Question

I encounter a java.lang.NullPointerException error when running my Java program, and after reading up on it I think I understand what the error means, but I’m still not sure how to fix it.

import java.util.ArrayList;
import java.io.FileInputStream;
import java.io.DataInputStream;
import java.io.BufferedReader;
import java.io.FileOutputStream;
import java.io.InputStreamReader;
import java.io.IOException;
import java.io.PrintStream;
public class readFile {


public static ArrayList<String> pullFirst(String fileName) throws IOException{
    String filen = "C:\\Users\\Steven\\Desktop\\Tests\\wunderground\\outputTweetsWeather.txt";
    ArrayList<String> arl = new ArrayList<String>(); // java 6 ArrayList 

    FileInputStream fs = new FileInputStream(filen); // this is how you access a file in java
    DataInputStream din = new DataInputStream(fs);
    BufferedReader bin = new BufferedReader(new InputStreamReader(din));

    String line;    
    while(!(line = bin.readLine()).startsWith("String index out of range")){ // read each line in the file while they exist
        arl.add(line);                // add them to the array list
    }

    return arl;                                  // return the array list
}

public static void main(String[] args, int j) {

    // this is just demo code to prove it works so you can check the output.
    try{
    ArrayList<String> rVal = pullFirst("testFile.txt");
        for(String a : rVal){
            //System.out.println(a + "\n"); // insert write to file code here
           if(a.startsWith("+t")){
       //          System.out.println(a);
           }
        }
        int i = 36;


        PrintStream out5 = new PrintStream(new FileOutputStream("forpopUp1.txt", true));

        System.setOut(out5);
     System.out.println(rVal.get(2 + j*i)); // Display Value


    }catch(Exception e){
        System.out.print("Problem in readFile" + e);
    }

}
}

Now I’m not sure if it’s to do with the fact that my main line is

public static void main(String[] args, int j) {

But, as far as I’m aware I have to have it this way, as this class is executed after an if condition from another class is ran, using the line

readFile.main(args, counterForreadFile);

This is the result of the StackTrace:

java.lang.NullPointerException
at readFile.pullFirst(readFile.java:22)
at readFile.main(readFile.java:33)

Any help would be appreciated!

Answer 1

I believe the error is when the file you read was exhausted:

While reading the file:

while(!(line = bin.readLine()).startsWith("String index out of range")){ 
  ...
}

When the file is exhausted the BufferedReader will return null.

When you do (line=bin.readLine()).startsWith(..) you actually try to invoke startsWith() on null in this cases – and thus the error.

To solve this, you should iterate as:

while ((line = bin.readLine()) != null) { 
  if (line.startsWith("String index out of range")) break; 
  //rest of the code
}

This provide you null safety when the reader is exhausted.

While it is definetly an issue – I cannot be sure it is the exact problem you are encountering until you provide a full stack trace.

---

P.S. As I already said in comments – getting the stack trace (which provides more information on the issue) you can add to your catch block, so it will be something like that:

}catch(Exception e){
    System.out.print("Problem in readFile" + e);
    e.printStackTrace();
}

An alternative is to declare main() as throws Exception, and remove the catch block (and try block as well).