Java：从ByteBuffer扩展

Question

Java will not allow me to extend from ByteBuffer because it implements abstract methods that I am not overriding, yet these methods work for any ByteBuffer objects that are created, such as asDoubleBuffer()...

byte[] bytes = new byte[256];
ByteBuffer buf = ByteBuffer.wrap(bytes);
DoubleBuffer db = buf.asDoubleBuffer();

Yet, if I extend a new class from ByteBuffer, it forces me to implement asDoubleBuffer(), even though the superclass already implements this method, obviously, since I can call it just fine. I'm totally not understanding what's going on here... Please explain this to me.

Answer 1

The ByteBuffer factory method returns a class which implements ByteBuffer.

byte[] bytes = new byte[256];
ByteBuffer buf = ByteBuffer.wrap(bytes);
System.out.println(buf.getClass());

prints

class java.nio.HeapByteBuffer

Using ByteBuffer is a relatively advanced tool, you need to understand Java pretty well before you try to extend it IMHO.

---

You can see this by looking at the code. In my IDE you can find it by doing a <shift>+<click> on the method.

public static ByteBuffer wrap(byte[] array) {
    return wrap(array, 0, array.length);
}

calls

public static ByteBuffer wrap(byte[] array,
                                int offset, int length)
{
    try {
        return new HeapByteBuffer(array, offset, length);
    } catch (IllegalArgumentException x) {
        throw new IndexOutOfBoundsException();
    }
}

BTW: I am a big fan of using ByteBuffer, however you really need to understand what you are doing to use it. esp when parsing data from a ByteBuffer. Developers with ten years experience find it tricky to use.

Answer 2

Nevermind. I see that when a new ByteBuffer is allocated it returns a subclass that uses those abstract methods. The object created is not really a ByteBuffer at all. Confusing to me. Not sure why they would use this methodology.