The file of BankAccount.java

 /**
    A bank account has a balance that can be changed by 
    deposits and withdrawals.
 */
 public class BankAccount
 {  
    /**
       Constructs a bank account with a zero balance.
    */
    public BankAccount()
    {   
       balance = 0;
    }
 
    /**
       Constructs a bank account with a given balance.
       @param initialBalance the initial balance
    */
    public BankAccount(double initialBalance)
    {   
       balance = initialBalance;
    }

    /**
       Deposits money into the bank account.
       @param amount the amount to deposit
    */
    public void deposit(double amount)
    {  
       double newBalance = balance + amount;
       balance = newBalance;
    }
 
    /**
       Withdraws money from the bank account.
       @param amount the amount to withdraw
    */
    public void withdraw(double amount)
    {   
       double newBalance = balance - amount;
       balance = newBalance;
    }
 
    /**
       Gets the current balance of the bank account.
       @return the current balance
    */
    public double getBalance()
    {   
       return balance;
    }
 
    private double balance;
 }