Splitting a Byte Array on A Particular Byte

Question

I am trying to read an old .dat file byte by byte, and have run into an issue: a record is terminated by \n (newline). I'd like to read in the whole byte array, then split it on the character.

I can do this by reading the whole byte array from the file, creating a String with the contents of the byte array, then calling String.split(), but find this to be inefficient. I'd rather split the byte array directly if possible.

Can anyone assist?

Update: Code was requested.

public class NgcReader {

public static void main(String[] args) {

    String location;
    if (System.getProperty("os.name").contains("Windows")) {
        location = "F:\\Programming\\Projects\\readngc\\src\\main\\java\\ngcreader\\catalog.dat";
    } else {
        location = "/media/My Passport/Programming/Projects/readngc/src/main/java/ngcreader/catalog.dat";
    }

    File file = new File(location);

    InputStream is = null;
    try {
        is = new FileInputStream(file);
    } catch (FileNotFoundException e) {
        System.out.println("It didn't work!");
        System.exit(0);
    }

    byte[] fileByteArray = new byte[(int) file.length() - 1];

    try {
        is.read(fileByteArray);
        is.close();
    } catch (IOException e) {
        System.out.println("IOException!");
        System.exit(0);
    }

    // I do NOT like this. I'd rather split the byte array on the \n character
    String bigString = new String(fileByteArray);
    List<String> stringList = Arrays.asList(bigString.split("\\n"));
    for (String record : stringList) {
        System.out.print("Catalog number: " + record.substring(1, 6));
        System.out.print(" Catalog type: " + record.substring(7, 9));
        System.out.print(" Right Ascension: " + record.substring(10, 12) + "h " + record.substring(13, 17) + "min");
        System.out.print(" Declination: " + record.substring(18, 21) + " " + record.substring(22, 24));
        if (record.length() > 50) {
            System.out.print(" Magnitude: " + record.substring(47, 51));
        }

        if (record.length() > 93) {
            System.out.print(" Original Notes: " + record.substring(54,93));
        }

        if (record.length() > 150) {
            System.out.print(" Palomar Notes: " + record.substring(95,150));
        }
        if (record.length() > 151) {
            System.out.print(" Notes: " + record.substring(152));
        }
        System.out.println();
    }

}

Another Update: Here's a README with a description of the file I'm processing:

http://cdsarc.u-strasbg.fr/viz-bin/Cat?VII/1B

It's not clear whether this is a text file, in which case you should load it as text, or a binary file, in which case you shouldn't be talking about characters. — Jon Skeet
– Jon Skeet, Commented Oct 4, 2011 at 13:34
yes, code! Also, is this .dat file a Text file or binary file? — Pintobean77
– Pintobean77, Commented Oct 4, 2011 at 13:35
The file is a binary file, but the bytes translate directly to ASCII. — Jason
– Jason, Commented Oct 4, 2011 at 13:37

Jon Skeet · Accepted Answer · 2011-10-04 13:42:10Z

2

It sounds like this might actually just be a text file to start with, in which case:

InputStream stream = new FileInputStream(location);
try {
    BufferedReader reader = new BufferedReader(new InputStreamReader(stream,
                                                                     "ASCII"));
    String line;
    while ((line = reader.readLine()) != null) {
        // Handle the line, ideally in a separate method
    }
} finally {
    stream.close();
}

This way you never need to have more than a single line of the file in memory at a time.

answered Oct 4, 2011 at 13:42

Jon Skeet

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1 Comment

Jason Over a year ago

Yes, that was it. The description in the text file described it byte by byte, so I was trying to process it that way. Stupid of me.

ratchet freak · Accepted Answer · 2011-10-04 13:50:25Z

if you're set on using byte arrays...

byte[] buff = new byte[1024];//smaller buffer

try {
    int ind=0,from=0,read;
    while((read=is.read(buff,ind,buff.length-ind))!=-1){
        for(int i=ind;i<ind+read;i++){
            if(buff[i]=='\n'){
                string record = new String(buff,from,i+1);
                //handle
                from=i+1;
            }
        }
        System.arraycopy(buff,from,buff,0,buff.length-from);
        ind=ind+read-from;
        from=0;
    }

} catch (IOException e) {
    System.out.println("IOException!");
    //System.exit(0);
    throw RunTimeException(e);//cleaner way to die
} finally{
    is.close();
}

this also avoids loading in the entire file and it puts the close inside a finally

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