Writing Files - Beginning Java 2 SDK - page 391

The buffer's capacity will be array.length and the position is set to the value of the second
argument, 9. The third argument specifies the number of buffer elements that can be read or written so
this value will be added to the position to define the limit. If either the second argument value or the
sums of the second and third argument values do not represent legal index values for the array, then an
exception of type IndexOutOfBoundsException will be thrown.
You can also wrap arrays of other basic types to create a buffer of the corresponding type. For example:
long[] numbers = { 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89};
LongBuffer numBuf = LongBuffer.wrap(numbers);
The buffer of type LongBuffer that we create will have a capacity of array.length , which will be
11. The buffer position will be set to 0 and the limit will be set to the capacity. In a similar manner we
can create buffers from arrays of any of the other basic types with the exception of type boolean .
If a buffer object has been created from an array, you can obtain a reference to the backing array that is
storing the data in the buffer by calling array() for the buffer. For example, for the buffer created by
the previous code fragment, we could obtain a reference to the original array like this:
long[] data = numBuf.array();
The variable data will now contain a reference to the original array, numbers , which we used to create
numBuf . If the buffer had not been created from an array, the array() method will throw an
exception of type UnsupportedOperationException .
If a buffer object is passed to a method as an argument, you might need to determine whether or not it
has a backing array - before you can call its array() method for example, if you plan to alter the
buffer's contents by changing the elements in the array. The hasArray() method for a buffer object
will return true if the buffer was created from an array, and false otherwise. Typical usage of this
method is something like this:
if(numBuf.hasArray()) {
long[] data = numBuf.array();
// Modify the data array directly to alter the buffer...
} else {
// Modify the buffer using put() methods for the buffer object...
}
Obviously you would only take the trouble to do this if modifying the backing array was a whole lot
more convenient or faster than using put() methods for the buffer. We will see how we use put()
methods to modify a buffer's contents very soon.
You can create buffers of type CharBuffer by wrapping an object of type String , as well as an array
of type char[] . For example:
String wisdom = "Many a mickle makes a muckle.";
CharBuffer charBuf = CharBuffer.wrap(wisdom);