我来说一下我具体的问题吧
 lastInterest += balance*annual/12;
我现在就是这个算下来小数点后面没有数字， 只有12.00，而没有12.12，我应该得12.12的，为什么后面是0呢？
 lastInterest += (double)balance*annual/12; 这样也没用

还是不行

这个是整个程序
我现在这个程序小数点后面的数字无法显示，比如12.12，他只显示12.00，谁帮我改一下阿(我在下面标出了我觉得有问题的地方)
 
 
public class SavingsAccount
{
   private double balance;    // To hold balance
   private double annualInterestRate;// annual interest rate
   private double lastInterest; // To hold total interest rate
   
    //Constructor
   public SavingsAccount(double bal, double rate)
   {
      balance = bal;
      annualInterestRate = rate;
   }
    //Add deposit to the current balance
   public void deposit(double amount)
   {
      balance += amount;
   }
    //Subtract withdraw to the balance
   public void withdraw(double amount)
   {
      balance -= amount;
   }
    //Calculate the total interest  
   public void addInterest()
   {
      lastInterest += balance*getInterestRate();              --------------我觉得这里有问题
   }
    //Return total balance
   public double getBalance()
   {
      return balance+lastInterest;
   }
    //Return monthly interest rate
   public double getInterestRate()
   {
      return annualInterestRate/12;
   }
    //Return total interest
   public double getLastInterest()
   {
      return lastInterest;
   }
}
 
——————————————————————————————————————————————————————————
 
import java.util.Scanner;//Needed for the Scanner class
import java.text.DecimalFormat;//Needed for decimal place
 
public class SavingsAccountTest
{
   public static void main(String[] args)
   {
      double balance1;   // To hold balance
      double aInterest;   // To hold annual interest
        double month, dep, with; //To hold total months, deposite, withdraw
        double dep1=0, with1=0;//count total deposite and withdraw
      
      Scanner keyboard = new Scanner(System.in);
        DecimalFormat f1 = new DecimalFormat("0.00");
      
      // Get the intial balance
      System.out.print("Enter the account's starting balance: ");
      balance1 = keyboard.nextDouble();
      
      // Get the annual interest rate
      System.out.print("Enter the savings account's annual "  
                         +"interest rate: ");
      aInterest = keyboard.nextDouble();
      
      // Get how many months have been passed
      System.out.print("How many months have passed since the "
                         + "account was opened? ");
      month = keyboard.nextDouble();
         
        // Create an instance of the CellPhone class,
      // passing the data that was entered as arguments
      // to the constructor.
      SavingsAccount acc = new SavingsAccount(balance1, aInterest);
         
        for (int n=1; n<=month; n++)
        {
            System.out.print("Enter the amount deposited during month "
                                 + n + ": ");
            dep = keyboard.nextDouble();
            dep1 +=dep;   
            acc.deposit(dep);
            
            System.out.print("Enter the amount withdrawn during month "
                            + n + ": ");
            with = keyboard.nextDouble();
            with1 +=with;
         acc.withdraw(with);
            acc.addInterest();
        }   
         
      // Get the data from the phone and display it.
      System.out.println("Total deposited: $"+ f1.format(dep1));
      System.out.println("Total withdrawn: $" + f1.format(with1));
      System.out.println("Interest earned: $" + f1.format(acc.getLastInterest()));
      System.out.println("Ending balance: $" + f1.format(acc.getBalance()));
   }
}

你如果输入 1000, 0.12, 100, 0, 110, 10, 120, 20结果就不对了,你会发现Interest earned小数点后面是零，其实应该有是36.36好像是

这个没有解决问题,只是换了一个method，没有用的，还是要注重我标出了的那个地方，我肯定问题就在那里