How to calculate the charge of all user-inputted data?

Question

I'm writing a program that can calculate the call charge made by its customers. It requires us to define and call a method which can calculate and return the call charge.

I have a bit of an error in the output: the total charge isn't detected. Is there something wrong while in the main method or the formula method that I created?

import java.util.Scanner;

public class Asg22 {

    static double formula(double rate) {
          double charge =+ rate;
          
        return charge;
    }

    public static void main(String[] args) {

        //variables
        int total = 0;
        int n;
        int choice = 0;
        int i = 0;
        double sum = 0;
        int min = 0;
        int cat = 0;
        String data = "";
        double rate = 0;
        double charge=0;

        //scanner
        Scanner scan = new Scanner(System.in);

        do{
            do{System.out.println("Enter the call duration (in minutes):   ");
        min = scan.nextInt();
        System.out.println("Enter category (1.Daytime, 2.Evening, 3.Off-peak):   ");
        cat = scan.nextInt();

        if (cat == 1) {
            rate = min * 0.07;
            formula(rate);
        } else if (cat == 2) {
            rate = min * 0.12;
            formula(rate);
        } else if (cat == 3) {
            rate = min * 0.05;
            formula(rate);
        } else {
            System.out.println("The number you've inserted is Invalid.");
           
        }}while(cat !=1 && cat !=2 && cat !=3);
        
        charge = formula(rate);
        
        System.out.printf("The amount you have to pay is = RM%.2f", rate);
        System.out.println("\nDo you want to continue? 1.Yes 2.No ");
        choice = scan.nextInt();
      i++;
    } while(choice == 1);
        
        System.out.println("Total Customer: "+i);
        charge += rate;
        System.out.printf("Total Charge: RM%.2f", charge);}
        
    }

Output:

Enter the call duration (in minutes):   
22
Enter category (1.Daytime, 2.Evening, 3.Off-peak):   
1
The amount you have to pay is = RM1.54
Do you want to continue? 1.Yes 2.No 
1
Enter the call duration (in minutes):   
56
Enter category (1.Daytime, 2.Evening, 3.Off-peak):   
2
The amount you have to pay is = RM6.72
Do you want to continue? 1.Yes 2.No 
1
Enter the call duration (in minutes):   
22
Enter category (1.Daytime, 2.Evening, 3.Off-peak):   
4
The number you've inserted is Invalid.
Enter the call duration (in minutes):   
13
Enter category (1.Daytime, 2.Evening, 3.Off-peak):   
3
The amount you have to pay is = RM0.65
Do you want to continue? 1.Yes 2.No 
2
Total Customer: 3
Total Charge: RM1.30

Answer 1

There are multiple problems with this code:

1. there is no operator =+ - this are 2 separate operators, it is interpreted as

    double charge = +rate;

where the + is the Unary Plus Operator + .

2. local variables don't retain any value across calls. You need to change it to a field, or calculate the sum in main .

Minimal change to make it work (though this is not necessarily the best solution):

static double charge = 0;

static double formula(double rate) {
    charge += rate;
    return charge;
}