为什么写入文件的输出为零？我的代码有什么问题？

Question

我尝试过input = nextLine（）并将输入附加到String数据但仍然返回null。这可能是什么问题？

[撤回方法]

public void Withdraw(){

    System.out.print("Enter withdrawal amount:" );
    double withdraw = input.nextDouble();
    System.out.println("Your withdrawal amount: " + withdraw);
        if(this.balance - withdraw < 100){
            System.out.println("The maintaining balance should not be less than 100. " );
        }else
    this.balance -= withdraw;
    System.out.println("Your new balance is:" + this.balance);
}

[附件文件]

public void withdrawWriter(){
try{
        String data = "Your withdrawal amount: " + withdraw;

        File file = new File(this.name + ".txt");

        //if file doesnt exists, then create it
        if(!file.exists()){
            file.createNewFile();
        }

        //true = append file
        FileWriter fileWritter = new FileWriter(file.getName(),true);
            BufferedWriter bufferWritter = new BufferedWriter(fileWritter);
            bufferWritter.write("\r\n");
            bufferWritter.write(data);
            bufferWritter.close();

        System.out.println("Done");

    }catch(IOException e){
        e.printStackTrace();
    }
}

以下是截图：

Answer 1

从屏幕截图中可以清楚地看到，您同时拥有本地变量double withdraw和类double withdraw上的字段。

如果你摆脱了本地的话，可能会工作，所以只需：

withdraw = input.nextDouble();

Answer 2

您正在接受局部变量withdraw中的输入并尝试在withdrawWriter方法中使用实例变量。尝试使用实例变量withdraw，同时接受这样的输入：

public void Withdraw(){

    System.out.print("Enter withdrawal amount:" );


**withdraw = input.nextDouble();**

    System.out.println("Your withdrawal amount: " + withdraw);
        if(this.balance - withdraw < 100){
            System.out.println("The maintaining balance should not be less than 100. " );
        }else
    this.balance -= withdraw;
    System.out.println("Your new balance is:" + this.balance);
}