Which data structure would you use: TreeMap or HashMap? (Java) [duplicate]

Answer 1

Why not use TreeSet?

Same ordering concept as a TreeMap, except it's a Set - which, by definition, is "A collection that contains no duplicate elements".

From your problem description, it sounds as if you need a Set, I don't see what keys and values you are mapping together.

This class implements the Set interface, backed by a TreeMap instance. This class guarantees that the sorted set will be in ascending element order, sorted according to the natural order of the elements (see Comparable), or by the comparator provided at set creation time, depending on which constructor is used.

Answer 2

TreeMap beats HashMap because TreeMap is already sorted for you.

However, you might want to consider using a more appropriate data structure, a bag. See Commons Collections - and the TreeBag class:

This has a nice optimised internal structure and API:

bag.add("big")
bag.add("small")
bag.add("big")
int count = bag.getCount("big")

EDIT: The question of HashMap vs TreeMap performance was answered by Jon - HashMap and sort may be quicker (try it!), but TreeBag is easier. The same is true for bags. There is a HashBag as well as a TreeBag. Based on the implementation (uses a mutable integer) a bag should outperform the equivalent plain map of Integer. The only way to know for sure is to test, as with any performance question.

Answer 3

Depending on what the speed requirements are, you could also use a Trie. But there's no point in implementing one of those if a TreeMap is quick enough.

Answer 4

I would definitely choose a TreeMap:

• TreeMap automatically sorts new keys on insertion, no sorting afterwards is needed.
• When a key already exists it has the same performance as a HashMap.

A TreeSet internally uses a TreeMap so why not use TreeMap directly.

Answer 5

import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.ObjectInputStream.GetField;
import java.util.Iterator;
import java.util.Map;
import java.util.StringTokenizer;
import java.util.TreeMap;

public class TreeMapExample {

    public static void main (String args[]){
        Map<String,Integer> tm = new TreeMap<String,Integer>();
        try {

            FileInputStream fis = new FileInputStream("Test.txt");
            DataInputStream in = new DataInputStream(fis);
            BufferedReader br = new BufferedReader(new InputStreamReader(in));
            String line;
            int countValue = 1;
            while((line = br.readLine())!= null ){
                line = line.replaceAll("[-+.^:;,()\"\\[\\]]","");
                StringTokenizer st = new StringTokenizer(line, " ");    
                while(st.hasMoreTokens()){
                    String nextElement = (String) st.nextElement();

                    if(tm.size()>0 && tm.containsKey(nextElement)){
                        int val = 0;
                        if(tm.get(nextElement)!= null){
                        val = (Integer) tm.get(nextElement);
                        val = val+1;
                        }
                        tm.put(nextElement, val);
                    }else{
                    tm.put(nextElement, 1);
                    }

                }
            }
            for(Map.Entry<String,Integer> entry : tm.entrySet()) {
            System.out.println(entry.getKey() + " : " + entry.getValue());
            }

        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

Answer 6

You can't assign a TreeMap<String,Number> to a variable with the type Map<String,Integer>. Double, Long, etc. can be "put" into a TreeMap<String,Number>. When I "get" a value from a Map<String,Integer>, it must be an Integer.

Completely ignoring any i18n issues, memory constraints, and error handling, here goes:

class Counter {

  public static void main(String... argv)
    throws Exception
  {
    FileChannel fc = new FileInputStream(argv[0]).getChannel();
    ByteBuffer bb = fc.map(FileChannel.MapMode.READ_ONLY, 0, fc.size());
    CharBuffer cb = Charset.defaultCharset().decode(bb);
    Pattern p = Pattern.compile("[^ \t\r\n\f.,!?:;\"()']+");
    Map<String, Integer> counts = new TreeMap<String, Integer>();
    Matcher m = p.matcher(cb);
    while (m.find()) {
      String word = m.group();
      Integer count = counts.get(word);
      count = (count == null) ? 1 : count + 1;
      counts.put(word, count);
    }
    fc.close();
    for (Map.Entry<String, Integer> e : counts.entrySet()) {
      System.out.printf("%s: %d%n", e.getKey(), e.getValue());
    }
  }

}