svn commit: r154265 - jakarta/commons/proper/digester/branches/digester2/src/java/org/apache/commons/digester2/MapKey.java

[sk...@apache.org]

Author: skitching
Date: Fri Feb 18 04:49:36 2005
New Revision: 154265

URL: http://svn.apache.org/viewcvs?view=rev&rev=154265
Log:
Create class to be base implementation for all "unique key to map entry" classes.

Added:
    jakarta/commons/proper/digester/branches/digester2/src/java/org/apache/commons/digester2/MapKey.java   (with props)

Added: jakarta/commons/proper/digester/branches/digester2/src/java/org/apache/commons/digester2/MapKey.java
URL: http://svn.apache.org/viewcvs/jakarta/commons/proper/digester/branches/digester2/src/java/org/apache/commons/digester2/MapKey.java?view=auto&rev=154265
==============================================================================
--- jakarta/commons/proper/digester/branches/digester2/src/java/org/apache/commons/digester2/MapKey.java (added)
+++ jakarta/commons/proper/digester/branches/digester2/src/java/org/apache/commons/digester2/MapKey.java Fri Feb 18 04:49:36 2005
@@ -0,0 +1,118 @@
+/* $Id$
+ *
+ * Copyright 2005 The Apache Software Foundation.
+ *
+ * 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 org.apache.commons.digester2;
+
+import java.util.List;
+import java.util.Map;
+import java.util.HashMap;
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.EmptyStackException;
+import java.lang.reflect.InvocationTargetException;
+
+import org.xml.sax.Locator;
+import org.xml.sax.ContentHandler;
+import org.xml.sax.SAXException;
+import org.xml.sax.SAXParseException;
+
+import org.apache.commons.logging.Log;
+
+/**
+ * Simple class whose instances can be used as keys into a map.
+ * <p>
+ * Classes such as Context provide generic data storage facilities which can
+ * be used by other instances to store data for later retrieval. This storage
+ * is in the form of a map, which requires that the objects that write and
+ * read objects within the map have some convention to avoid stomping on each
+ * other's data. This class provides a mechanism for creating unique keys 
+ * for such a purpose.
+ * <p>
+ * The idea is that some object declares an instance of this class as a
+ * member. It then uses that member as a key into a map held elsewhere.
+ * There can never be any conflicts with other users of that map, because
+ * the object is unique. An alternative would be to use strings as
+ * keys into the shared map, but that would involve inventing a convention
+ * for creating unique string values, which this approach doesn't need.
+ * <p>
+ * The important technical point that makes this object usable as a key
+ * into a map is that it does <i>not</i> override the default Object.equals
+ * method, ie two instances are equal only if they are the same object.
+ * <p>
+ * Example usage:
+ * <br>
+ * If an instance of class Foo wants to be able to store data in a shared map
+ * without conflicting with any other class, or even with other instances of
+ * the Foo class, then it should declare a private non-static id instance:
+ * <pre>
+ *    private final MapKey WIDGET_ID
+ *      = new MapKey(Foo.class, "Widget", this);
+ *    ....
+ *    someCommonMap.put(WIDGET_ID, theWidget);
+ *    ....
+ *    Widget savedWidget = (Widget) someCommonMap.get(WIDGET_ID);
+ * </pre>
+ * <p>
+ * If an class Bar wishes to share a particular map entry across all instances
+ * of itself, then it should declare a static id:
+ * <pre>
+ *    private static final MapKey GADGET_ID
+ *      = new MapKey(Bar.class, "Gadget");
+ * </pre>
+ * <p>
+ * If a class wishes to share an item in a shared map with objects that are
+ * not of the same class, then it can follow the above example but declare
+ * access to be protected or public. Other classes can then access it via:
+ * <pre>
+ *   someCommonMap.get(Bar.WIDGET_ID);
+ * </pre>
+ */
+
+public class MapKey {
+
+    /**
+     * A string used only for generating debug output.
+     */
+    private String desc;
+
+    /**
+     * Create an instance which has no specific owner object.
+     */
+    public MapKey(Class sourceClass, String desc) {
+        this.desc = sourceClass.getName() + ":" + desc;
+    }
+
+    /**
+     * Create an instance which has an owner object. This is equivalent
+     * to the constructor without an owner object, except that the 
+     * diagnostic output (toString) doesn't show the identity of the
+     * object associated with this key.
+     */
+    public MapKey(Class sourceClass, String desc, Object owner) {
+        this.desc = sourceClass.getName() + ":" + desc
+           + ":" + System.identityHashCode(owner);
+    }
+
+    /**
+     * Provides a nice string which shows what class declares this StackId,
+     * what it is intended to be used for ("desc") and what specific
+     * owner (if any) the stack is associated with.
+     */
+    public String toString() {
+        return desc;
+    }
+}

Propchange: jakarta/commons/proper/digester/branches/digester2/src/java/org/apache/commons/digester2/MapKey.java
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