CODEWITHSUNDEEP
javahashmap
Toran Billups
Toran Billups

Reputation: 27399

How to avoid items being re-ordered when put into java HashMap

I'm creating a new Map and pushing strings into it (no big deal) -but I've noticed that the strings are being re-ordered as the map grows. Is it possible to stop this re-ordering that occurs so the items in the map retain the order the were put in with?

Map<String,String> x = new HashMap<String, String>();
x.put("a","b");
x.put("a","c");
x.put("a","d");

x.put("1","2");
x.put("1","3");
x.put("1","4");

//this shows them out of order sadly...
for (Map.Entry<String, String> entry : x.entrySet()) {
    System.out.println("IN THIS ORDER ... " + entry.getValue());
}

Upvotes: 10

Views: 14935

Answers (4)

bittersweetryan
bittersweetryan

Reputation: 3443

A HashMap in java is not sorted http://download.oracle.com/javase/1,5.0/docs/api/java/util/HashMap.html. If you want predictable iteration order use a LinkedHashMap instead: http://download.oracle.com/javase/1.4.2/docs/api/java/util/LinkedHashMap.html

Heres a good discussion on the difference: How is the implementation of LinkedHashMap different from HashMap?

Upvotes: 9

John B
John B

Reputation: 32949

The previous answers are correct in that you should use an implementation of Map that maintains ordering. LinkedHashMap and SortedMap each do these things.

However, the takeaway point is that not all collections maintain order and if order is important to you, you should choose the appropriate implementation. Generic HashMaps do not maintain order, do not claim to do so and cannot be set to do so.

Upvotes: 4

ssedano
ssedano

Reputation: 8432

You can keep it with LinkedHashMap.

Upvotes: 17

dlev
dlev

Reputation: 48596

If you care about order, you can use a SortedMap. The actual class which implements the interface (at least for most scenarios) is a TreeMap. Alternatively, LinkedHashMap also maintains its order, while still utilizing a hashtable-based container.

Upvotes: 24

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