I have a file which I write some java objects. After writing, sometimes I need to delete some of these objects placed at ramndom positions in the file. Is there a way to delete a selected object from the file without replacing the whole file?
Thank you.
No, this is fundamentally something that most file systems don't support. (There may be some with special data structures to make it cheap, but most don't - and the standard APIs don't support the idea.)
The simplest way of "deleting" part of a file is to copy just the bits you want to a new file. If you're only removing bits near the end, you could potentially avoid copying the start of it again, but that would require more complicated code.
In the case of a stream of objects, you'd probably be best to just read objects from one stream, and write the ones you still want into a new one. (Obviously you can rename the output file afterwards.) It's possible that by looking at the serialization format yourself you could avoid some of the processing required by repeated deserialization/serialization, but I'd go for the simple route to start with.
@Jon Skeet is correct.
I was wondering if it would be technically possible to do something that to turn an object in a serial stream into ... something else ... by overwriting it with an appropriate byte pattern. I think that the answer is no.
The best I could think of was to overwrite it with a sequence of 0x73 bytes ... the representation of a null; refer to the Java Object Serialization Specification Chapter 6. But this has three serious problems:
0x73 bytes would turn one object into multiple nulls, which would make life difficult for the application trying to read the serialized object stream.So, in practice the best answer is to rewrite the object stream, omitting the objects that you want to delete.
You can try using RandomAccessFile and overwrite the object with blanks. Then in offline mode you could truncate it.