How to correctly access ArrayList to iterate over within HashMap Java?

Question

I am attempting to access an ArrayList within a HashMap and iterate over it. However I am currently running into an issue with the following setup.

Map<String, List<String>> map = new HashMap<String, List<String>>();
List<String> list = Arrays.asList("One", "Two", "Three");
map.put("key_one", list);

Iterator it = map.entrySet().iterator();

while (it.hasNext()) {
    Map.Entry pair = (Map.Entry)it.next();
    log.info(pair.getKey());
    log.info(pair.getValue().getClass());
    for (String element : pair.getValue()) {
        log.info(element);
    }
}

There is something with accessing the ArrayList that is giving me issues.

[ERROR] required: array or java.lang.Iterable

[ERROR] found: java.lang.Object

If I access the list outside of the map, everything works fine. Its just accessing within the HashMap.

Any guidance would be most appreciated. Thank you for your time.

Warm Regards

No, really, it's because you're using raw types. If you [ask the same question with the same problem](https://stackoverflow.com/questions/52919831/how-do-you-iterate-over-inner-list-on-arrays-object-is-defining-as-arrays-aslis?noredirect=1#comment92749277_52919831), I'll close it for the same reason. — Andy Turner, Oct 21 '18 at 21:53

Mark · Accepted Answer · 2018-10-21T22:06:12.390

1

I would iterate over a setup like that as follows:

Map<String, List<String>> map = new HashMap<>();
List<String> list = Arrays.asList("One", "Two", "Three");

for (Map.Entry<String, List<String>> mapValue : map.entrySet()) {
    for (String listValue : mapValue.getValue()) {
        // Do stuff

    }
}

The reason your code is not working is because you're defining pair as Map.Entry pair = (Map.Entry)it.next();, without any types. This causes pair.getValue() to return Object and not some sort of List, which is why you cannot iterate over it in the for loop.

A fix using your code would be to change:

Map.Entry pair = (Map.Entry)it.next();

to:

Map.Entry<String, List<String>> pair = (Map.Entry<String, List<String>>) it.next();

edited Oct 21 '18 at 22:06

answered Oct 21 '18 at 21:37

Mark

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Minor detail, the OP could also define the `iterator` with the type info, `Iterator> it...` and avoid the cast in `pair`. – StaticBeagle Oct 21 '18 at 21:48
(But if you are going to cast, refer to types consistently: `Map.Entry` on LHS vs `Entry` on RHS). – Andy Turner Oct 21 '18 at 22:04
Thanks I've made it more consistent. – Mark Oct 21 '18 at 22:06
1

I ended up using the `Map.Entry> pair = (Entry>) it.next();` within the while loop. Thank you. – Mike Croteau Oct 22 '18 at 20:38

Pushpesh Kumar Rajwanshi · Answer 2 · 2018-10-22T04:47:08.363

0

You just had to use generics in your code and then it works perfect. Here is your code with generics,

Map<String, List<String>> map = new HashMap<String, List<String>>();
List<String> list = Arrays.asList("One", "Two", "Three");
map.put("key_one", list);

Iterator<Entry<String, List<String>>> it = map.entrySet().iterator();

while (it.hasNext()) {
    Map.Entry<String, List<String>> pair = it.next();
    System.out.println(pair.getKey());
    System.out.println(pair.getValue().getClass());
    for (String element : pair.getValue()) {
        System.out.println(element);
    }
}

edited Oct 22 '18 at 04:47

answered Oct 21 '18 at 21:42

Pushpesh Kumar Rajwanshi

18,127
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There's no need for the cast on the `it.next()` line any longer. – Andy Turner Oct 21 '18 at 21:56
Oh yes, that was redundant. Thanks for pointing out. – Pushpesh Kumar Rajwanshi Oct 22 '18 at 04:48

How to correctly access ArrayList to iterate over within HashMap Java?

2 Answers2