Hashmap in java

[Rajendra Prakash]

Hi friends, i need sample code to retrieve objects from HashMap in ascending order . Thanks Prakash

[Sebastian Janisch]

I suppose you want to sort by the keys which are Strings.

You cannot directly sort a HashMap, it's not made for that. The only workaround would be extracting the keySet() and sorting that, then iterating over the keySet.

But since there is a TreeMap, which does sorting for you, you can simply use that.

[Rob Spoor]

As Sebastian said, HashMap does not preserve any order at all. You can use TreeMap which uses Comparable's compareTo method or Comparator's compare method for determining the order, or LinkedHashMap which uses insertion order. The latter can be changed to be the last-accessed order instead by specifying this in the constructor.

[Salil Vverma]

You can use this sample code -

  

import java.util.HashMap;
import java.util.TreeMap;

public class Test {

	public static void main(String[] s) {

		HashMap<Integer, String> testmap = new HashMap<Integer, String>();
		new Test().addValues(testmap);
		// tmp would return values in shorted (ascending) order of keys
		TreeMap<Integer, String> tmp = new TreeMap<Integer, String>(testmap);
		System.out.println(tmp.toString());
	}

	private void addValues(HashMap<Integer, String> tmp) {
		tmp.put(20, "First");
		tmp.put(30, "Second");
		tmp.put(10, "Sample");
		tmp.put(1, "Testing");
	}

}
  

The result of this program is {1=Testing, 10=Sample, 20=First, 30=Second}.
In this case I have used Integer as a key to it is giving the result in ascending order by key. But if you are using any other object they you shall have to check that the object has implemented the comparable interface in it

Regards
Salil Verma