尝试使用File类加载文件时出现空指针异常

Question

I have 2 files Details.java and TestDetails.java and a data file called Details.dat

Details.java

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

public class Details {
    private String path;
    File myFile = new File(path);

    public void setMyFile(String path) {
        this.path = path;

    }

    public void load() throws IOException, FileNotFoundException {
        Scanner input = new Scanner(myFile);

        int numberOfMembers = input.nextInt();

        String[] members = new String[numberOfMembers];

        for (String s : members) {

            String name = input.next();

            String age = input.next();

            String qualification = input.next();

            System.out.println("The name of the family member is " + name + " the age of the family member is" + age
                    + " the qualification of the " + "family member is" + qualification);

        }
    }
}

TestDetails.java

import java.io.IOException;

public class TestDetails {

    public static void main(String[] args) {

        Details myDetails = new Details();

        myDetails.setMyFile(args[0]);

        try {
            myDetails.load();
        } catch (IOException i) {

            System.out.println(i.getMessage());
        }
    }
}

Details.dat

4

a 26 bsc

b 22 bcom

c 50 ba

d 60 bsc

Whenever i try to run the TestDetails.java file i get a NullPointerException and the stack trace points the stack trace towards the File object.

So what is the problem here? Why am i getting a NullPointerException?

ps in the setFile() method argumnet, i pass in Details.dat in the args[0] position on the command prompt

Answer 1

The problem is this:

public class Details{
    private String path;
    File myFile = new File(path);
    ...

The File myFile = new File(path) line will be executed when the object is constructed. This means that path is null at the time this line is executed.

You should change your code so that the File object is instantiated only when you need it.

Answer 2

You initialize the File first and after that set the file path. Try to use a constructor in the Details class:

public Details(String path)
        this.path = path;
        myFile = new File(path);
}

Answer 3

public class Details{
   private  String path;
   File myFile =new File(path);
   ...

How is myFile ever going have its path set to anything other than "" ?

Answer 4

You are trying to make a File object with path name without initializing path. Because

File myFile = new File(path); 

line will be executed before setting path and at that time it is null. So first set value of path and then make object of File.