java nio 创建buffer的疑问

Question

以bytebuffer为例
allocate方法的源码

  public static ByteBuffer allocate(int capacity) {
        if (capacity < 0)
            throw new IllegalArgumentException();
        return new HeapByteBuffer(capacity, capacity);
    }

heapbytebuffer的源码 。只不过调用的bytebuffer的构造器

 HeapByteBuffer(int cap, int lim) {            // package-private

        super(-1, 0, lim, cap, new byte[cap], 0);
        /*
        hb = new byte[cap];
        offset = 0;
        */




    }

为什么不直接调用bytebuffer的构造器，而是要通过heapbytebuffer调用bytebuffer构造器