Android Array list filter and group items which have the same key value

Question

I got this data from firebase and I want to group the items based on the deadline key value and get an array of grouped data.

{
  "-MJTP2ZF4-Qrn3xdK3x4" : {
    "comment" : "Clean ",
    "deadline" : "9/11/2020",
    "firstDeadline" : "12/10/2020",
    "interval" : 7,
    "owner" : "Mika",
    "parentTaskID" : "-MJTP2ZCNDXlgMsw6bHy",
    "subtask" : true,
    "title" : "Some "
  },
  "-MJTP2ZF4-Qrn3xdK3x5" : {
    "comment" : "Clean ",
    "deadline" : "16/11/2020",
    "firstDeadline" : "12/10/2020",
    "interval" : 7,
    "owner" : "Mika",
    "parentTaskID" : "-MJTP2ZCNDXlgMsw6bHy",
    "subtask" : true,
    "title" : "Some "
  },
  "-MJTPy8Q7MyGM5mO0b_N" : {
    "comment" : "7.3 C",
    "deadline" : "9/11/2020",
    "firstDeadline" : "12/10/2020",
    "interval" : 7,
    "owner" : "Mika",
    "parentTaskID" : "-MJTPy8PdX4DmWWC_DfE",
    "subtask" : true,
    "title" : "Temp"
  },
  "-MJTPy8Q7MyGM5mO0b_O" : {
    "comment" : "7.3 C",
    "deadline" : "16/11/2020",
    "firstDeadline" : "12/10/2020",
    "interval" : 7,
    "owner" : "Mika",
    "parentTaskID" : "-MJTPy8PdX4DmWWC_DfE",
    "subtask" : true,
    "title" : "Temp"
  },
  "-MLHTHmzx8J7Ifh0Mecx" : {
    "comment" : "Finally ",
    "deadline" : "4/11/2020",
    "owner" : "Mika",
    "repetition" : false,
    "title" : "Clean "
  },
  "-MLHTx_ZK4n5dhkBb8TE" : {
    "comment" : "Dirty ",
    "deadline" : "04/12/2020",
    "owner" : "Mika",
    "repetition" : false,
    "title" : "Clean "
  }
}

and I want to group items based on the deadline key and got something like this

{

    "9/11/2020":{
        {
  "-MJTP2ZF4-Qrn3xdK3x4" : {
    "comment" : "Clean ",
    "deadline" : "9/11/2020",
    "firstDeadline" : "12/10/2020",
    "interval" : 7,
    "owner" : "Mika",
    "parentTaskID" : "-MJTP2ZCNDXlgMsw6bHy",
    "subtask" : true,
    "title" : "Some "
  },
    "-MJTPy8Q7MyGM5mO0b_N" : {
    "comment" : "7.3 C",
    "deadline" : "9/11/2020",
    "firstDeadline" : "12/10/2020",
    "interval" : 7,
    "owner" : "Mika",
    "parentTaskID" : "-MJTPy8PdX4DmWWC_DfE",
    "subtask" : true,
    "title" : "Temp"
  },
    },

    "16/11/2020":{
         "-MJTPy8Q7MyGM5mO0b_O" : {
    "comment" : "7.3 C",
    "deadline" : "16/11/2020",
    "firstDeadline" : "12/10/2020",
    "interval" : 7,
    "owner" : "Mika",
    "parentTaskID" : "-MJTPy8PdX4DmWWC_DfE",
    "subtask" : true,
    "title" : "Temp"
  },
"-MJTPy8Q7MyGM5mO0b_O" : {
    "comment" : "7.3 C",
    "deadline" : "16/11/2020",
    "firstDeadline" : "12/10/2020",
    "interval" : 7,
    "owner" : "Mika",
    "parentTaskID" : "-MJTPy8PdX4DmWWC_DfE",
    "subtask" : true,
    "title" : "Temp"
  }
    },
    "04/12/2020":{
        "-MLHTx_ZK4n5dhkBb8TE" : {
    "comment" : "Dirty ",
    "deadline" : "04/12/2020",
    "owner" : "Mika",
    "repetition" : false,
    "title" : "Clean "
  },
  "-MLHTHmzx8J7Ifh0Mecx" : {
    "comment" : "Finally ",
    "deadline" : "4/11/2020",
    "owner" : "Mika",
    "repetition" : false,
    "title" : "Clean "
  }
    }
}

and this is how I got the data from the firebase I just iterate them and store array list then I want that item list not to be just a list of items but grouped and have a chunk of the items array list

 ValueEventListener valueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                for (DataSnapshot singleSnapshot : snapshot.getChildren()) {
                    Task task = singleSnapshot.getValue(Task.class);
                    mTaskArrayList.add(task);
                }
                mLogBookRecyclerView.setAdapter(mAdapter);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        };
        mDatabaseReference.addValueEventListener(valueEventListener);

Answer 1

The data structure you want to use is a Map, specifically a HashMap.

I assumed your Task class has a getter getDeadline for the deadline date and it is a String. If you are using one of the many Java inbuilt Date features, modify the class accordingly.

Map<String,List<Task>> groupedTasks=new HashMap<>();
.
.
.
public void onDataChange(@NonNull DataSnapshot snapshot) {
        
        for (DataSnapshot singleSnapshot : snapshot.getChildren()) {
            Task task = singleSnapshot.getValue(Task.class);
            if(!groupedTasks.containsKey(task.getDeadline())
                groupedTasks.put(task.getDeadline(),new ArrayList<Task>());
            groupedTasks.get(task.getDeadline()).add(task);
        }
        mLogBookRecyclerView.setAdapter(mAdapter);
}

You can later access them using the get method of the Map. For example groupedData.get("16/11/2020") will return a List<Task> containing the Tasks of that deadline.