Learning Java - Chapter 9 : Java

We have seen that Java is very strict with data types. Every data element must be explicitly assigned to a specific type. Casting can convert data of one type into another but a data value can never simultaneously act as more than one type. This differs from C/C++ where, say, a 32-bit integer type value can easily be accessed as two short values or four separate bytes.

For binary I/O with files as discussed previously, the underlying flow of data occurs as a sequence of bytes. We saw that by wrapping FileOutputStream with a DataOutputStream we obtained methods such as writeInt (int) and writeDouble (double) that put the bytes of wider types like int or double onto the outgoing stream.

Similarly, by wrapping a FileInputStream with a DataInputStream, the readInt() and readDouble() methods convert four bytes in the stream into an int value and eight bytes into a double value, respectively.

Note that on all platforms, Java uses the so-called big-endian representation. This means that for any primitive data value longer than one byte, the most significant byte is at the lowest address (i.e. big end first.) So, for example, in an output stream of int values, the high order byte goes out first and in an input stream the high order byte arrives first. This order is preserved by the JVM even on underlying platforms (such as Intel's) that use the reverse representation known as little-endian.

What if you obtain an array of bytes and need to extract data values of different types from it. For example, a byte array of 19 bytes might contain one char type, two int values, one double value, and one byte type. In C/C++ you can use memory pointers to reference different parts of the array and cast a group of bytes there to a particular type. In Java that type of direct memory referencing is not allowed.

Instead, you can make the array the source of a stream with the ByteArrayInputStream class. You then wrap this stream with DataInputStream and use its selection of methods for reading different data types. For example, to extract our data from the 19 elements of a byte array, we could use the following:

Conversely, you can send a stream of data of different types to a ByteArrayOutputStream and then extract its internal byte array. It is convenient to wrap this stream with DataOutputStream, which provides a selection of methods for different types of such as writeInt (int), writeDouble (double), and writeChars().

In Chapter 9: Tech we use these techniques for I/O with our histogram classes where we want to save a mix of data types into one big byte array. Also, in Chapter 23 we discuss serial I/O with a Java hardware processor that requires low level I/O with bytes. In an example program, we use a byte array streams to convert the a set of data bytes to wider types.