Question

Here is the code I have.

This is my PrintToFile class

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

public class PrintToFile{
        File f;
        FileWriter fw;
        PrintWriter pw;

    public void PrintToFile()throws Exception{//remove void from constructor
      File f = new File ("Output.txt");//dont reinitialize 
      FileWriter fw = new FileWriter(f, true);//dont reinitialize 
     PrintWriter pw = new PrintWriter(fw);//dont reinitialize 
   }

    public void printExp(ArrayList<Expense> expList){
        for(int i = 0; i < expList.size(); i++){
         pw.println("---------------------------------------");//exception here
         pw.println(expList.get(i));
      }
        pw.close();
    }
}

in my main class here is my call to print my ArrayList

    PrintToFile printer = new PrintToFile();
    printer.printExp(expList);   

I have defined expList as an ArrayList of objects

The exception I get is a

Exception in thread "main" java.lang.NullPointerException

occuring where marked. My question is what is causing this exception? Thanks

Answer 1

You are not creating object of pw which is class field but you are creating object of pw which is local to method PrintToFile(). So by default PrintToFile.pw is null and you get NPE.

Change your method to following or initialize pw,f and fw in constructor(recommended):

public void PrintToFile() throws Exception {
      f = new File ("Output.txt");
      fw = new FileWriter(f, true);
      pw = new PrintWriter(fw);
}