup(String)} method always returns a String, regardless of
122      * the underlying data, by converting it as necessary. For example:
123      * <pre>
124      * Map&lt;String, Object&gt; map = new HashMap&lt;String, Object&gt;();
125      * map.put("number", Integer.valueOf(2));
126      * assertEquals("2", StrLookup.mapLookup(map).lookup("number"));
127      * </pre>
128      * @param key  the key to be looked up, may be null
129      * @return the matching value, null if no match
130      */
131     public abstract String lookup(String key);
132 
133     //-----------------------------------------------------------------------
134     /**
135      * Lookup implementation that uses a Map.
136      */
137     static class MapStrLookup<V> extends StrLookup<V> {
138 
139         /** Map keys are variable names and value. */
140         private final Map<String, V> map;
141 
142         /**
143          * Creates a new instance backed by a Map.
144          *
145          * @param map  the map of keys to values, may be null
146          */
147         MapStrLookup(final Map<String, V> map) {
148             this.map = map;
149         }
150 
151         /**
152          * Looks up a String key to a String value using the map.
153          * <p>
154          * If the map is null, then null is returned.
155          * The map result object is converted to a string using toString().
156          *
157          * @param key  the key to be looked up, may be null
158          * @return the matching value, null if no match
159          */
160         @Override
161         public String lookup(final String key) {
162             if (map == null) {
163                 return null;
164             }
165             final Object obj = map.get(key);
166             if (obj == null) {
167                 return null;
168             }
169             return obj.toString();
170         }
171     }
172 }