Java - Iterating over a Map which contains a List
for(List<String> valueList : map.values()) {
for(String value : valueList) {
...
}
}
That's really the "normal" way to do it. Or, if you need the key as well...
for(Map.Entry<String, List<String>> entry : map.entrySet()) {
String key = entry.getKey();
for (String value : entry.getValue()) {
...
}
}
That said, if you have the option, you might be interested in Guava's ListMultimap
, which is a lot like a Map<K, List<V>>
, but has a lot more features -- including a Collection<V> values()
that acts exactly like what you're asking for, "flattening" all the values in the multimap into one collection. (Disclosure: I contribute to Guava.)
I recommend iterating over Map.entrySet()
as it is faster (you have both, the key and the value, found in one step).
Map<String, List<String>> m = new HashMap<String, List<String>>();
m.put("list1", Arrays.asList("s1", "s2", "s3"));
for (Map.Entry<String, List<String>> me : m.entrySet()) {
String key = me.getKey();
List<String> valueList = me.getValue();
System.out.println("Key: " + key);
System.out.print("Values: ");
for (String s : valueList) {
System.out.print(s + " ");
}
}
And the output is, as expected:
Key: list1 Values: s1 s2 s3