Question

I would like to shuffle values in an HashMap. I have following type of HashMap

Map<Integer,ArrayList<String> > trainDataSet = new HashMap<Integer, ArrayList<String>>();

I would like to shuffle the values in the Map. How would i do it?

Following is my attempt:

    collections.shuffle(trainDataSet.values());

Got an error:

    Values cannot be cast to java.util.List

Yes this make sense, because my values are in Arraylist not in List. Would it be possible to shuffle collections of arraylist?

EDIT

If i have following order::

key1  [aa,bb,cd]
key2  [xx,xy,sfr]

Into something like

key1  [xx,xy,sfr]
key2  [aa,bb,cd]

Answer 1

You actually want to randomly reassociate keys and values. Here’s an example how that can be achieved:

final Map<String, Object> x = new HashMap<String, Object>();
x.put("x", 1); x.put("y", 2); x.put("z", 3); x.put("w", 4);
System.out.println(x);
final List<Object> vs = new ArrayList<Object>(x.values());
Collections.shuffle(vs);
System.out.println(vs);
final Iterator<Object> vIter = vs.iterator();
for (String k : x.keySet()) x.put(k, vIter.next());
System.out.println(x);