此代码保持格式化文件而不是写入文件。我如何解决它？

Question

我在这里声明了一个数组和变量。 productList 数组、名称、价格和数量

import java.io.*;
import java.util.*;

public class ReadingAndWritting {

  public String name;
  public double price;
  public int number;
  public ReadingAndWritting[] productList = new ReadingAndWritting[3];

  public ReadingAndWritting() {

  }

  public ReadingAndWritting(String name, double price, int number) {

    this.name = name;
    this.price = price;
    this.number = number;

  }

  public void printContents() {

    int i = 0;

    try {

      FileReader fl = new FileReader("Product List.txt");

      Scanner scn = new Scanner(fl);

      while (scn.hasNext()) {

        String productName = scn.next();

        double productPrice = scn.nextDouble();

        int productAmount = scn.nextInt();

        System.out.println(productName + " is " + productPrice + " pula. There are " + productAmount + " items left in stalk.");

        productList[i] = new ReadingAndWritting(productName, productPrice, productAmount);

        i = i + 1;

      }

      scn.close();

    } catch (IOException exc) {

      exc.printStackTrace();

    } catch (Exception exc) {

      exc.printStackTrace();

    }

  }

  public void writeContents() {

    try {

      //FileOutputStream formater = new FileOutputStream("Product List.txt",true);

      Formatter writer = new Formatter(new FileOutputStream("Product List.txt", true));

      for (int i = 0; i < productList.length; ++i) {

        writer.format(name, (price + 100.0), (number - 1), "\n");

      }

      writer.close();

    } catch (IOException exc) {

      exc.printStackTrace();

    }

  }

  public static void main(String[] args) {

    ReadingAndWritting obj = new ReadingAndWritting();

    System.out.println("_____THIS IS THE BEGINNING OF THE PRODUCT LIST_____");

    obj.printContents();

    System.out.println("_____THIS IS THE END OF THE PRODUCT LIST_____");

    System.out.println("_____THIS IS THE BEGINNING OF THE PRODUCT LIST_____");

    obj.writeContents();

    obj.printContents();

    System.out.println("_____THIS IS THE END OF THE PRODUCT LIST_____");

  }

}

每次我运行代码都会格式化现有文件，然后报告 NullPointerException。我完全不知道如何解决这个问题，我们的讲师也从未涉及过。请

the productList[] array has name,price and number value contained in each identifier. I want to wipe the contents of the existing file and then write with the new values to update contents from the array

Answer 1

您正在使用的构造函数截断了输入文件，Javadoc 中明确指出：

fileName The name of the file to use as the destination of this formatter. If the file exists then it will be truncated to zero size; otherwise, a new file will be created. The output will be written to the file and is buffered.

您可以使用其他构造函数，例如 Formatter(OutputStream os) 来避免截断。

Formatter writer = new Formatter(new FileOutputStream("Product List.txt",true));

至于NullPointerException，看到你的评论：

 public String name;

name没有初始化，所以默认是null，这就导致了NullPointerException.

P.S。我不知道 productList 是什么，根据它的名称和你正在迭代它的事实，也许你应该从该列表的元素中获取 price 和 name .

编辑：

根据您的其余代码，您应该从 productList 数组中获取值：

  for (int i = 0; i < productList.length; ++i) {

    writer.format(productList[i].name, (productList[i].price + 100.0), (productList[i].number - 1), "\n");

  }

Answer 2

writer.format(name, (price + 100.0), (number - 1), "\n");

参数不正确。由于它给出了 NullPointerException。阅读 Java 文档以了解应该传递的正确参数。

做这样的事情：

public String str=null; //declare class variable str.

在 printContents() 方法中，将以下内容分配给字符串：

str = "\n"+productName +" "+ (productPrice + 100.0)+" "+ (productAmount -   1)+"\n";

最后是方法，改变传递给 writer.format() 方法的参数，如下所示：

Formatter writer = new Formatter(new FileOutputStream("Product List.txt", true));

  for (int i = 0; i < productList.length; ++i) {

    writer.format(str);
  }

这对我有用。您想要的值已正确附加到文件 "ProductList.txt".

此代码保持格式化文件而不是写入文件。我如何解决它？

This code keeps formatting the file instead of writing to it. How do i fix it?

java

arrays

formatter

java-io