Java, need a while loop to reach eof. i.e.while !eof, keep parsing

Question

I currently have a working parser. It parses a file once(not what I want it to do) and then outputs parsed data into a file. I need it to keep parsing and appending to the same output file until the end of the input file. Looks something like this.

try {
// my code parsing the data and appending to eof of output. (works)
}
catch (EOFException eof){
}

Everything is done except the while loop. It only parses once when I need it to keep parsing. I'm looking for a while loop function to reach eof.

I'm also using a DataInputStream. Is there some sort of DataInputStream.hasNext function?

DataInputStream dis = new DataInputStream(new FileInputStream(inFile));
i.e. dis.read();

.

//Need a while !eof while loop
try {
// my code parsing the data and appending to eof of output. (works)
}
catch (EOFException eof){
}

FThompson · Accepted Answer · 2020-09-05T07:39:20.567

6

Warning: This answer is incorrect. See the comments for explanation.

Instead of looping until an EOFException is thrown, you could take a much cleaner approach, and use available().

DataInputStream dis = new DataInputStream(new FileInputStream(inFile));
while (dis.available() > 0) {
    // read and use data
}

Alternatively, if you choose to take the EOF approach, you would want to set a boolean upon the exception being caught, and use that boolean in your loop, but I do not recommend it:

DataInputStream dis = new DataInputStream(new FileInputStream(inFile));
boolean eof = false;
while (!eof) {
    try {
        // read and use data
    } catch (EOFException e) {
        eof = true;
    }
}

edited Sep 05 '20 at 07:39

answered Jun 05 '13 at 16:39

FThompson

28,352
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12

Although this is an old answer, I'd like to comment that `available()` does not tell you that you reached end-of-file. It merely tells you how many bytes you can read before being blocked (e.g. waiting for the next network packet). Thus the above recommended method is plain wrong. – RealSkeptic Apr 06 '17 at 14:07
@RealSkeptic While this isn't the right approach for reaching network packets, is there any case in which reading from a FIS will be blocked before reading EOF? – FThompson Apr 06 '17 at 18:05
1

If your filesystem is network based, it definitely can. Or if you're reading from a fifo. Or from /dev/random. It's simply wrong to ignore the contract, and such bugs are hard to track down. – RealSkeptic Apr 06 '17 at 21:22

Ravi K Thapliyal · Answer 2 · 2013-06-05T16:54:40.830

3

DataInputStream has a lot of readXXX() methods that do throw EOFException but the method that you're using DataInputStream.read() does not throw EOFException.

To correctly identify the EOF while using read() implement your while loop as follows

int read = 0;
byte[] b = new byte[1024];
while ((read = dis.read(b)) != -1) { // returns numOfBytesRead or -1 at EOF
  // parse, or write to output stream as
  dos.write(b, 0, read); // (byte[], offset, numOfBytesToWrite)
}

edited Jun 05 '13 at 16:54

answered Jun 05 '13 at 16:49

Ravi K Thapliyal

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That works but then it's pointless to have wrapped the original stream with a DataInputStream! – Dave Moten May 16 '19 at 20:39

score 0 · Answer 3 · answered Jul 10 '17 at 21:18

If you are using FileInputStream, here's an EOF method for a class that has a FileInputStream member called fis.

public boolean isEOF() 
{ 
    try { return fis.getChannel().position() >= fis.getChannel().size()-1; } 
    catch (IOException e) { return true; } 
}

Java, need a while loop to reach eof. i.e.while !eof, keep parsing

3 Answers3

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