The ByteArrayOutputStream class in Java provides several methods for writing data to an array of bytes. Some of the commonly used methods of the ByteArrayOutputStream class are:

1. write(int b) - Writes a single byte of data to the output stream.

2. write(byte[] b) - Writes a sequence of bytes to the output stream.

3. write(byte[] b, int off, int len) - Writes a subarray of bytes to the output stream, starting at the specified offset off, and writing up to len bytes.

4. toByteArray() - Returns the current contents of the output stream as a byte array.

5. size() - Returns the current size of the output stream.

6. reset() - Resets the output stream to the beginning, discarding any currently held data.

7. toString() - Converts the contents of the output stream to a string, using the platform's default character encoding.

It's important to note that the ByteArrayOutputStream class is not thread-safe, so you should not use it in a multithreaded environment without taking appropriate precautions.

Overall, the ByteArrayOutputStream class provides a flexible and efficient way of writing data to a byte array in Java, and the various methods it provides can be used to write data in a variety of different ways depending on your specific needs. By using the various methods of the ByteArrayOutputStream class, you can write data to byte arrays in a variety of formats, including text data, binary data, and more.