How to use ByteArrayOutputStream and DataOutputStream simultaneously in 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.

Answer 2

Could you make a variable to hold on to the ByteArrayOutputStream and pass it into the DataOutputStream.

ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream dos = new DataOutputStream(baos);
dos.writeInt(1);
byte[] result = dos.toByteArray();

Answer 3

Use the former case - wrap DataOutputStream around the ByteArrayOutputStream . Just make sure you save the reference to the ByteArrayOutputStream . When you are finished, close() or at least flush() the DataOutputStream and then use the toByteArray method of the ByteArrayOutputStream .

Answer 4

You could use a stream approach if you connect your outputstream to an inputstream through a PipedInputStream / PipetOutputStream . Then you will consume the data from the inputstream.

Anyway if what you need to do is simple and doesn't not require a stream approach I would use a java.nio.ByteBuffer on which you have

• put(byte[] src) for your byte[]
• putInt(int value)
• and byte[] array() to get the content

Answer 5

You don´t need more like this

Example exampleExample = method(example); 
ByteArrayOutputStream baos = new ByteArrayOutputStream(); marshaller.marshal(exampleExample , baos);
Message message = MessageBuilder.withBody(baos.toByteArray()).build();

Answer 6

The Integer class has a method to get the byte value of an int. Integer.byteValue()