如何在帐户上存储和修改余额：

Question

I have a small acount in which to deposit, withdrawl, view balance, and exit. Everything runs perfectly however I would like the balance to ajust according to what method I do (withdrawal, deposit, etc.) I started with joptionpane but had the same problem and thought I should just start over. Anyone point me in the right direction?

Main file:

import java.util.Scanner;

public class Bank1
{
    public static void main(String[] args) 
    {
        HomeBank obj1 = new HomeBank();

        Scanner keyboard = new Scanner(System.in);
        String option;
        char object;

        System.out.println("This is your home banking account" + "\n");

        do 
        {
            System.out.println("What would you like to do today?"+ "\n" +
                    "\nSelect the following: " +
                    "\n'B' To View Balance." +
                    "\n'D' To Deposit" +
                    "\n'W' To Withdrawal" +
                    "\n'E' To Exit");

            System.out.print("Your selection: ");       

            option = keyboard.next().toUpperCase();
            object = option.charAt(0);

            System.out.print("\n");
            System.out.println("you entered: " +object+ "\n");

            if(object =='D')
                obj1.deposit();
            else if(object =='W')
                obj1.withdrawal();
            else if(object =='B')
                obj1.balance();
            else
                System.out.println("**Invalid input**" +
                        "\n Please try again");            
        } while(object !='E');

        System.out.println("The End");    
    }
}

Class:

import java.util.InputMismatchException;
import java.util.Scanner;

public class HomeBank 
{
    private double withdrawal, deposit, balance;

    public HomeBank() 
    {
        balance = 100;
        withdrawal = 50;
        deposit = 150;
    }

    public HomeBank(double bal, double with, double de) 
    {
        balance = bal;
        withdrawal = with;
        deposit = de;
    }

    public static void balance() 
    {
        double balance = 250.00d;

        System.out.println("You have $" + balance+"dollars");  
    }

    public static void deposit() 
    {
        Scanner keyboard = new Scanner(System.in);
        double deposit = 0.00d;
        double balance = 250.00d;
        boolean goodput =true;

        do 
        {
            try 
            {
                System.out.print("How much would you like to deposit today? :");
                deposit = keyboard.nextDouble();

                if(deposit > 0.00) 
                {
                    System.out.println("you entered $" +deposit+" dollars");
                    System.out.println("you now have $" + (deposit + balance)+" dollars");
                    goodput = false;
                }
                else
                {
                    System.out.println("\n**Error**\nYou cannot deposit a "
                            + "negative amount\n");
                    System.out.println("Please try again\n");
                    goodput = true;
                }
            }
            catch (InputMismatchException x) 
            {
                System.out.println("I'm sorry but that is an invalid input" +
                        "\nYou will be redirected to the main menu shortly..."+
                        "\n");
                goodput = false;
            }
        } while (goodput == true); 
    }

    public static void withdrawal() 
    {
        Scanner keyboard = new Scanner(System.in);
        double withdrawal = 0.00d;
        double balance = 250.00d;
        boolean goodput = true;

        do 
        {
            try
            {
                System.out.println("How much would you like to withdrawal?");
                withdrawal = keyboard.nextDouble();

                if (withdrawal < 0 || withdrawal > balance) 
                {
                    System.out.println("You have either entered a negative number"
                            + "or trying to withdrawal more than is in your account");
                    System.out.println("You cannot withdrawal more than $"+balance);
                    System.out.println("Please try again");
                    goodput = true;
                }              
                else 
                {
                    System.out.println("You now have $" +(balance-withdrawal)+ "dollars");
                    System.out.println("Thank you for choosing HomeBank\n"
                            + "\nYou will be redirected to the main menu\n");
                    goodput =false;
                }
            }
            catch (InputMismatchException x) 
            {
                System.out.println("I'm sorry but that is an invalid input" +
                        "\nYou will be redirected to the main menu shortly..."+
                        "\n");
                goodput = false;
            }
        } while (goodput == true);    
    }
}

Answer 1

The issue is how you are updating the values. In your methods, you have something that goes like the following:

double deposit = 0.00d;
double balance = 250.00d;

In there, notice how you are putting the double keyword before the variable. By doing that, you are telling Java to create a new variable named deposit and balance in the local scope.

If you want to modify the fields named deposit and balance , you would definitely have to remove the double keyword before them, and optionally put this. in front of the variable. It would look something like the following:

deposit = 0.00d;
balance = 250.00d;

Or

this.deposit = 0.00d;
this.balance = 250.00d;

You would replace all instances of the variables withdraw , balance , and deposit like that, but only the declaration.

The reason you don't need to specify the type is because you already define it in the field declaration. Therefore when you again define it in the method, you are saying to create a new variable with that name and type in the current scope.

Edit: As Suyash Limaye said, you will have to remove the static keywords from the methods. Having the static keywords prevents the methods from being able to access the non-static fields, which will cause conflicts.