How to sort a Java Hashtable?

Question

I inserted some data into a Java Hashtable. If I read the data from the Hashtable it doesn't come back in the same order that I inserted it in. How do I get the ordered data from the Hashtable?

I use the following code to get the values from the hashtable:

// Get a set of the entries
Set set = hsUpdateValues.entrySet();
// Get an iterator
Iterator i = set.iterator();
// Display elements
while (i.hasNext()) {
    Map.Entry me = (Map.Entry) i.next();            
    System.out.print(
        "Key : " + me.getKey()
        + ", Value: " + me.getValue()
    );
}

Don't use Vector, Enumeration or Hashtable. They are ancient and should be avoided. Info: http://stackoverflow.com/questions/453684/use-hashtable-vector-or-hashmap-or-arraylist-in-java http://stackoverflow.com/questions/2601602/what-does-it-mean-when-we-say-hashtable-or-vector-is-synchronized http://stackoverflow.com/questions/2873254/difference-between-a-deprecated-and-legacy-api — Sean Patrick Floyd, Dec 02 '10 at 10:35

score 23 · Accepted Answer · edited Jun 20 '20 at 09:12

If you want an order-preserving map, you should use LinkedHashMap:

Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map. (A key k is reinserted into a map m if m.put(k, v) is invoked when m.containsKey(k) would return true immediately prior to the invocation.)

This implementation spares its clients from the unspecified, generally chaotic ordering provided by HashMap (and Hashtable), without incurring the increased cost associated with TreeMap.

Note that this is usually compared with HashMap rather than Hashtable - I don't know of an order-preserving equivalent to Hashtable; the latter isn't usually used these days anyway (just as ArrayList is usually used in preference to Vector).

I've assumed you want insertion order rather than key-sorted order. If you want the latter, use TreeMap.

@Tester: you'd likely want to "accept" an answer, if it's okay for you :) — GreyCat, Dec 02 '10 at 09:20

score 8 · Answer 2 · edited Dec 23 '14 at 18:49

Although a Hashtable cannot be sorted, he asked how to get sorted data, that can be done sorting the list of keys extracted from the HashTable and retrieving values in that order. Something like:

List<'your_type'> tmp = Collections.list('your_hashtable'.keys());
Collections.sort(tmp);
Iterator<'your_type'> it = tmp.iterator();

while(it.hasNext()){
    'your_type' element =it.next();
    //here you can get ordered things: 'your_hashtable'.get(element);
}

will be fine.

score 7 · Answer 3 · edited Dec 23 '14 at 18:50

7

A Hashtable has no predictable iteration order, and cannot be sorted. If you only want predictable iteration order you should use a LinkedHashMap. If you want to be able to sort your Map, you should use a TreeMap.

edited Dec 23 '14 at 18:50

msrd0

7,816
9
47
82

answered Dec 02 '10 at 09:10

Björn

29,019
9
65
81

Peter Lawrey · Answer 4 · 2015-03-19T06:11:44.227

7

Hashtable is a legacy collection which was replaced by Java 1.2 collections in 1998. I suggest you avoid it, along with Vector and Enumeration.

Instead of Hashtable use HashMap where possible. You can add synchronization using Collections.synchronizedMap(map) if you need it.

Instead of Vector, use ArrayList where possible. You can add synchronization using Collections.synchronizedList(map) if you need it.

Instead of Enumeration you can use Iterator or even a for-each loop.

edited Mar 19 '15 at 06:11

answered Dec 02 '10 at 09:13

Peter Lawrey

525,659
79
751
1,130

1

What would you use instead? – Grumblesaurus Mar 19 '15 at 02:19
1

Example can be found at [link](https://www.javatpoint.com/difference-between-hashmap-and-treemap) – Thiam Hooi May 20 '21 at 06:03

score 3 · Answer 5 · answered Dec 02 '10 at 09:09

3

Use TreeMap for sorting it:

Map<String, String> yourMap = new HashMap<String, String>();
    yourMap.put("1", "one");
    yourMap.put("2", "two");
    yourMap.put("3", "three");

Map<String, String> sortedMap = new TreeMap<String, String>(yourMap);

answered Dec 02 '10 at 09:09

Mohamed Saligh

12,029
19
65
84

2

That won't give *insertion* order though. – Jon Skeet Dec 02 '10 at 09:09

score 0 · Answer 6 · answered Dec 02 '10 at 09:07

If i read the data from the hash table it's not coming in the same order what i inserted.

Your question does not make sense. A Hashtable does not have an "order", it is unordered (Edit: Some implementations do have an order, but it's not common for a hashtable)..

In what order would you expect the entries to be?

If you want to store elements in a certain order, you need to use a list (e.g. a subclass of java.util.List).

And BTW, your code sample does not even contain a hash table.

How to sort a Java Hashtable?

6 Answers6

Linked