Iterating a treemap for fixed number of times

Question

If I have a treemap that contains string and double. Is there anyway to retrieve say first 10 keys from map?

Answer 1

Fetch the iterator via treeMap.entrySet().iterator() if you want the key/value pairs or treeMap.keySet().iterator() if you just care about the keys, then call iterator.next() 10 times or as long as iterator.hasNext() returns true.

List<Map.Entry<String, Double>> firstTen = new ArrayList<Map.Entry<String, Double>>(10);
Iterator<String, Double> iterator = treeMap.entrySet().iterator();
for (int i = 0; iterator.hasNext() && i < 10; i++) {
    firstTen.add(iterator.next());
}

Answer 2

Not in the JDK but there are methods in 3rd-party libraries that do this.

For example Iterables.limit from guava

Answer 3

Since a TreeMap is a NavigableMap, you should take a look at the methods in that interface. There's nothing that lets you select the first n, but you can select all entries with a key less than a given key, etc.

Otherwise, I would just use an Iterator and stop after n iterations.

Answer 4

Wrap the entrySet() in a List and create a subList():

SortedMap<String,Double> treeMap = new TreeMap<String, Double>();
List<Entry<String, Double>> firstTen = 
    new ArrayList<Map.Entry<String,Double>>(treeMap.entrySet())
    .subList(0, 10);
for(Entry<String, Double> entry : firstTen){
    // do something with entries
}

Drawback: the entire map will be copied to the list.

Answer 5

It worked with the code snippets provided by dkarp and sean. However, it worked with the suggestion of DJClayworth as well. Here is how I solved it:

    c=sorted_map.keySet();

    Iterator itr=c.iterator();
    String s="";
    while (i<iteratesize&&itr.hasNext()){
        s=(String) itr.next();
        System.out.println(s);
        s="";
        i++;
    }

Thank you very much everyone for your help. I really appreciate it.