Java Reading File

Question

i am reading csv file.and in that i have created 2 methods 1)getHeaders and 2)getContent so assuming 1 line as a header file i am take 1 line in header and storing in one variable and hence when i am reading again that file i am not getting the expected output.

public static String getHeaderLine(File file) throws IOException {
    BufferedReader reader = new BufferedReader(new FileReader(file));
    String headerLine = reader.readLine();
    return headerLine;  
}

public void convertCSVtoJSON() throws IOException {
    File f = new File("C:\\user\\Desktop\\Abc.csv");
    String headerLine = CSVtoJSON.getHeaderLine(f);
    System.out.println(getHeaderLine(f));

    BufferedReader in = new BufferedReader(new FileReader(f));
    while (in.readLine() != null) {
        System.out.println(in.readLine());
    }
}

Abc.csv:

name,add,sal
john,SF,25k
Roy,Chicago,30k
jason,SF,35k
joy,Sf,50K

Output:

name,add,sal
john,SF,25k
jason,SF,35k
null

Expected output:

name,add,sal
john,SF,25k
Roy,Chicago,30k
jason,SF,35k
joy,Sf,50K

Please don't use File anymore. Instead use the new modern NIO API of Java which revolves around the classes Paths, Files and Path. With it you'll shrink your method to 1-2 lines. Also you should never forget to close your resources. Use reader.close() and in.close() if you are finished reading. — Zabuzard
– Zabuzard, Commented Oct 23, 2017 at 14:37
@Zabuza thanks for responding.and its been verified code you can also check by running the same..it gives the same output as i have shown.. — Murtaza Zaveri
– Murtaza Zaveri, Commented Oct 23, 2017 at 19:20

Zabuzard · Accepted Answer · 2017-10-23 20:16:53Z

Explanation

It is because you use readLine to check whether there is something to read and then again use readLine inside the loop.

So you actually read a line, reject it (inside the condition) and then read again a line. So every iteration you are reading two lines but only print one and you also only check for null with one of those two lines. That is why you see the print of null in the last iteration since there is no line left.

Considering your example:

name,add,sal     // $1
john,SF,25k      // $2
Roy,Chicago,30k  // $3
jason,SF,35k     // $4
joy,Sf,50K       // $5

And your code:

System.out.println(getHeaderLine(f));   // #1
while (in.readLine() != null) {         // #2
    System.out.println(in.readLine());  // #3
}

Line #1 reads the head line $1 and prints it.

Then you create a new BufferedReader whose pointer is placed at the start of the file again, so the head line. Line #2 is executed which again reads the head line $1 and checks it for null. It is not null and the code starts the first loop iteration.

Line #3 is executed, reading and printing the second line $2. The loop checks again the condition in line #2, it reads the next line $3 and checks it for null.

It's not and line #3 gets executed which reads and prints line $4. The condition is checked again in line #2, thus it reads line $5 and checks it for null. It is again not null thus it continues.

Again line #3 is executed which reads and prints a line ($6). However as there is no line left anymore to read, it reads and prints null. The condition is checked again, trying to read a line ($7) but there is no, the loop terminates.

All in all we have printed line $1 (due to #1) and $2, $4, null (due to #3):

name,add,sal   // $1
john,SF,25k    // $2
jason,SF,35k   // $4
null           // ($6)

Which is exactly what you got.

Solution

You need to fix your logic. If you want to check whether there is something to read, don't read it, just check for it. Or alternatively read it, store it inside a variable and then check this variable.

while (true) {
    // Read a line
    String line = in.readLine();
    if (line == null) {
        // Abort, nothing to read anymore
        break;
    }

    // Print what we have read
    System.out.println(line);
}

Modern solution: NIO

However I recommend you to not use BufferedReader and this stuff for reading files anymore. Java has updated its I/O API, we now use NIO which revolves around the classes Path, Paths and Files. With NIO you print every line elegantly and platform independent like this:

Path fileToRead = Paths.get(System.getProperty("user.home"), "Desktop", "Abc.csv");
Files.lines(fileToRead).forEach(System.out::println);

That's all. Path substitutes the old File class. Paths and Files provide utility methods. Here are their documentations:

achAmháin · Accepted Answer · 2017-10-23 14:34:30Z

1

You can use hasNext() to check if the input has a next line, then next() to print each line

Note: you should probably use a bit of try-catch in this to check the file exists etc

Scanner scanner = new Scanner(new File("C:\\user\\Desktop\\Abc.csv"));
String headerLine = scanner.next();
//this now has the first line in 'headerLine'
System.out.print(headerLine);
System.out.print(" ");
while (scanner.hasNext()) {
    System.out.print(scanner.next() + " ");
}
scanner.close();

answered Oct 23, 2017 at 14:34

achAmháin

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