How to compare two maps by their values

Question

How to compare two maps by their values? I have two maps containing equal values and want to compare them by their values. Here is an example:

    Map a = new HashMap();
    a.put("foo", "bar"+"bar");
    a.put("zoo", "bar"+"bar");

    Map b = new HashMap();
    b.put(new String("foo"), "bar"+"bar");
    b.put(new String("zoo"), "bar"+"bar");

    System.out.println("equals: " + a.equals(b));            // obviously false

How should I change the code to obtain a true?

Answer 1

The correct way to compare maps for value-equality is to:

1. Check that the maps are the same size(!)
2. Get the set of keys from one map
3. For each key from that set you retrieved, check that the value retrieved from each map for that key is the same (if the key is absent from one map, that's a total failure of equality)

In other words (minus error handling):

boolean equalMaps(Map<K,V>m1, Map<K,V>m2) {
   if (m1.size() != m2.size())
      return false;
   for (K key: m1.keySet())
      if (!m1.get(key).equals(m2.get(key)))
         return false;
   return true;
}