Update doc string.

Author: Mike Davis

core-api/src/main/java/com/optimizely/ab/config/audience/match/MatchRegistry.java

@@ -1,40 +1,66 @@
+/**
+ *
+ *    Copyright 2020, Optimizely and contributors
+ *
+ *    Licensed 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 com.optimizely.ab.config.audience.match;
 
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 
+/**
+ * MatchRegistry maps a string match "type" to a match implementation.
+ * All supported Match implementations must be registed with this registry.
+ * Third-party {@link Match} implementations may also be registered to provide
+ * additional functionality.
+ */
 public class MatchRegistry {
 
     private static final Map<String, Match> registry = new ConcurrentHashMap<>();
-    private static final String EXISTS = "exists";
-    private static final String EXACT = "exact";
-    private static final String GREATER_THAN = "gt";
-    private static final String LESS_THAN = "lt";
-    private static final String SUBSTRING = "substring";
-    private static final Match LEGACY = new DefaultMatchForLegacyAttributes();
+    public static final String EXISTS = "exists";
+    public static final String EXACT = "exact";
+    public static final String GREATER_THAN = "gt";
+    public static final String LESS_THAN = "lt";
+    public static final String SUBSTRING = "substring";
+    public static final String LEGACY = "legacy";
 
     static {
-        registry.put(EXISTS, new ExistsMatch());
-        registry.put(EXACT, new ExactMatch());
-        registry.put(GREATER_THAN, new GTMatch());
-        registry.put(LESS_THAN, new LTMatch());
-        registry.put(SUBSTRING, new SubstringMatch());
+        register(EXISTS, new ExistsMatch());
+        register(EXACT, new ExactMatch());
+        register(GREATER_THAN, new GTMatch());
+        register(LESS_THAN, new LTMatch());
+        register(SUBSTRING, new SubstringMatch());
+        register(LEGACY, new DefaultMatchForLegacyAttributes());
     }
 
     // TODO rename Match to Matcher
     public static Match getMatch(String name) throws UnknownMatchTypeException {
-        if (name == null) {
-            return LEGACY;
-        }
-
-        Match match = registry.get(name);
+        Match match = registry.get(name == null ? LEGACY : name);
         if (match == null) {
             throw new UnknownMatchTypeException();
         }
 
         return match;
     }
 
+    /**
+     * register registers a Match implementation with it's name.
+     * NOTE: This does not check for existence so default implementations can
+     * be overridden.
+     * @param name
+     * @param match
+     */
     public static void register(String name, Match match) {
         registry.put(name, match);
     }