retrieving data from cloud firestore

Question

Hi im trying to retrieve values from a cloud firestore document in a collection based on a string query to filter the keys but cant seem to find any help online. Please assist or direct to existing solutions. Thanks. this is in java as well.

• I havent tried anything yet as I'm not too sure what functions provide this

im trying to filter a collection called food emissions with the key from this function

UPDATE

ive retrieved the data from the firestore collection and stored in a hashmap called food data. im now trying to filter this hashmap based on a string stored in the variable key and assign the value that matches to the variable co2EqTemp. however when I make a toast of the value thats stored in co2EqTemp it just says it is referencing a null value

        private void addMeal() {
            mealname =  mealName.getText().toString();
            if(mealname.isEmpty()){
                mealName.setError("Please name your meal!");
                mealName.requestFocus();
                return;
            }

            String UserID = FirebaseAuth.getInstance().getCurrentUser().getUid();
            FirebaseDatabase fDatabase = FirebaseDatabase.getInstance();
            fDbRef = fDatabase.getReference("MealLists");

            for(Map.Entry<String,Double> entry: food.entrySet()){

                key = entry.getKey();
                values = entry.getValue();

                foodAmount = values.toString();
                mealData meals = new mealData(key, UserID, foodAmount);
                fDbRef.child(mealname).setValue(meals);


                makeToast(mealname);
                 foodRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<DocumentSnapshot> task) {
                        if(task.isSuccessful()){
                            Map<String, Object> map = task.getResult().getData();
                            for(Map.Entry<String, Object> entry : map.entrySet()){
                                foodData.put(entry.getKey(),entry.getValue());
                            }
                            for(Map.Entry<String, Object> entry: foodData.entrySet()){
                                if (entry.getKey().equals(key)){
                                    Co2eqTemp = (Double) entry.getValue();
                                }
                                Co2eq = Co2eq + Co2eqTemp;
                            }

                        }
                    }

                });


            }
            storeMeal();
        }
    });

Answer 1

ok so I fixed this by just avoiding the hashmap completely and filtering the data returned from the collection like this.

   public void onComplete(@NonNull Task<DocumentSnapshot> task) {
                        if(task.isSuccessful()){
                            Map<String, Object> map = task.getResult().getData();
                            for(Map.Entry<String, Object> entry : map.entrySet()){
                                if(entry.getKey().equals(key)) {
                                    Double co2 = Double.parseDouble(entry.getValue().toString());
                                    Co2eqTemp = co2 * values;
                                }
                                Co2eq = Co2eq + Co2eqTemp;
                            }
                           
                       }