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copied!<p>I doubt the differences can be explained significantly better than what's already written in the JavaDocs for these classes:</p> <ul> <li><a href="http://download.oracle.com/javase/6/docs/api/java/util/Map.html" rel="noreferrer">Map</a> is the basic interface common to all these classes</li> <li>a <a href="http://download.oracle.com/javase/6/docs/api/java/util/Hashtable.html" rel="noreferrer">Hashtable</a> is one implementation of that interface, for the "old" days when it was thought that having everything synchronized is a good idea (ref. <a href="http://download.oracle.com/javase/6/docs/api/java/util/Vector.html" rel="noreferrer">Vector</a>). It offers "kind of" thread-safety, if you know what you are doing. If you are serious about a map which can be used from multiple threads you should absolutely check out the <a href="http://download.oracle.com/javase/6/docs/api/java/util/concurrent/ConcurrentHashMap.html" rel="noreferrer"><code>ConcurrentHashMap</code></a> and <a href="http://download.oracle.com/javase/6/docs/api/java/util/concurrent/ConcurrentSkipListMap.html" rel="noreferrer"><code>ConcurrentSkipListMap</code></a>.</li> <li>a <a href="http://download.oracle.com/javase/6/docs/api/java/util/HashMap.html" rel="noreferrer">HashMap</a> is almost the same as a Hashtable, but with the synchronization removed. It's the preferred general-purpose Map implementation.</li> <li>a <a href="http://download.oracle.com/javase/6/docs/api/java/util/LinkedHashMap.html" rel="noreferrer">LinkedHashMap</a> additionally maintains a linked list of it's entries, which allows to maintain an ordering or use it as a LRU cache easily, just read the JavaDoc.</li> </ul> <p>All of the aforementioned <code>Map</code> implementations have their basic get/put operations in (amortized) <em>O(1)</em> time complexity. There are minor differences in the handling of <code>null</code> values, it's inevitable to check the JavaDoc for details.</p> <p>To get an idea of how these classes are implemeted, have a look at their inheritance tree:</p> <ul> <li><code>Map</code> (just the interface) <ul> <li><code>Dictionary</code> (obsoleted abstract class) <ul> <li><code>Hashtable</code> (the "old" map implementation lives on it's own)</li> </ul></li> <li><code>AbstractMap</code> (the basic functionality of the "new" map implementations) <ul> <li><code>HashMap</code> (the first concrete map implementation for general purpose use) <ul> <li><code>LinkedHashMap</code> (extends <code>HashMap</code> by mainaining the linked list)</li> </ul></li> </ul></li> </ul></li> </ul>

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