Question

HashMap selections = new HashMap<Integer, Float>();

How can i get the Integer key of the 3rd smaller value of Float in all HashMap?

Edit
im using the HashMap for this

for (InflatedRunner runner : prices.getRunners()) {
       for (InflatedMarketPrices.InflatedPrice price : runner.getLayPrices()) {
           if (price.getDepth() == 1) {
             selections.put(new Integer(runner.getSelectionId()), new Float(price.getPrice()));
           }
         }                    

}

i need the runner of the 3rd smaller price with depth 1

maybe i should implement this in another way?

Answer 1

Michael Mrozek nails it with his question if you’re using HashMap right: this is highly atypical scenario for HashMap. That said, you can do something like this:

• get the Set<Map.Entry<K,V>> from the HashMap<K,V>.entrySet().
• addAll to List<Map.Entry<K,V>>
• Collections.sort the list with a custom Comparator<Map.Entry<K,V>> that sorts based on V.
  • If you just need the 3rd Map.Entry<K,V> only, then a O(N) selection algorithm may suffice.

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//after edit

It looks like selection should really be a SortedMap<Float, InflatedRunner>. You should look at java.util.TreeMap.

Here’s an example of how TreeMap can be used to get the 3rd lowest key:

TreeMap<Integer,String> map = new TreeMap<Integer,String>();
map.put(33, "Three");
map.put(44, "Four");
map.put(11, "One");
map.put(22, "Two");

int thirdKey = map.higherKey(map.higherKey(map.firstKey()));
System.out.println(thirdKey); // prints "33"

Also note how I take advantage of Java’s auto-boxing/unboxing feature between int and Integer. I noticed that you used new Integer and new Float in your original code; this is unnecessary.

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//another edit

It should be noted that if you have multiple InflatedRunner with the same price, only one will be kept. If this is a problem, and you want to keep all runners, then you can do one of a few things:

• If you really need a multi-map (one key can map to multiple values), then you can:
  • have TreeMap<Float,Set<InflatedRunner>>
  • Use MultiMap from Google Collections
• If you don’t need the map functionality, then just have a List<RunnerPricePair> (sorry, I’m not familiar with the domain to name it appropriately), where RunnerPricePair implements Comparable<RunnerPricePair> that compares on prices. You can just add all the pairs to the list, then either:
  • Collections.sort the list and get the 3rd pair
  • Use O(N) selection algorithm