What is the best way to iterate two hashmap in same loop in java?

Question

What's the best way to iterate over the below two maps together? I want to compare two maps values which are strings and have to get the keys and values.

HashMap<String, String> map1;
HashMap<String, String> map2;

Answer 1

There really isn't a better option than

for (Map.Entry<String, String> entry1 : map1.entrySet() {
  String key = entry1.getKey();
  String value1 = entry1.getValue();
  String value2 = map2.get(key); 
  // do whatever with value1 and value2 
}

Answer 2

Depending on what exactly you're trying to do, there are several reasonable options:

1. Just compare the contents of two maps

   Guava provides a Maps.difference() utility that gives you a MapDifference instance letting you inspect exactly what is the same or different between two maps.

2. Iterate over their entries simultaneously

   If you just want to iterate over the entries in two maps simultaneously, it's no different than iterating over any other Collection. This question goes into more detail, but a basic solution would look like this:

   Preconditions.checkState(map1.size() == map2.size());
   Iterator<Entry<String, String>> iter1 = map1.entrySet().iterator();
   Iterator<Entry<String, String>> iter2 = map2.entrySet().iterator();
   while(iter1.hasNext() || iter2.hasNext()) {
     Entry<String, String> e1 = iter1.next();
     Entry<String, String> e2 = iter2.next();
     ...
   }
   

   Note there is no guarantee these entries will be in the same order (and therefore e1.getKey().equals(e2.getKey()) may well be false).

3. Iterate over their keys to pair up their values

   If you need the keys to line up, iterate over the union of both maps' keys:

   for(String key : Sets.union(map1.keySet(), map2.keySet()) {
     // these could be null, if the maps don't share the same keys
     String value1 = map1.get(key);
     String value2 = map2.get(key);
     ...
   }