How to Sort Hashtable in Java?

Given a Hashtable, the task is to sort this Hashtable. Hashtable is a data structure that stores data in key-value format. The stored data is neither in sorted order nor preserves the insertion order. 

Example

Java

import java.io.*;
import java.util.*;

public class SortHashtable {
    public static void main(String[] args)
    {

        // create a hashtable
        Hashtable<Integer, String> ht
            = new Hashtable<Integer, String>();

        // insert data into hashtable
        ht.put(2, "mango");
        ht.put(3, "orange");
        ht.put(1, "apple");

        Set<Integer> keys = ht.keySet();
        Iterator<Integer> itr = keys.iterator();

        // traverse the TreeMap using iterator
        while (itr.hasNext()) {
            Integer i = itr.next();
            System.out.println(i + " " + ht.get(i));
        }
    }
}

Output

3 orange
2 mango
1 apple

The Hashtable mappings can be sorted using the following two ways: 

1. Using TreeMap
2. Using LinkedHashMap

Examples:

Input: Hashtable: {2: “mango”, 1: “apple”, 3: “orange”}

Output: 1 apple

                2 mango

               3 orange

Input: Hashtable: {3: “three”, 2: “second”, 1:”first”}

Output: 1 first

                2 second

                3 third

Approach 1:

TreeMap stores the data in sorted order. We can use the TreeMap constructor and convert the Hashtable object into a TreeMap object. Now the resultant TreeMap object is in sorted order.

Syntax:

TreeMap<K, V> tm = new TreeMap<K, V>(Map m);

Parameters: m is the Hashtable in our program.

Example

Java

import java.io.*;
import java.util.*;

public class SortHashtable {
    public static void main(String[] args)
    {

        // create a hashtable
        Hashtable<Integer, String> ht
            = new Hashtable<Integer, String>();

        // insert data into hashtable
        ht.put(2, "mango");
        ht.put(3, "orange");
        ht.put(1, "apple");

        // create a TreeMap
        TreeMap<Integer, String> tm
            = new TreeMap<Integer, String>(ht);

        // create a keyset
        Set<Integer> keys = tm.keySet();
        Iterator<Integer> itr = keys.iterator();

        // traverse the TreeMap using iterator
        while (itr.hasNext()) {
            Integer i = itr.next();
            System.out.println(i + " " + tm.get(i));
        }
    }
}

Output

1 apple
2 mango
3 orange

Approach 2:

LinkedHashMap stores the data in the order in which it is inserted. As when the data comes insert it into LinkedHashMap which has a property to preserve the insertion order.

Syntax:

LinkedHashMap<K, V> lhm = new LinkedHashMap<K, V>();

Example

Java

import java.io.*;
import java.util.*;

public class SortHashTable {
    public static void main(String[] args)
    {

        // create a LinkedHashMap
        LinkedHashMap<Integer, String> lhm
            = new LinkedHashMap<Integer, String>();

        // insert data into LinkeHashMap
        lhm.put(2, "mango");
        lhm.put(3, "orange");
        lhm.put(1, "apple");

        // prepare a keyset
        Set<Integer> keys = lhm.keySet();
        Iterator<Integer> itr = keys.iterator();

        // traverse the LinkedHashMap using iterator
        while (itr.hasNext()) {
            Integer i = itr.next();
            System.out.println(i + " " + lhm.get(i));
        }
    }
}

Output

2 mango
3 orange
1 apple