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copied!<p>Here are the characteristic differences of some important <a href="http://java.sun.com/javase/6/docs/api/java/util/Map.html" rel="noreferrer"><code>Map</code></a> implementations:</p> <ul> <li><a href="http://java.sun.com/javase/6/docs/api/java/util/LinkedHashMap.html" rel="noreferrer"><code>LinkedHashMap</code></a>: "with predictable iteration order [...] which is normally the order in which keys were inserted into the map (<em>insertion-order</em>)."</li> <li><a href="http://java.sun.com/javase/6/docs/api/java/util/HashMap.html" rel="noreferrer"><code>HashMap</code></a>: "makes no guarantees as to the order of the map"</li> <li><a href="http://java.sun.com/javase/6/docs/api/java/util/TreeMap.html" rel="noreferrer"><code>TreeMap</code></a>: "is sorted according to the natural ordering of its keys, or by a <code>Comparator</code>" <ul> <li>i.e. it's a <a href="http://java.sun.com/javase/6/docs/api/java/util/SortedMap.html" rel="noreferrer"><code>SortedMap</code></a></li> </ul></li> </ul> <p>So it looks like <code>LinkedHashMap</code> is what you need in this case.</p> <p>Here's a snippet to illustrate the differences; it also shows a common way to iterate over all entries of a <code>Map</code>, and how using an interface to refer to objects allow great flexibility of choice of implementation.</p> <pre><code>import java.util.*; public class MapExample { public static void main(String[] args) { populateThenDump(new HashMap<String,Integer>()); populateThenDump(new TreeMap<String,Integer>()); populateThenDump(new LinkedHashMap<String,Integer>()); } static void populateThenDump(Map<String,Integer> map) { System.out.println(map.getClass().getName()); map.put("Zero", 0); map.put("One", 1); map.put("Two", 2); map.put("Three", 3); map.put("Four", 4); for (Map.Entry<String,Integer> entry : map.entrySet()) { System.out.println(entry.getKey() + " => " + entry.getValue()); } } } </code></pre> <p>The output of the above snippet is (<a href="http://ideone.com/ADouC" rel="noreferrer">as seen on ideone.com</a>):</p> <pre><code>java.util.HashMap // unordered, results may vary Three => 3 Zero => 0 One => 1 Four => 4 Two => 2 java.util.TreeMap // ordered by String keys lexicographically Four => 4 One => 1 Three => 3 Two => 2 Zero => 0 java.util.LinkedHashMap // insertion order Zero => 0 One => 1 Two => 2 Three => 3 Four => 4 </code></pre> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/46898/iterate-over-map">Iterate Over Map</a></li> <li><a href="https://stackoverflow.com/questions/1884889/iterating-over-and-removing-from-a-map">iterating over and removing from a map</a> <ul> <li>If you want to modify the map while iterating, you'd need to use its <a href="http://java.sun.com/javase/6/docs/api/java/util/Iterator.html" rel="noreferrer"><code>Iterator</code></a>.</li> </ul></li> </ul> <h3>Similar questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/1419708/how-to-keep-the-order-of-elements-in-hashtable">How to keep the order of elements in hashtable</a></li> <li><a href="https://stackoverflow.com/questions/1190083/does-entryset-in-a-linkedhashmap-also-guarantee-order">Does entrySet() in a LinkedHashMap also guarantee order?</a></li> <li><a href="https://stackoverflow.com/questions/683518/java-class-that-implements-map-and-keeps-insertion-order">Java Class that implements Map and keeps insertion order?</a></li> <li><a href="https://stackoverflow.com/questions/1212198/ordered-list-map-implementation-in-java">Ordered List Map implementation in Java</a></li> </ul>

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