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)); } } } |
3 orange 2 mango 1 apple
The Hashtable mappings can be sorted using the following two ways:
- Using TreeMap
- 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)); } } } |
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)); } } } |
2 mango 3 orange 1 apple
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