How to get total of data based on different child from Firebase in Android

There are multiples dates and data (Course and Scale) for each date. Each date and data store under a unique key. The dates are inserted by Date picker.

"Data":{
"K2ngvpioRUYF4bRM07Da5cbAjE53":{   //UID
"-M3jNjCuGdMCwt1Czpwz":{        //unique key
"Date":"2020-3-30",
"Course":"A",
"Scale":"3"
},
"-M5hxnQrCJdCUvRcMZJu":{
"Date":"2020-4-24",
"Course":"A",
"Scale":"3"
}
"-M3jQWxm7z0EQYgkVenX":{
"Date":"2020-4-29",
"Course":"B",
"Scale":"4"
},
"-M5hxn-rCJICUvRcMZJu":{
"Date":"2020-4-24",
"Course":"B",
"Scale":"2"
}
}
}

I would like to calculate the total number of scales based on Course monthly. For scale, if the number is 3, it means 0.6. If the number is 2, it means 0.4. If the number is 4, it means 0.8. So, for the Course A, the total number is 1.2(0.6+0.6). For Course B, the total number is 1.2 (0.8+0.4).

   Calendar mCalendar = Calendar.getInstance();
int year = mCalendar.get(Calendar.YEAR);
int month = mCalendar.get(Calendar.MONTH);
final int[] Currmonth = {month + 1};         //this is array because there are other method using this but unnecessary for this question
int lastDay = mCalendar.getActualMaximum(Calendar.DAY_OF_MONTH);
int firstDay=1;
String start=year+"-"+Currmonth[0]+"-"+firstDay;
String end=year+"-"+Currmonth[0]+"-"+lastDay;
DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("Users Mood");
Query range = ref.child(user.getUid()).orderByChild("Date").startAt(start).endAt(end);
range.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
ArrayList<String> courses=new ArrayList<>();
Map<String, Double> hm = new HashMap<>();
for(DataSnapshot ds : dataSnapshot.getChildren()) {
String course=ds.child("Course").getValue(String.class);
courses.add(course);
String scale=ds.child("Scale").getValue(String.class);
for(String i :courses) {
double num=0;
if (hm.get(i).equals("1")) {
num=0.2;
}else if(hm.get(i).equals("2")){
num=0.4;
}else if(hm.get(i).equals("3")){
num=0.6;
}else if(hm.get(i).equals("4")){
num=0.8;
}else if(hm.get(i).equals("5")){
num=1;
}
double total = hm.containsKey(course) ? hm.get(course) : 0;
total += num;
hm.put(course, total);
}
}
for(String key:hm.keySet()) {
Log.d("tag", key+hm.get(key));
}
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
}
});

In this case, the query is from the start date of month to end date of the month. The logic for mapping two children is incorrect. I don't think hashmap is suitable for this case because there might be few data with the same course with same scale (For example, there are two same data, Course A with scale 3 or num 0.6). Please help me to correct my logic.

The way you are using your date is not very efficient. Because that would mean 2020-4-12 comes before 2020-4-4. I suggest

//year is integer year
String monthStr = month; //month is integer month
if(month<=9)monthStr="0"+monthStr;
String dateStr = date; //date is integer date
if(date<=9)dateStr="0"+dateStr;
String finalStr = year+"-"+monthStr+"-"+dateStr;

Now that being said, You are using an ArrayList unnecessarily. On contrary to what you said the HashMap can be used here.
Also hm.get(i) would return Double and I dont know why you are comparing it to a string, when you should actually be comparing the string scale. Anyway, Look at this modification I made to your loop body. I hope you know parseDouble which converts a String to its Double equivalent.

for(DataSnapshot ds : dataSnapshot.getChildren()) {
String course=ds.child("Course").getValue(String.class);
String scale=ds.child("Scale").getValue(String.class);
double num= Double.parseDouble(scale)/5.0;
double total = hm.containsKey(course) ? hm.get(course) : 0;
total += num;
hm.put(course, total);
}
for(String key:hm.keySet()) {
Log.d("tag", key+hm.get(key));
}

If you have any doubts fell free to comment.