### Question:
JimfsFileChannel extends FileChannel { @Override public long size() throws IOException { checkOpen(); long size = 0; boolean completed = false; try { if (!beginBlocking()) { return 0; } file.readLock().lockInterruptibly(); try { size = file.sizeWithoutLocking(); completed = true; } finally { file.readLock().unlock(); } } catch (InterruptedException e) { Thread.currentThread().interrupt(); } finally { endBlocking(completed); } return size; } JimfsFileChannel(
      RegularFile file, Set<OpenOption> options, FileSystemState fileSystemState); AsynchronousFileChannel asAsynchronousFileChannel(ExecutorService executor); @Override int read(ByteBuffer dst); @Override long read(ByteBuffer[] dsts, int offset, int length); @Override int read(ByteBuffer dst, long position); @Override int write(ByteBuffer src); @Override long write(ByteBuffer[] srcs, int offset, int length); @Override int write(ByteBuffer src, long position); @Override long position(); @Override FileChannel position(long newPosition); @Override long size(); @Override FileChannel truncate(long size); @Override void force(boolean metaData); @Override long transferTo(long position, long count, WritableByteChannel target); @Override long transferFrom(ReadableByteChannel src, long position, long count); @Override MappedByteBuffer map(MapMode mode, long position, long size); @Override FileLock lock(long position, long size, boolean shared); @Override FileLock tryLock(long position, long size, boolean shared);  }### Answer:
@Test public void testSize() throws IOException { RegularFile file = regularFile(10); FileChannel channel = channel(file, READ); assertEquals(10, channel.size()); file.write(10, new byte[90], 0, 90); assertEquals(100, channel.size()); }