CODEWITHSUNDEEP
javafilepathbufferedreader
u3l
u3l

Reputation: 3412

NullPointerException when trying to read a file line by line (Java)?

I'm trying to read a file line by line, but every time I run my program I get a NullPointerException at the line spaceIndex = line.indexOf(" "); which obviously means that line is null. HOWEVER. I know for a fact that the file I'm using has exactly 7 lines (even if I print the value of numLines, I get the value 7. And yet I still get a nullpointerexception when I try to read a line into my string.

// File file = some File I take in after clicking a JButton

Charset charset = Charset.forName("US-ASCII");
try (BufferedReader reader = Files.newBufferedReader(file.toPath(), charset)) {

    String line = "";

    int spaceIndex;
    int numLines = 0;
    while(reader.readLine()!=null) numLines++;

    for(int i = 0; i<numLines; i++) {
        line = reader.readLine();
        spaceIndex = line.indexOf(" ");
        System.out.println(spaceIndex);
}

PS: (I'm not actually using this code to print the index of the space, I replaced the code in my loop since there's a lot of it and it would make it longer to read)

If i'm going about reading the lines the wrong way, it would be great if someone could suggest another way, since so far every way I've tried gives me the same exception. Thanks.

Upvotes: 1

Views: 8530

Answers (2)

But I&#39;m Not A Wrapper Class
But I&#39;m Not A Wrapper Class

Reputation: 14276

You have two options.

First, you could read number of lines from a file this way:

LineNumberReader  lnr = new LineNumberReader(new FileReader(new File("File1")));
lnr.skip(Long.MAX_VALUE);
System.out.println(lnr.getLineNumber());

Then read the file right after:

while((line = reader.readLine())!=null)
{
    spaceIndex = line.indexOf(" ");
    System.out.println(spaceIndex);
}

This first option is an alternative (and in my my opinion, cooler) way of doing this.

Second option (and probably the more sensible) is to do it all at once in the while loop:

while((line = reader.readLine())!=null) 
{
    numLines++;
    spaceIndex = line.indexOf(" ");
    System.out.println(spaceIndex);
}

Upvotes: 1

SLaks
SLaks

Reputation: 887365

By the time you start your for loop, the reader is already at the end of the file (from the while loop).

Therefore, readLine() will always return null.

You should get rid of the for loop and do all of your work in the while loop as you first read the file.

Upvotes: 5

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