CODEWITHSUNDEEP
javajava.util.scannerfilereader
Nazarii Moshenskyi
Nazarii Moshenskyi

Reputation: 2108

How to read hex values from file into byte array?

I have a file which consists of comments (look like Java single-line comments which start with double slash, //) and space-separated hex values.

File looks like this:

//create applet instance
0x80 0xB8 0x00 0x00 0x0c 0x0a 0xa0 0x00 0x00 0x00 0x62 0x03 0x01 0xc 0x01 0x01 0x00 0x7F;

How can I convert line where hex values go from string to byte array?

I use the following approach:

List<byte[]> commands = new ArrayList<>();
Scanner fileReader = new Scanner(new FileReader(file));
while (fileReader.hasNextLine()) {
      String line = fileReader.nextLine();
      if (line.startsWith("0x")) {
          commands.add(line.getBytes());
      }
}

But definitely, this shows a byte representation of symbols as they were chars and doesn't convert it to byte. That's right. But how can it be converted properly?

Thanks in advance.

Upvotes: 2

Views: 1047

Answers (2)

mypetlion
mypetlion

Reputation: 2524

You're on the right track. Just remove the trailing ; and then use the methods provided for you in the Integer class.

while ( fileReader.hasNextLine() ) {
    String line = fileReader.nextLine();
    if ( line.startsWith( "0x" ) ) {
        line = line.replace( ";", "" );
        List<Byte> wrapped = Arrays
                .asList( line.split( " " ) )
                .stream()
                // convert all the string representations to their Int value
                .map( Integer::decode )
                // convert all the Integer values to their byte value
                .map( Integer::byteValue )
                .collect( Collectors.toList() );
        // if you're OK with changing commands to a List<Byte[]>, you can skip this step
        byte[] toAdd = new byte[wrapped.size()];
        for ( int i = 0; i < toAdd.length; i++ ) {
            toAdd[i] = wrapped.get( i );
        }
        commands.add( toAdd );
    }
}

Upvotes: 2

markspace
markspace

Reputation: 11030

Just thought I'd point out that if you loosen the specification a little bit, you can essentially do this in one line with splitAsStream.

List<Integer> out = Pattern.compile( "[\\s;]+" ).splitAsStream( line )
       .map( Integer::decode ).collect( Collectors.toList() );

I use Integer here and Integer::decode because Byte::decode will throw an error on 0x80, the OP's first input. And you'd have to do a bit more work if you really need an array of primitives, but often boxed numbers will do in practice.

Here's the whole code:

public class ScannerStream {

   static String testVector = "//create applet instance\n" +
"0x80 0xB8 0x00 0x00 0x0c 0x0a 0xa0 0x00 0x00 0x00 0x62 0x03 0x01 0xc 0x01 0x01 0x00 0x7F;";

   public static void main( String[] args ) {
      List<List<Integer>> commands = new ArrayList<>();
      Scanner fileReader = new Scanner( new StringReader( testVector ) );
      while( fileReader.hasNextLine() ) {
         String line = fileReader.nextLine();
         if( line.startsWith( "0x" ) ) {
            List<Integer> out = Pattern.compile( "[\\s;]+" ).splitAsStream( line )
                    .map( Integer::decode ).collect( Collectors.toList() );
            System.out.println( out );
            commands.add( out );
         }
      }
      System.out.println( commands );
   }
}

Upvotes: 1

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