CODEWITHSUNDEEP
javadictionarytreemap
Anil
Anil

Reputation: 2455

Retrieving last n values from treemap

I merely want to retrieve the last n values from a tree map without considering any keys or values from the map. But i really don't have any clue on how to do this. Below is a sample piece of code which can pull out first 5 records from the map. How can i modify it to retrieve last n records from the map?

Map startValues = new ArrayList<Map.Entry<String, Integer>>(5);
iterator = sortedMap.entrySet().iterator();
for (int i1 = 0; iterator.hasNext() && i1 < 5; i1++) {
    startValues.add(iterator.next());
}

Upvotes: 3

Views: 2557

Answers (3)

P3arl
P3arl

Reputation: 391

Get map.keySet() and convert it into ArrayList, then you can get last n values using loop as in example below:

Map<Integer, String> map = new HashMap<>();

map.put(1, "Praveen");
map.put(2, "shristi");
map.put(3, "tarang");
Set<Integer> keys = map.keySet();
ArrayList<Integer> list = new ArrayList<>(keys);
for(int i=1;i<list.size();i++) {
  System.out.println(map.get(list.get(i)));
}

Upvotes: 0

Saul
Saul

Reputation: 18041

If sortedMap variable contains an instance of a TreeMap then use descendingKeySet(). It returns a reverse order of the keys contained in there.

Map startValues = new ArrayList<Map.Entry<String, Integer>>(5);
iterator = sortedMap.descendingKeySet().iterator();
for (int i1 = 0; iterator.hasNext() && i1 < 5; i1++) {
   startValues.add(sortedMap.get(iterator.next()));
}

Note that in contrast to original code this iterates over keys, not entries. I have not test-compiled it so there might be minor issues with generics and type casting but hopefully the main idea is clear enough.

Upvotes: 4

mishadoff
mishadoff

Reputation: 10789

Ordering in TreeMap maintained by Comparator object at the creation time. If Comparator not provided - it is natural ordering.

The most straightforward solution to create Comparator that implements reverse sorting, pass this comparator to new TreeMap, and add all elements from old treemap to new one. That way you can reuse you code sample for retrieving last 5 records.

Upvotes: 1

Related Questions