Question

I have this HashMap that I need to print out in ascending order according to the values contained in it (not the keys).

But the order when I print it out is seemingly random.

What’s the best way to print it out in ascending value order?

Map<String, String> codes = new HashMap<String, String>();

codes.put("A1", "Aania");
codes.put("X1", "Abatha");
codes.put("C1", "Acathan");
codes.put("S1", "Adreenas");

In other words, the example above should print out as this:

A1, Aania
X1, Abatha
C1, Acathan
S1, Adreenas

Answer 1

You aren’t going to be able to do this from the HashMap class alone.

I would take the Map<String, String> codes, construct a reverse map of TreeMap<String, String> reversedMap where you map the values of the codes Map to the keys (this would require your original Map to have a one-to-one mapping from key-to-value). Since the TreeMap provides Iterators which returns entries in ascending key order, this will give you the value/key combination of the first map in the order (sorted by values) you desire.

Map<String, String> reversedMap = new TreeMap<String, String>(codes);

//then you just access the reversedMap however you like...
for (Map.Entry entry : reversedMap.entrySet()) {
    System.out.println(entry.getKey() + ", " + entry.getValue());
}

There are several collections libraries (commons-collections, Google Collections, etc) which have similar bidirectional Map implementations.