How to fetch first 10 key value pairs in HashMap

java collections hashmap

14,271

Solution 1

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.

Solution 2

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.

Solution 3

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());
}

14,271

Author by

Vijay Selvaraj

Updated on June 17, 2022

Comments

Vijay Selvaraj almost 2 years
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
default locale about 12 years

OP stated that he knows how to do this with counter variable
Churk about 12 years

Just looked at both Hashmap and Set, there is no subList method, care to tell us where that came from?
NPE about 12 years

@Churk: It came from the ArrayList that got constructed with the contents of map.entrySet().
Churk about 12 years

@aix map.entrySet() returns a set, not an arraylist
NPE about 12 years

@Churk: Right. And the new ArrayList<...>(...) creates the ArrayList on which subList() is then called.
aioobe about 12 years

@Churk, I go via the ArrayList(Collection<? extends E> c) constructor. Clarified code in the answer slightly.
Vijay Selvaraj about 12 years

I tried it out but the compiler complains "The constructor ArrayList<String>(Set<Map.Entry<String,String>>) is undefined"
aioobe about 12 years

Sorry. ... should be Map.Entry<String,String>. I'll update the answer to be more complete.
Churk about 12 years

@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.