How to swap keys and values in a Map elegantly

Question

I already know how to do it the hard way and got it working - iterating over entries and swapping \"manually\". But i wonder if, like so many tasks, this one can be solved

Answer 1

This will work for duplicate values in the map also, but not for HashMap as values.

package Sample;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

public class Sample {
    public static void main(String[] args) { 
        Map map = new HashMap(); 
        Map > newmap = new HashMap >(); 

        map.put("1", "a"); 
        map.put("2", "a"); 
        map.put("3", "b"); 
        map.put("4", "b"); 
        System.out.println("before Reversing \n"+map.toString()); 

        for (Map.Entry entry : map.entrySet()) 
        { 
            String oldVal = entry.getValue(); 
            String oldKey = entry.getKey(); 
            Set newVal = null; 

            if (newmap.containsKey(oldVal)) 
            { 
                newVal = newmap.get(oldVal); 
                newVal.add(oldKey); 
            } 
            else 
            { 
                newVal= new HashSet<>(); 
                newVal.add(oldKey); 
            } 
            newmap.put(oldVal, newVal); 
        } 
        System.out.println("After Reversing \n "+newmap.toString()); 
    } 
}