java - 将对象声明为 null 的问题

Question

这是我的代码的一部分（后面有一个 BankAccount 类）：

import java.util.Scanner; //This library is added

public class BankSystem_
{
    public static void main(String[] args)
    {
        Scanner keyboard = new Scanner(System.in);//Scanner object to read input

        BankAccount ba = null;


        //Define variables
        int task=0;
        int accountNumber=0;
        String accountName="unknown";
        double balance=0;
        double interestRate=.05;

        do
        {       
        System.out.println("1.\t Open new account");
        System.out.println("2.\t Current Balance");
        System.out.println("3.\t Deposit");
        System.out.println("4.\t Withdrawal");
        System.out.println("5.\t Change Interest Rate");
        System.out.println("6.\t Bank Statement");
        System.out.println("7.\t Exit");
        System.out.println("Please create an account and choose a task: ");
        task=keyboard.nextInt();
            switch (task)
            {
                case 1:
                    System.out.print("What is the account name?: ");
                    accountName=keyboard.nextLine();
                    System.out.print("\nWhat is the account number?: ");
                    accountNumber=keyboard.nextInt();
                    System.out.print("\nAmount of money to start your account?: ");
                    balance=keyboard.nextDouble();
                        while (balance < 100)
                        {
                            System.out.println("The minimum amount to open a new account should be $100");
                            System.out.print("Amount of money to start your account?: ");
                            balance=keyboard.nextDouble();
                        }   
                    BankAccount ba = new BankAccount(accountName,accountNumber,balance,interestRate);
                    System.out.print(ba.NewAccount());  
                    break;
                case 2:
                    if (ba != null)
                    {
                    System.out.print(ba.Balance());
                    }
                    break;
                case 3:
                    if (ba != null)
                    {
                    System.out.print(ba.Deposit());
                    }
                    break;
                case 4:
                    if (ba != null)
                    {
                    System.out.print(ba.Withdrawal());
                    }
                    break;
                case 5: 
                    if (ba != null)
                    {       
                    System.out.print(ba.ChangeInterestRate());
                    }
                    break;
                case 6:
                    if (ba != null)
                    {
                    System.out.print(ba.BankStatement());
                    }
                    break;
                case 7:
                    System.exit(0);
                    break;
            }
        }while(task >=1 && task <=7);
    }
}

*代码的目的是在选择任务1之前警告选择任务2-6的用户。所以，我必须检查BankAccount的对象是否不为空才能继续。

每当我包括：

BankAccount ba = null;

我稍后在代码中包含：

BankAccount ba = new BankAccount(accountName,accountNumber,balance,interestRate);

声明对象，编译器告诉我variable ba is already defined in method main(String[])

但是当我删除空值时，编译器告诉我：

variable ba might not have been initialized if (ba != null)

我这里有什么问题吗？

Answer 1

只是改变

BankAccount ba = new BankAccount(accountName,accountNumber,balance,interestRate);

至

ba = new BankAccount(accountName,accountNumber,balance,interestRate);

因为你想改变 ba 的内容，它作为一个变量已经在你的方法开始时被定义了。因此您不能再次声明具有相同名称的变量，您想更改已经存在的变量。

Answer 2

BankAccount ba = new BankAccount(accountName,accountNumber,balance,interestRate);

尝试重新声明ba已声明的内容。

只需将其更改为：

ba=new BankAccount(accountName,accountNumber,balance,interestRate);

此外，您应该声明BankAccount ba = null;为 just BankAccount ba;。ba在这种情况下无论如何都会为空。

Answer 3

而不是使用

BankAccount ba = new BankAccount(accountName,accountNumber,balance,interestRate);

利用 ：

ba = new BankAccount(accountName,accountNumber,balance,interestRate);`