Java Cookbook

Binary Data

Problem

You need to read or write binary data, as opposed to text.

Solution

Use a DataInputStream or DataOutputStream.

Discussion

The Stream classes have been in Java since the JDK 1.0 release and are optimal for reading/writing bytes, rather than characters. The “data” layer over them, comprising DataInputStream and DataOutputStream, are configured for reading and writing binary values, including all of Java’s built-in types. Suppose that you want to write a binary integer plus a binary floating-point value into a file and read it back later. This code shows the writing part:

import java.io.*;
/** Write some data in binary. */
public class WriteBinary {
    public static void main(String argv[]) throws IOException {
        int i = 42;
        double d = Math.PI;
        String FILENAME = "binary.dat";
        DataOutputStream os = new DataOutputStream(
            new FileOutputStream(FILENAME));
        os.writeInt(i);
        os.writeDouble(d);
        os.close(  );
        System.out.println("Wrote " + i + ", " + d + " to file " + FILENAME);
    }
}

The reading part is left as an exercise for the reader. Should you need to write all the fields from an object, you should probably use an ObjectDataStream ; see Section 9.17.