继续显示:"java.lang.NullPointerException"

Question

我想从文本文件中读取内容,然后将其设置为car class中的一些变量.但它一直向我显示"java.lang.NullPointerException".我不知道它有什么问题.有人能告诉我该怎么办？

错误行是 cars[0].setRegion(tokens[2]);

这是文本文件.

CarInLot KLM456 ND Meter4 120

CarInLot VMK123 ME移动0

CarInLotDKC003 WA Meter5 30

仪表1无10

CarInLot IML84U ND Meter6 800

这是测试类.

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


public class test
{
   public static void main(String[] args) throws IOException
   {

      // Get the filename.

      String filename = "input.txt";

      // Open the file.
      File file = new File(filename);

      Scanner inputFile = new Scanner(file);

      Car[] cars = new Car[4];


      while (inputFile.hasNext())
      {

         String filecotent = inputFile.nextLine();

         String[] tokens = filecotent.split(" ");

         if(filecotent.startsWith("CarInLot")){

            cars[0].setRegion(tokens[2]);

            cars[0].setMinutes(Integer.parseInt(tokens[4]));
         }

         if(filecotent.startsWith("Meter")){

            cars[0].setPlate(tokens[1]);      
         }


      }
       System.out.println(cars[0].toString());

      // Close the file.
      inputFile.close();
   }


}

这是汽车类.

public class Car {

    private String plate;
    private String region;
    private int minutes;

    public Car(String carPlate, String carRegion, 
               int carMinutes) {

        plate = carPlate;
        region = carRegion;
        minutes = carMinutes;
    }


    public Car(Car object2) {

        plate = object2.plate;
        region = object2.region;
        minutes = object2.minutes;
    }

    public void setPlate(String pl) {
        plate = pl;
    }

    public void setRegion(String re) {
        region = re;
    }

    public void setMinutes(int mi) {
        minutes = mi;
    }


    public String getPlate() {
        return plate;
    }

    public String getRegion() {
        return region;
    }

    public int getMinutes() {
        return minutes;
    }

    public String toString() {

        String string = "Car's information: " 
                        + "\n"
                        + "\nLicense Plate:    " + plate
                        + "\nLicense Plate Resgistration Region:    " + region
                        + "\nParked time" + minutes
                        + "\n";

        return string;

    }


}

Answer 1

所以你有这个代码

Car[] cars = new Car[4];

while (inputFile.hasNext())
{
    String filecotent = inputFile.nextLine();

    String[] tokens = filecotent.split(" ");

    if(filecotent.startsWith("CarInLot")){
        cars[0].setRegion(tokens[2]);
        cars[0].setMinutes(Integer.parseInt(tokens[4]));
    }
    ...

cars已初始化,但其中的元素不是.你需要先将它们初始化,否则它们就是null你得到的NullPointerException.

cars[someIndex] = new Car(...);

此外,您现在拥有代码的方式,您将始终覆盖Car数组中的相同引用,即.索引为0的那个.您可能希望使用递增索引来初始化每个元素.