How to convert object class to arrayList ? java

Question

I have this code :

ArrayList<Integer> users = new ArrayList<Integer>();

Map <String,Object> hashMap = new HashMap <String,Object> ();

hashMap.put("users",user);

// now I want to get users as array list
 hashMap.get("users")[i];//error

How to users as array ? thank you

http://docs.oracle.com/javase/7/docs/api/java/util/List.html#toArray() — Luca Basso Ricci, Jul 31 '14 at 09:36
1) Change the value type of the map to `List`. 2) User `hashMap.get("users").get(0)` — Konstantin Yovkov, Jul 31 '14 at 09:36
@Andreas no it's not duplicated, I want to convert specific value — david, Jul 31 '14 at 09:41

EpicPandaForce · Answer 1 · 2014-07-31T09:44:31.203

3

List<Integer> users = new ArrayList<Integer>();

Map<String, Object> hashMap = new HashMap<String, Object>();
hashMap.put("users",user);
....
....
....
List<Integer> users = (List<Integer>)hashMap.get("users"); //assuming this is another method and 'users' is not defined here yet
Integer[] usersArray = users.toArray(new Integer[users.size()]);
System.out.println("" + usersArray[i]);

Please note that this is an atrocity and you shouldn't be storing that List as an Object, you are pretty much circumventing what Generics are for.

edited Jul 31 '14 at 09:44

answered Jul 31 '14 at 09:39

EpicPandaForce

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agreed, but he already has users as a variable, I'm not even sure why he wants to put it into a map and get it out of it like that. Or store it as an object. I'm confused about the purpose, really, but this should work. – EpicPandaForce Jul 31 '14 at 09:40
@user2864740 'mkay done – EpicPandaForce Jul 31 '14 at 09:44

Simon Verhoeven · Answer 2 · 2014-07-31T09:42:27.017

2

This code will work if you want to access it as an ArrayList:

List<Integer> users = new ArrayList<Integer>();
Map<String, List<Integer>> hashMap = new HashMap<String, List<Integer>> ();
hashMap.put("users",users);
hashMap.get("users");

Your code won't work since you were storing it as an Object. So you would need a cast. And even then you use [] for arrays, not ArrayLists

If you want to use it as an array you can change the last line to:

hashMap.get("users").toArray()

And then you can use [1], or just do .get(1) on the initial code.

Alternatively, if you want to use hashMap as a property bag as someone validly mentioned you would need to do a cast:

((ArrayList<Integer>) hashMap.get("users"))

Then you can use users as an ArrayList as you are getting an Object by default from the HashMap.

edited Jul 31 '14 at 09:42

answered Jul 31 '14 at 09:40

Simon Verhoeven

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If it's a property bag, he should use a class unless he is sending this to JavaScript world, which I doubt he is. – EpicPandaForce Jul 31 '14 at 09:42
@user2864740 you raise a valid point so I've updated my answer to also help him if that is his intention. – Simon Verhoeven Jul 31 '14 at 09:47

Ninad Pingale · Answer 3 · 2014-07-31T09:44:19.070

Because it is ArrayList and not array,You can get it like this-

public static void main(String args[]) throws Exception
{
  ArrayList<Integer> users = new ArrayList<Integer>();
  users.add(5);
  users.add(10);

  Map <String,Object> hashMap = new HashMap <String,Object> ();

  hashMap.put("users",users);

  ArrayList<Integer> no =  ((ArrayList<Integer>) hashMap.get("users"));

   for (Integer integer : no) {
    System.out.println(integer);
   }
}

output-

5
10

Now to convert ArrayList to Array -

Integer[] arr = no.toArray(new Integer[no.size()]);

score 1 · Answer 4 · answered Jul 31 '14 at 09:43

You can try something like following

List<Users> usersList = new ArrayList<>();
Map<String,List<Users>> hashMap = new HashMap <> ();
hashMap.put("user",usersList);

Now

hashMap.get("user")

Will return a List of Users, Now you can use

hashMap.get("user").get(0); // to get 0th index user

How to convert object class to arrayList ? java

4 Answers4