How to fix Exception in thread "main" java.lang.NullPointerException - exception

Question

I have this error when I Run the code but no when I Compile it.
Exception in thread "main" java.lang.NullPointerException
at java.util.StringTokenizer.<init>(StringTokenizer.java:199)
at java.util.StringTokenizer.<init>(StringTokenizer.java:221)
at WillinsLaMarkusFileInputOutput.main(WillinsLaMarkusFileInputOutput.java:23)
Here is my complete code
import java.io.*;
public class WillinsLaMarkusFileInputOutput {
public static void main(String[] args) throws IOException {
/* open the files */
// Scanner sc = new Scanner(System.in);
BufferedReader r = new BufferedReader(new FileReader("input.txt"));
BufferedWriter w = new BufferedWriter(new FileWriter("output.txt"));
float[] values = new float[10];
String str = r.readLine();
int i = 0;
float sum = 0.0f, avg = 0.0f;
/* tokenize the string into floats separated by spaces */
java.util.StringTokenizer tk = new java.util.StringTokenizer(str, " ");
while (tk.hasMoreElements()) {
values[i] = Float.valueOf(tk.nextToken()).floatValue();
/* compute sum */
sum += values[i];
i++;
}
/* calculate average */
avg = sum / 10.0f;
/* write results to output.txt */
w.write("Sum: " + sum);
w.newLine();
w.write("Average: " + avg);
w.flush();
/* close the files */
r.close();
w.close();
}
}
Any suggestions on how to fix this ?

Answer 1

The exception goes from this line:
java.util.StringTokenizer tk = new java.util.StringTokenizer(str, " ");
So first let's read the documentation when this constructor may throw NullPointerException:
public StringTokenizer(String str, String delim)
Throws:
NullPointerException - if str is null
Fine. It means that str is null. The str is assigned in this line:
String str = r.readLine();
Now you should read the documentation when r.readLine() may return null:
Returns:
A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached
So it signals that end of file is reached. As it's the very first attempt to read something from this file it seems that you are reading an empty file.
In general careful documentation reading may greatly help to understand why your program doesn't work.