Question

Following is a snippet that throws java.lang.NullPointerException.

else if(jRadioButton2.isSelected()) {
             // chrome selected
             String chrome_count_S="0";
             int chrome_count_I=0;
             FileWriter writer = new FileWriter("D:\\UnderTest\\MyFirstPoll\\build\\classes\\poll_count\\Chrome.txt");
             FileReader reader = new FileReader("D:\\UnderTest\\MyFirstPoll\\build\\classes\\poll_count\\Chrome.txt");
             BufferedReader br = new BufferedReader(reader);
             while((chrome_count_S = br.readLine()) != null) {
                  chrome_count_I = Integer.parseInt(chrome_count_S);
                  chrome_count_I++;
                  chrome_count_S = Integer.toString(chrome_count_I);
             }
             writer.write(chrome_count_S);
             writer.close();

When this snippet is encountered NullPointerException is thrown. If I replace the argument of writer.write(chrome_count_S); to writer.write("chrome_count_S"); I.E. a String, I don’t get any exception. Otherwise why do I get the exception when I have initialized the string chrome_count_S?

Answer 1

The while loop stops when the readline() is null and writes the current value to the variable chrome_count_S.

while((chrome_count_S = br.readLine()) != null)

So chrome_count_S is be null after the loop and at the the write command.

=== UPDATE ===

Remove the chrome_count_S row in the loop and take the value from the chrome_count_I during writing:

while((chrome_count_S = br.readLine()) != null) {
    chrome_count_I = Integer.parseInt(chrome_count_S);
    chrome_count_I++;
}
writer.write(Integer.toString(chrome_count_I));