(Java) Write decimal/hex to a file, and not string

Question

If I have a file, and I want to literally write '42' to it (the value, not the string), which for example is 2a in hex, how do I do it? I want to be able to use something like outfile.write(42) or outfile.write(2a) and not write the string to the file.

(I realize this is a simple question but I can't find the answer of google, probably because I don't know the correct search terms)

Answer 1

For writing binary data you'll want to use a OutputStream (such as a FileOutputStream).

If you find that your data is written as strings, then you're probably using a Writer (such as a FileWriter or a OutputStreamWriter wrapped around a FileOutputStream). Everything named "*Writer" or "*Reader" deals exclusively with text/Strings. You'll want to avoid those if you want to write binary data.

If you want to write different data types (and not just plain bytes), then you'll want to look into the DataOutputStream.

Answer 2

There you go http://java.sun.com/javase/6/docs/api/java/io/DataOutputStream.html#writeInt%28int%29

Answer 3

If you just want to write bytes, the following will suffice:

import java.io.*;
...
OutputStream out = new FileOutputStream("MyFile");
try {
    // Write one byte ...
    out.write((byte) 42);
    // Write multiple bytes ...
    byte[] bytes = ...
    int nosWritten = out.write(bytes, 0, bytes.length);

} finally {
    out.close();
}

Exception handling is left as an exercise for the reader :-)

Answer 4

    OutputStream os = new FileOutputStream(fileName);
    String text = "42";
    byte value = Byte.parseByte(text);
    os.write(value);
    os.close();