how to use ByteArrayOutputStream and DataOutputStream simultaneously (Java)

Question

I'm having quite a problem here, and I think it is because I don't understand very much how I should use the API provided by Java.

I need to write an int and a byte[] into a byte[]

I thought of using a DataOutputStream to solve the data writing with writeInt(int i) and write(byte[] b), and to be able to put that into a byte array, I should use ByteArrayOutputStream method toByteArray().

I understand that this classes use the Wrapper pattern, so I had two options:

DataOutputStream w = new DataOutputStream(new ByteArrayOutputStream());

or

ByteArrayOutputStream w = new ByteArrayOutputStream(new DataOutputStream());

but in both cases, I "loose" a method. in the first case, I can't access the toByteArray() method, and in the second, I can't access the writeInt() method.

How should I use this classes together?

Answer 1

Like this:

ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream w = new DataOutputStream(baos);

w.writeInt(100);
w.write(byteArray);

w.flush();

byte[] result = baos.toByteArray();

Actually your second version will not work at all. DataOutputStream requires an actual target stream in which to write the data. You can't do new DataOutputStream(). There isn't actually any constructor like that.