What is the initial "mode" of ByteBuffer?

Question

While studying the ByteBuffer class I got to thinking about an array wrapped ByteBuffer that might be constructed as follows:

byte data[] = new byte[10];
// Populate data array
ByteBuffer myBuffer = ByteBuffer.wrap(data);
int i = myBuffer.getInt();

Which, I thought, might retrieve the first 4 bytes of my byte array as an int value. However, as I studied further, I seemed to find that the ByteBuffer has two modes which are read and write and we can flip between them using the flip() method. However, since flip is basically a toggle, it pre-supposes than one knows the initial value to meaningfully flip between the read and write states.

What is the definition of the initial state of a ByteBuffer?

• write?
• read?
• A function of how it was created (eg. allocate vs wrap)?

Answer 1

Strictly speaking the ByteBuffer itself doesn't track if it is "read" or "write"; that's merely a function of how it is used. A ByteBuffer can read and write at any time. The reason why we say flip switches the "mode" is because it is part of the common task of writing to the buffer, flipping it, then reading from the buffer.

Indeed, both allocate and wrap set the limit and capacity to be equal to the array size, and the position to zero. This means that read operations can read up to the whole array, and write operations can fill the whole array. You can therefore do either reading or writing with a newly allocated or wrapped ByteBuffer.