HashMap, Iteraring problem

[Max Vandenburg]

Hi guys,

I have a HashMap, and it stored a bunch of Strings... the key is a String and the value is a Set of String...

and it looks something like this

{ D=[E,C], C=[B], E=[B], B=[A] }

what is the best way to iterate through this map so i get the two list. i.e.

D-E-B-A and the other would be D-C-B-A

[Stan James]

I guess we found D->E, then E->B then B->A? Try writing down what we did in plain English:

print the first key
get the value - looks like an array
for each value in the array
   print it
   use it as key to get another value
   for each value in that array
      print it
      use it as key to get another value
         etc until there is no value for the key

Hmmm, that didn't turn out very "plain" because it could keep repeating itself like that.

The solution that comes to mind for me involves "recursion". Are you familiar with how that works? If not, we could go there next.

[Max Vandenburg]

not really... someone mentioned something about recursion method... im not familiar with this

[Joanne Neal]

OP is getting some useful help here, so probably not worth explaining recursion again.

[Max Vandenburg]

I modified the code(here) slightly so it compiles with 1.4 (without the generics)
and i also havent come accross the

Arrays.asList

method so i rebuild the graphI'm sticking to what i know for now. so now the code looks like this...

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
 
public class Path {
    
    // { "D"=["E","C"], "C"=["B"], "E"=["B"], "B"=["A"] }
    
    static String[] nodes= { "A", "B", "C", "D", "E" };
    static String[][] to= { {}, { "A" }, { "B" }, { "E", "C" }, { "B" }};
    static List d = new ArrayList();
    static List c = new ArrayList();
    static List e = new ArrayList();
    static List b = new ArrayList();
    static List a = new ArrayList();
    
    static Map graph= new HashMap();
    static Map amap = new HashMap();
    static {
        
        d.add("C");
        d.add("E");
        c.add("B");
        e.add("B");
        b.add("A");
        amap.put("D", d);
        amap.put("C", c);
        amap.put("E", e);
        amap.put("B", b);
        amap.put("A", a);
        
    }
    
    private static void deliver(ArrayList path) {
        
        System.out.println(path);
    }
    
    public static void findAll(String source, String dest, ArrayList path) {
        
        if (path.contains(source)) return;
        path.add(source);
        if (source.equals(dest)) deliver(path);
        for (Iterator i = ((HashSet)graph.get(source)).iterator(); i.hasNext(); ) {
            findAll((String) i.next(), dest, path);
        }
        
        path.remove(source);
    }
    public static void main(String[] args) {
        
        System.out.println(amap);
        findAll("D", "A", new ArrayList());
    }
}

and the output i get is null pointer exception...
why is that?

{D=[C, E], A=[], C=[B], B=[A], E=[B]}
Exception in thread "main" java.lang.NullPointerException
        at Path.findAll(Unknown Source)
        at Path.main(Unknown Source)
Java Result: 1

[ October 02, 2006: Message edited by: Firman Drage ]

[Max Vandenburg]

Ok its solved again... here