public class DateActivity extends ExpandableListActivity {
   // date, isAdmin and name are passed to this activity using shared preferences
       private String date, name;
       private boolean isAdmin;
       // Adapter declaration
       private CustomExpandableListAdapter mAdapter;
       // ArrayList to store group items
       private ArrayList<ParentHeader> parents = new ArrayList<>();

       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_date);
    // initialize date, isAdmin, name here

           // Fetch all shifts for selected date from Firebase
           Validator.fetchShiftsPerDayFromDB(date, new Callback()
           {
               @Override
               void shiftsCallback(ArrayList<Shift> shifts)
               {
                   super.shiftsCallback(shifts);
                   if (shifts == null)
                       return;

                   for(Shift shift : shifts){
                       parents.add(new ParentHeader(shift));
                   }

                   mAdapter = new CustomExpandableListAdapter();
                   setListAdapter(mAdapter);
               }
           });
}

   private class CustomExpandableListAdapter extends BaseExpandableListAdapter
   {
       private LayoutInflater inflater;

       public CustomExpandableListAdapter()
       {
           // Create Layout Inflator
           inflater = LayoutInflater.from(DateActivity.this);
       }

       @Override
       public int getGroupCount()
       {
           return parents.size();
       }

       @Override
       public int getChildrenCount(int groupPosition)
       {
           int size = 0;
           if(parents.get(groupPosition).getEmployeesList() != null)
               size = parents.get(groupPosition).getEmployeesList().size();
           return size;
       }

       @Override
       public Object getGroup(int groupPosition)
       {
           return parents.get(groupPosition);
       }

       @Override
       public Object getChild(int groupPosition, int childPosition)
       {
           return parents.get(groupPosition).getEmployeesList().get(childPosition);
       }

       @Override
       public long getGroupId(int groupPosition)
       {
           return groupPosition;
       }

       @Override
       public long getChildId(int groupPosition, int childPosition)
       {
           return childPosition;
       }

     @Override
       public boolean hasStableIds()
       {
           return true;
       }

       @Override
       public View getGroupView(int groupPosition, boolean isExpanded, View convertView, ViewGroup parentView)
       {
               final ParentHeader parentHeader = parents.get(groupPosition);

               // Inflate group_row.xml file for group rows
               convertView = inflater.inflate(R.layout.group_row, parentView, false);
// Get group_row.xml file elements and set values here

           //Add User to selected shift
           convertView.findViewById(R.id.add).setOnClickListener(v -> {
               //TODO insert constraints
              Validator.addUserToShiftInDB(parentHeader.getShift().getKey(), date, name, new Callback() {
                  @Override
                  void onUserAssignedToShiftCallback() {
                      super.onUserAssignedToShiftCallback();
                       // TODO attach as child-item to chosen group-item
                  }
              });

           });

               convertView.findViewById(R.id.delete).setOnClickListener(v -> {
                   if(isAdmin){
                       //TODO open confirmation window
                       Validator.deleteShiftFromDateInDB(parentHeader.getShift().getKey(), date, new Callback() {
                           @Override
                           void onDeletedShiftCallback() {
                               super.onDeletedShiftCallback();
// shift gets deleted in Firebase but the expandable list does not refresh
                               parents.remove(groupPosition);
                               mAdapter.notifyDataSetChanged();
                           }
                       });

                   }
                   else{
                       Toast.makeText(getApplicationContext(),"Admin privileges required", Toast.LENGTH_SHORT).show();
                   }
               });
               return convertView;
       }

         @Override
       public View getChildView(int groupPosition, int childPosition, boolean isLastChild, View convertView, ViewGroup parentView)
       {
           final ParentHeader parentHeader = parents.get(groupPosition);
           final String childString = parentHeader.getEmployeesList().get(childPosition);

           // Inflate child_row.xml file for child rows
           convertView = inflater.inflate(R.layout.child_row, parentView, false);

           // Get child_row.xml file elements and set values here

           return convertView;
       }

       @Override
       public boolean isChildSelectable(int groupPosition, int childPosition)
       {
           return true;
       }

       @Override
       public boolean areAllItemsEnabled()
       {
           return true;
       }

       @Override
       public boolean isEmpty()
       {
           return ((parents == null) || parents.isEmpty());
       }

       @Override
       public void notifyDataSetChanged()
       {
           // Refresh List rows
           super.notifyDataSetChanged();
       }
   }
}

public class ParentHeader {
   private Shift shift;
   private boolean isDeleteButtonClicked, isAddButtonClicked, isHeaderClicked;
   // Array to store child objects
   private ArrayList<String> employeesList = new ArrayList<>();

   public ParentHeader() { }

   public ParentHeader(Shift shift) {
       this.shift = shift;
   }

// Getters/Setters here
}

public class Shift {
   private String startTime, endTime, name, key;
   private Integer wage, numOfEmps, employees;


   public Shift(){}

   public Shift(String name, String startTime, String endTime, Integer wage, Integer numOfEmps, Integer employees) {
       this.startTime = startTime;
       this.endTime = endTime;
       this.name = name;
       this.wage = wage;
       this.numOfEmps = numOfEmps;
       this.employees = employees;
// key field is assigned a value when fetching each shift from firebase using .getKey() method

   }
//Getters/Setters here
}

"20190528" : {
    "-LfvCgavw5bYUJzULE6E" : {
      "employees" : 0,
      "endTime" : "00:00",
      "name" : "test1",
      "numOfEmps" : 2,
      "startTime" : "00:00",
      "wage" : 2
    }
  }

"20190528" : {
    "-LfvCgavw5bYUJzULE6E" : {
      "employees" : {
        "generatedKey1" : "John Smith",
        "generatedKey2" : "Jane Doe"
      },
      "endTime" : "00:00",
      "name" : "test1",
      "numOfEmps" : 2,
      "startTime" : "00:00",
      "wage" : 2
    }
  }