How can I iterate starting at a particular key in a LinkedHashMap?

Question

If I have a data structure

Stock
{
  String Symbol;
  LinkedHashMap<Date,Double> DateForPrice;  
}

I know in the LinkedHashMap, I can get the stock price of specific date without traversing the whole list.

However, if I want to iterate through the LinkedHashMap of DateForPrice starting from a specific date, are there any way to do it without traversing the whole list?

Answer 1

LinkedHashMap doesn't offer a way to start iterating in the middle of its ordered view of the map's data. Supposing your use case is really that you want all dates after some Date d and to iterate those, then you should probably store your map as a TreeMap . An important distinction here is that LinkedHashMap 's ordering is the insertion-order , and our supposed use-case here is that you want the natural key-order . TreeMap s maintain such a view, sorting the contents of the map by the map's key.

TreeMap s have the additional benefit of allowing you to create slices of the map based on the key, so you can call tailMap(K k) , to return the map with all keys occurring after k . In this case, you can call tailMap with your starting point, d .

eg:

TreeMap<Date, Double> dateForPrice;

// load up dateForPrice

Date start = // the point to start your iteration

for(Entry<Date, Double> entry : dateForPrice.tailMap(start).entrySet()){
    // loop code
}

tailMap method returns SortedMap , which is not iterable. But it has entrySet method returning Set , which is subinterface of Iterable .

Conveniently, if you want to keep storing your data in a LinkedHashMap you can simply load up a TreeMap with your current instance (with some performance tradeoff, of course):

TreeMap<Date, Double> dateSortedDateForPrice = new TreeMap<Date, Double>(dateForPrice);

Answer 2

I'd suggest to use TreeMap instead - it will be sorted by date and you can use tailMap to get the required portion