BufferedReader中的NullPointerException

Question

I'm trying to use BufferedReader to import strings from a .txt file into an Arraylist , then using a random method to randomly pick a string inside the Arraylist .

But whenever I run this code, it gives me a java.lang.NullPointerException .

What should I do to fix this problem? Thank you in advance.

java.lang.NullPointerException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)

the .txt file in question consists of a few lines of words.

import java.util.ArrayList;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.FileReader;
import java.util.ArrayList;
import java.util.Random;

public class WordList{

    private static ArrayList<String> words =new ArrayList<String>();


    public void main(String[] args) throws IOException {
        ArrayListCon("Majors.txt");
        System.out.println(words);
    }

    private void ArrayListCon(String filename) throws IOException{
        String line;
        BufferedReader br = null; 
                 br = new BufferedReader(new FileReader(filename));

        while (( line = br.readLine()) != null){
            words.add(line);      
        }
        br.close();
    }

    public static String getRandomWord(){
        Random r = new Random();
        String randomWord = words.get(r.nextInt(words.size()));
        return randomWord;
    }
}

Answer 1

After making the following changes, your code worked perfectly for me, and i never saw the null pointer exception error.

1 ) I first made the method main static, as I was getting an error that there was no main method found:

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

2 ) I also made the ArrayListCon method static

private static void ArrayListCon(String filename) throws IOException{

3 ) I made a file called Majors.txt with the contents:

hello
hi
there
my
words
are
cool

4 ) Finally, I just compiled and ran the program, with the following output:

javac WordList.java
java WordList
[hello, hi, there, my, words, are, cool]

I believe the issue is coming up with how you are running the code (edu.rice.cs.drjava.model.compiler.JavacCompiler)

Answer 2

The exception is arising as a result of a bug in both your code and in the DrJava code.

In your code, you need to make your main method static.

In the DrJava code, they need to add a check for Modifier.isStatic(m.getModifiers()) in the JavacCompiler.runCommand method.