Question

I need to get the byte array out of everything I send to the output stream. But instead I get 4 bytes of rubbish. Why?

ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
ObjectOutputStream objectOutputStream = new ObjectOutputStream(byteArrayOutputStream);
objectOutputStream.write(new byte[]{1,2,3,4,5,6,7,8,9});
byte[] original = byteArrayOutputStream.toByteArray(); 
System.out.println(Arrays.toString(original)); // why not [1,2,3,4,5,6,7,8,9]?

Answer 1

There are several flaws in your code. First of all you should use writeObject():

objectOutputStream.writeObject(new byte[]{1,2,3,4,5,6,7,8,9});

then you should use symmetric ObjectInputStream for reading:

final ObjectInputStream objectInputStream = new ObjectInputStream(
        new ByteArrayInputStream(
                byteArrayOutputStream.toByteArray()
        )
);
byte[] original = (byte[]) objectInputStream.readObject();

However if you already have a byte[], there is no point in using Java serialization to convert it to byte array (which it already is!) Just write and read it directly:

ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
byteArrayOutputStream.write(new byte[]{1, 2, 3, 4, 5, 6, 7, 8, 9});
final ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(
    byteArrayOutputStream.toByteArray()
);
final byte[] original = new byte[9];
byteArrayInputStream.read(original);
System.out.println(Arrays.toString(original));