You can save byte array in this way:

  FileOutputStream fos = new FileOutputStream(strFilePath);
  String strContent = "Content";

  /*
   * To write byte array to a file, use
   * void write(byte[] bArray) method of Java FileOutputStream class.
   *
   * This method writes given byte array to a file.
   */

   fos.write(strContent.getBytes());

  /*
   * Close FileOutputStream using,
   * void close() method of Java FileOutputStream class.
   *
   */

   fos.close();

It will create a file with your byte array

Just write the byte[] to a FileOutputStream pointing to the file:

private static void writeBytesToFile(byte[] b, String f) {

    try (FileOutputStream out = new FileOutputStream(f)){
        out.write(b);
    }

    catch (IOException e) {
        e.printStackTrace();
    }          
}

You could write custom encoder and decoder e.g. by mapping String to byte[] with String.getBytes() and then writing each byte with OutputStream.write() method. The question is what do you mean by "the best way" because:

1. Do you need to support different locales? What is the String locale as getBytes() uses platform's default charset.
2. Do you need to compress the binary data to save space as String might be either very long or very short.
3. Do you plan to share the binary files with other applications? If so it's best to use a format supported in few languages e.g. Google Protobuf.

If the files are ever to be read outside of your application then Protobuf seems like a sane choice. It would however be an overkill for a Hello World application.

The simpliest approach would be to use ObjectOutputStream and ObjectInputStream but it's far from pretty:

javaCopyString fileName = "data.set";

try (ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(fileName))) {
  for (int n : number) {
    out.writeInt(n);
  }
  for (String n : name) {
    out.writeUTF(n);
  }
  for (int a : age) {
    out.writeInt(a);
  }
}

try (ObjectInputStream in = new ObjectInputStream(new FileInputStream(fileName))) {
  for (int i = 0; i < number.length; i++) {
    number[i] = in.readInt();
  }
  for (int i = 0; i < name.length; i++) {
    name[i] = in.readUTF();
  }
  for (int i = 0; i < age.length; i++) {
    age[i] = in.readInt();
  }
}

Use Apache Commons IO

FileUtils.writeByteArrayToFile(new File("pathname"), myByteArray)

Or, if you insist on making work for yourself...

try (FileOutputStream fos = new FileOutputStream("pathname")) {
   fos.write(myByteArray);
   //fos.close(); There is no more need for this line since you had created the instance of "fos" inside the try. And this will automatically close the OutputStream
}

Use Integer#toBinaryString():

byte b1 = (byte) 129;
String s1 = String.format("%8s", Integer.toBinaryString(b1 & 0xFF)).replace(' ', '0');
System.out.println(s1); // 10000001

byte b2 = (byte) 2;
String s2 = String.format("%8s", Integer.toBinaryString(b2 & 0xFF)).replace(' ', '0');
System.out.println(s2); // 00000010

DEMO.

You can use FileOutputStream#write(byte[] b).