NullPointerException

[Feng]

BufferedReader br = null;
        try {  
        	String inputFileLine = JOptionPane.showInputDialog("Insert text file: ");
        	while((inputFileLine = br.readLine()) != null) {

What am i doing wrong? It says Exception in thread "main" java.lang.NullPointerException due to the line

while((inputFileLine = br.readLine()) != null) {

[Nickk]

Well, you need to instatiate the BufferedReader.

br = new BufferedReader(new FileReader(filename));

See Read from a file using a BufferedReader - A Java Code Example

[Feng]

thanks man!

[hardwired]

import java.io.*;
import javax.swing.JOptionPane;

public class ExpTest {
    public static void main(String[] args) {
        BufferedReader br = null;
        try {  
            String inputFileLine = JOptionPane.showInputDialog("Insert text file: ");
            // Do we have a file to read?
            if(!new File(inputFileLine).exists()) {
                System.out.println("Can not find " + inputFileLine);
            }
            System.out.println("Attempting to read " + inputFileLine);
            // Do we have a BufferedReader to which "br" is pointing?
            // In other words, has "br" been instantiated which means
            // is it pointing to an object that has been created with
            // the "new" operator? Let's see:
            System.out.println("br = " + br);
            // Instantiate "br":
            br = new BufferedReader(
                 new InputStreamReader(
                 new FileInputStream(inputFileLine)));
            // Do we have a BufferedReader to which "br" is pointing?
            System.out.println("br = " + br);
            while((inputFileLine = br.readLine()) != null) {
                System.out.println(inputFileLine);
            }
            br.close();
        } catch(IOException e) {
            System.out.println("Read error: " + e.getMessage());
        }
    }
}

[Feng]

@hardwired

a)why we say

 BufferedReader br=null;

before the try? and generally why we (need to) write it?

b)you typed

br = new BufferedReader(
                 new InputStreamReader(
                 new FileInputStream(inputFileLine)));

why you didnt type

BufferedReader br=new BufferedReader(new FileReader (inputFileLine));

?

[staykovmarin]

If you read the API docs, you would see that BufferedReader took a Reader in its constructor. InputStreamReader is a subclass of Reader and FileReader is a subclass of InputStreamReader. That means that Reader is a superclass for both InputStreamReader and FileReader.

Simply put, it doesn't matter which one he puts in there, but in InputStreamReader he has to specifically open an InputStream to the file, while with FileReader, the file is already opened.