How to get a limited number of values from a HashMap or LinkedHashMap?

Question

假设我有一个包含216个条目的LinkedHashMap ，我如何从LinkedHashMap<Integer, Object>获取前100个值（此处为Object类型）。

Answer 1

Well to start with, doing this for HashMap as per your title, doesn't make much sense - HashMap has no particular order, and the order may change between calls. It makes more sense for LinkedHashMap though.

There, I'd use Guava 's Iterables.limit method:

Iterable<Object> first100Values = Iterables.limit(map.values(), 100);

or

// Or whatever type you're interested in...
Iterable<Map.Entry<Integer, Object>> firstEntries =
    Iterables.limit(map.entrySet(), 100);

You can then create a list from that, or iterate over it, or whatever you want to do.

Answer 2

You can do:

Map<Integer, Object> records;
List<Entry<Integer, Object>> firstHundredRecords
    = new ArrayList<Entry<Integer, Object>>(records.entrySet()).subList(0, 100);

Although note that this will copy all the entries from the map.

Answer 3

To copy only the records you need with using a library.

Map<Integer, Object> records;

List<Entry<Integer, Object>> firstHundredRecords = new ArrayList<>();
for(Entry<Integer, Object> entry : records.entrySet()) {
    firstHundredRecords.add(entry);
    if (firstHundredRecords.size()>=100) break;
}

Answer 4

Ugly One-Liner

This ugly one-liner would do (and return a ArrayList<Object> in the question's case):

Collections.list(Collections.enumeration(lhMap.values())).subList(0, 100)

This would work for a HashMap as well, however HashMap being backed by a HashSet there's not guarantee that you will get the first 100 values that were entered; it would work on other types, with similar limitations.

Notes:

• relatively unefficient (read the Javadoc to know why - though there's worse!),
• careful when using views (read the Javadoc to know more),
• I did mention it was ugly.

Step-By-Step Usage Example

(as per the OP's comment)

Map<Integer, Pair<Double, SelectedRoad>> hashmap3 =
  new LinkedHashMap<Integer, Pair<Double, SelectedRoad>>();

// [...] add 216 elements to hasmap3 here somehow

ArrayList<Pair<Double,SelectedRoad>> firstPairs = 
  Collections.list(Collections.enumeration(hashmap3.values())).subList(0, 100)

// you can then view your Pairs' SelectedRow values with them with:
//  (assuming that:
//    - your Pair class comes from Apache Commons Lang 3.0
//    - your SelectedRoad class implements a decent toString() )
for (final Pair<Double, SelectedRoad> p : firstPairs) {
    System.out.println("double: " + p.left);
    System.out.println("road  : " + p.right);
}

Answer 5

You can use counter. Your foreach loop will exit when your counter reached 100.

Answer 6

Write a loop which uses a Iterator.next() 100 times, and then stops.

I was going to say something about NavigableMap and SortedMap - but their interfaces are defined in terms of keys, not indexes. But they may be useful nevertheless, depending on what your actual underlying problem is.