Say you have a Hashtable with : number for keys, and a comma deliminated list of letters of values. Now you want to make another hash table which is the opposite. The keys for the second Hashtable will be a distinct list of all possible letters from the first Hashtable, and the values will be a comma deliminated list of all the numbers associated with a particular letter.
import java.util.Enumeration; import java.util.Hashtable; public class HashInversion { public static void main(String[] args) { //hashtable : numbers with lists of letters Hashtable num2letter_hash = new Hashtable(); //hashtable letters with list of numbers Hashtable letters2num_hash = new Hashtable(); num2letter_hash.put(1,"a,b,c,d"); num2letter_hash.put(22,"g,e,d"); num2letter_hash.put(73,"e,g,d"); num2letter_hash.put(41,"a,v,c,d,b,e"); num2letter_hash.put(5,"a,l"); num2letter_hash.put(17,"a"); num2letter_hash.put(18,"a,l,e"); //loop through all the elements in num2letter_hash Enumeration keys = num2letter_hash.keys(); while( keys.hasMoreElements() ) { int key = (int) keys.nextElement(); String content = (String) num2letter_hash.get(key); //break up the content by commas String [] letters = content.split(","); for( int i = 0 ; i < letters.length; i++) { //if this letter hasn't been added to the new hash, // add it, and set its value to the current key //if it already exists in the new table, obtain the current value string // and append a new number/key to it. if( letters2num_hash.containsKey(letters[i].toString()) ) { String previous_num = (String) letters2num_hash.get(letters[i].toString()); letters2num_hash.put(letters[i].toString(), previous_num+","+key); } else { letters2num_hash.put( letters[i].toString(), ""+key); } } } //before (numbers,letter-list) System.out.println(num2letter_hash.toString()); //after (letters,number-list) System.out.println(letters2num_hash.toString()); } } |
