How to fetch first 10 key value pairs in HashMap

Question

I am trying to iterate only the first "n" values in my Map, is there any method available or i need to control it only with a count variable.

Below is an example, i have sorted a group of names belong to the same city. Now i only want the first 10 city and the person names in it.

for (Map.Entry<String, List<String>> entry : map.entrySet()) {
    List<String> list = entry.getValue();
    // Display list of people in City
}

Is there a Map implementation that can hold fixed number of key,value pairs? Please get some directions.

Thanks,

-Vijay Selvaraj

Do you want to prevent adding more than 10 entries, or do you want any 10 entries, or do you want the 10 first entries added? More details. — Christoffer Hammarström, Mar 29 '12 at 10:35
My target is to have 10 entries. Now i am making it full and then taking a subset of the whole. But i can very well stop when i reach 10 entries. — Vijay Selvaraj, Mar 29 '12 at 11:12

score 9 · Accepted Answer · edited May 23 '17 at 10:29

How to fetch first 10 key value pairs in HashMap

HashMap is unordered. This makes the question ill-posed (unless by "first" you mean "arbitrary").

If you want a consistent ordering of keys, you need to change the type of your map to a SortedMap, such as TreeMap.

Alternatively, if it's the oldest elements you're after (i.e. the ones you've inserted first), then LinkedHashMap is the answer.

As to actually getting the first n elements, a loop with a counter is a pretty reasonable way to do it.

aioobe · Answer 2 · 2012-03-29T11:13:21.747

3

I am trying to iterate only the first "n" values in my Map, is there any method available or i need to control it only with a count variable.

The closest thing you'll find using only the standard Collections API (which still is slightly worse than a counter variable IMO) is the following:

List<Map.Entry<String, List<String>> entryList =
        new ArrayList<Map.Entry<String, List<String>>(map.entrySet());

for (Map.Entry<String, List<String>> entry : entryList.subList(0, 10)) {
    List<String> list = entry.getValue();
    // Display list of people in City
}

The lengthy type parameters could be avoided either by using the fancy diamond from Java 7:

List<Map.Entry<String, List<String>> entryList = new ArrayList<>(map.entrySet());

or by using iterating over the keys and .get the corresponding values.

edited Mar 29 '12 at 11:13

answered Mar 29 '12 at 10:35

aioobe

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Just looked at both Hashmap and Set, there is no subList method, care to tell us where that came from? – Churk Mar 29 '12 at 10:47
1

@Churk: It came from the `ArrayList` that got constructed with the contents of `map.entrySet()`. – NPE Mar 29 '12 at 10:48
@aix map.entrySet() returns a set, not an arraylist – Churk Mar 29 '12 at 10:51
@Churk: Right. And the `new ArrayList<...>(...)` creates the `ArrayList` on which `subList()` is then called. – NPE Mar 29 '12 at 10:57
@Churk, I go via the [`ArrayList(Collection extends E> c)`](http://docs.oracle.com/javase/6/docs/api/java/util/ArrayList.html#ArrayList%28java.util.Collection%29) constructor. Clarified code in the answer slightly. – aioobe Mar 29 '12 at 11:00
I tried it out but the compiler complains "The constructor ArrayList(Set>) is undefined" – Vijay Selvaraj Mar 29 '12 at 11:09
Sorry. `...` should be `Map.Entry`. I'll update the answer to be more complete. – aioobe Mar 29 '12 at 11:12
@aioobe so there is an implied casting from a set to arraylist. I been away from the java for a few years, and just haven't seen it done this way before. – Churk Mar 29 '12 at 11:32

Churk · Answer 3 · 2012-03-29T10:40:43.860

2

List<List<string>> list = new ArrayList<List<String>>();
for (Map.Entry<String, List<String>> entry : map.entrySet()) {
  if (list.size() > 9) break;
    list.add(entry.getValue());
}

edited Mar 29 '12 at 10:40

answered Mar 29 '12 at 10:35

Churk

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OP stated that he knows how to do this with counter variable – default locale Mar 29 '12 at 10:37

How to fetch first 10 key value pairs in HashMap

3 Answers3