How can I convert a HashMap<String, ArrayList<String>> to a HashMap<String, String[]>?

Question

I have a HashMap<String, ArrayList<String>>. I am trying to convert it to a HashMap<String, String[]>.

HashMap<String, ArrayList<String>> arrayListMap = new HashMap<>();
HashMap<String, String[]> arrayMap = new HashMap<>();
for (Map.Entry<String, ArrayList<String>> entry : arrayListMap.entrySet()) {
    arrayMap.put(entry.getKey(), entry.getValue().toArray());
}

However, for entry.getValue().toArray(), my IDE is giving me the error:

Wrong 2nd argument type. Found: 'java.lang.Object[], required 'java.lang.String[]'.

I don't know why, because the arrayListMap specifies that I will be working with Strings.

Why is this not working, and how can I fix it?

Can you post a snippet of what is stored in entry (keys and values)? — lmcphers
– lmcphers, Commented Jun 29, 2015 at 15:42
@DmitryZaitsev: I don't see an XY problem; wanting to convert between a list and an array was such a specific use case that they included support for it in the API. — Makoto
– Makoto, Commented Jun 29, 2015 at 15:46
@Makoto in general, using collections is preferred over using arrays. And you can do more with a list than you can do with arrays. So doing the transformation is probably a bad idea. The code expecting an array in the map should instead be modified to expect a list. — JB Nizet
– JB Nizet, Commented Jun 29, 2015 at 15:47
I agree. But if the OP explained why he wants to do that transformation, we could guide him towards the best strategy. It could be an XY problem. — JB Nizet
– JB Nizet, Commented Jun 29, 2015 at 15:52
Converting a ArrayList to an array takes more time than not doing it (It's O(n)). And accessing elements of an ArrayList (which is backed by an array), is not significantly slower than accessing the array directly. So I would keep using the List, and only try to optimize if you really have a perfrmance problem and can prove that transforming to an array would solve it. — JB Nizet
– JB Nizet, Commented Jun 29, 2015 at 17:26

Makoto · Accepted Answer · 2015-06-29 15:43:20Z

6

ArrayList has overloaded the toArray method.

The first form, toArray(), will return an Object[] back. This isn't what you want, since you can't convert an Object[] into a String[].

The second form, toArray(T[] a) will return an array back that is typed with whatever array you pass into it.

You need to use the second form here so that the array is correctly typed.

arrayMap.put(entry.getKey(), entry.getValue()
                                  .toArray(new String[entry.getValue().size()]));

answered Jun 29, 2015 at 15:43

Makoto

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Sotirios Delimanolis Over a year ago

Let us continue this discussion in chat.

Jeet · Accepted Answer · 2015-06-29 15:52:59Z

-1

HashMap<String, ArrayList<String>> arrayListMap = new HashMap<>();
        HashMap<String, String[]> arrayMap = new HashMap<>();
        for (Map.Entry<String, ArrayList<String>> entry : arrayListMap.entrySet()) {
            arrayMap.put(entry.getKey(), entry.getValue().toArray(new String[entry.getValue().size()]));
        }

entry.getValue().toArray() will return object array "object[]"

But need String[].

So need to use the method entry.getValue().toArray(T[] a) that returns T[] //generic arg type

edited Jun 29, 2015 at 15:52

answered Jun 29, 2015 at 15:43

Jeet

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1 Comment

Makoto Over a year ago

A little explanation would go a long way.

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