Java Hashtable: Sorted by Values

When writing a program, we occasionally encounter a situation where we need to use Hashtable for storing data and then sort data in the Hashtable. In this blog, I demonstrate how to sort data in a Hashtable based on values.

Suppose that we need to count number of unique words in a text document and then sort those words in descending order based on their frequencies. If two words have the same frequency, they will be sorted in alphabetical order. So, we can use Hashtable where keys are unique words or String and values are frequencies or Integer.

There are two important things: 1) Create an inner (or ordinary) class that implements Comparator class and 2) Use Collections.sort() method for sorting. Here is a working source code.

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;

public class Testing {

public static void main(String[] args) {

//Create a hashtable
Hashtable myhash=new Hashtable();

//Put things in Hashtable
myhash.put("AAB", 2);
myhash.put("ABC", 2);
myhash.put("AAX", 3);

//Put keys and values in to an arraylist using entryset
ArrayList myArrayList=new ArrayList(myhash.entrySet());

//Sort the values based on values first and then keys.
Collections.sort(myArrayList, new MyComparator());

//Show sorted results
Iterator itr=myArrayList.iterator();
String key="";
int value=0;
int cnt=0;
while(itr.hasNext()){

cnt++;
Map.Entry e=(Map.Entry)itr.next();

key = (String)e.getKey();
value = ((Integer)e.getValue()).intValue();

System.out.println(key+”, “+value);
}

}

static class MyComparator implements Comparator{

public int compare(Object obj1, Object obj2){

int result=0;Map.Entry e1 = (Map.Entry)obj1 ;

Map.Entry e2 = (Map.Entry)obj2 ;//Sort based on values.

Integer value1 = (Integer)e1.getValue();
Integer value2 = (Integer)e2.getValue();

if(value1.compareTo(value2)==0){

String word1=(String)e1.getKey();
String word2=(String)e2.getKey();

//Sort String in an alphabetical order
result=word1.compareToIgnoreCase(word2);

} else{
//Sort values in a descending order
result=value2.compareTo( value1 );
}

return result;
}

}

}