Why does the data read loop read a "null" as the last element? (After the last of all item of the inputlist.) But only in some cases, and in other cases doesn't.

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A data read loop threw a nullPointerException in some cases. The input list contained numbers and strings mixedly, but I read them all as strings one after the other, into a string ArrayList:

while (true) {
    String s = reader.readLine();
    if (s.isEmpty()) break;
    list.add(s);
}

The nullPointerException is understandable: I checked the "s" elements with a System.out.println(s) command, and the last of all "s" contained a "null" after the last item of the inputlist. But why?... Therefore, the cycle had to be corrected to:

while (true) {
    String s = reader.readLine();
    if (s==null || s.isEmpty()) break;
    list.add(s);
}

This way it worked already nicely:) (without any Exception.) But the questions are: 1) Why does the program read such a "null" in some cases after the end of the inputlist? 2) Is this second cycle correct or should it still be refined?

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Guadalupe Gagnon

Level 37 , Tampa, United States

3 July 2020, 13:27

solution

Whenever a BufferedReader finishes reading to the end of a data source, such as a file, any subsequent read requests past that will return null. So if you have a file that had 5 lines in it, and you read the file (line by line) 7 times, the last 2 times would return null. The way that you handled it works fine. I use this method:

String s = "";
while ((s = reader.readline()) != null){
   if (s.isEmpty()) break;
   list.add(s);
}
// This code first calculates the value inside the parenthesis to give 's' a value
// then checks that that value does not equal 'null'
// if it does then the loop breaks.

The code progression in this code is the same as the code progression in the code you shared, so either method works just as good as the other.

Gellert Varga

Level 23 , Szekesfehervar, Hungary

3 July 2020, 14:54

Thank You very much!:)