Return HashMap added to List<Object>

Question

I have a problem in JAVA when i'm trying to return a HashMap that I have added to a list of type: List<Object> . I know I can use other type of lists, but I need to use List<Object>

List<Object> listOfObjects = new ArrayList<Object>();

    HashMap<String, String> hashmap = new HashMap<String,String>();
    hashmap.put("x", "foo");
    hashmap.put("y", "bar");

    listOfObjects.add(hashmap);

    for (int i = 0; i < listOfObjects.size(); i++) {
        System.out.println(listOfObjects.get(i));
    }

I have added my hashmap to my listOfObject, but how do I get the HashMap from the listOfObject such that I can use the HashMap-commands. fx: hashmap.get("x) and it will return "foo".

Normally i thought i could just write: listOfObjects.get(0).get("x") and it would return "foo" but that does not work.

If anyone know another work around that's find but I just need to use a List.

Answer 1

Normally i thought i could just write: listOfObjects.get(0).get("x") and it would return "foo" but that does not work.

No, it wouldn't - because the type of listOfObjects.get(0) is just Object . How do you expect the compiler to know that it's meant to be a map?

You can use:

HashMap<String, String> map = (HashMap<String, String>) listOfObjects.get(0);
// Use map...

... but be aware that due to the nature of generics in Java, that cast isn't really ensuring that all the key/value pairs in the map are "string to string". The cast would work even if you'd originally used:

Map<Integer, Integer> badMap = new HashMap<Integer, Integer>();
badMap.put(0, 10);
listOfObjects.add(badMap);

You'll get a warning for this, but it's important to understand what it means. It's not clear what your use case is, but if you can make it more strongly typed (perhaps create a new class which contains a Map<String, String> ?) that would be good. Is every element of your list going to be a map? If so, why are you using List<Object> rather than a more strongly-typed list? If some elements aren't going to be maps, how can you tell which ones will be? (These are the sort of things you should be thinking about carefully.)

Answer 2

I hope this will help u..

List<Object> listOfObjects = new ArrayList<Object>();

        HashMap<String, String> hashmap = new HashMap<String,String>();
        hashmap.put("x", "foo");
        hashmap.put("y", "bar");

        listOfObjects.add(hashmap);

        for (int i = 0; i < listOfObjects.size(); i++) {
            System.out.println(((HashMap<String, String>)listOfObjects.get(i)).get("x"));
        }

Normally as your list is of type of object . so first cast it to HashMap type and then get the value from map

please notice the following code

 System.out.println(((HashMap<String, String>)listOfObjects.get(i)).get("x"));