Android Firestore, Query Array

To solve this, please use the following code:

ref.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
    @Override
    public void onComplete(@NonNull Task<QuerySnapshot> task) {
        for (DocumentSnapshot document : task.getResult()) {
            List<String> list = (List<String>) document.get("card");
            for (String item : list) {
                    Log.d("TAG", item);
            }
        }
    }
});

The output will be:

123
456
789
//and so on

Remember, document.get("card") does not return an array, it returns an ArraList.

If you want use a Query using whereEqualTo() method then I suggest you change your database a little bit. Your database structure shoould look like this:

Firestore-root
   |
   --- company
         |
         --- companyDocumentId
                |
                --- address: "P18/A CIT Road Palit"
                |
                --- card
                     |
                     --- 123: true
                     |
                     --- 456: true
                     |
                     --- 789: true

In this case the your query should look like this:

Query = ref.whereEqualTo("card.123", true);

Edit:

According to your commend the query should be:

Query = ref.whereEqualTo("card.E20040806709002620760CE82", true);

Edit 13 Aug 2018:

According to the updated documentation regarding array membership, now it is possible to filter data based on array values using whereArrayContains() method. A simple example would be:

CollectionReference citiesRef = db.collection("cities");
citiesRef.whereArrayContains("regions", "west_coast");

This query returns every city document where the regions field is an array that contains west_coast. If the array has multiple instances of the value you query on, the document is included in the results only once.