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javafileiorandom-access

Java - overwriting specific parts of a file

I would like to update specific part of a text file using Java. I would like to be able to scan through the file and select specific lines to be updated, a bit like in a database, for instance given the file:

ID Value
1  100
2  500
4  20

I would like to insert 3 and update 4, e.g.

ID Value
1  100
2  500
3  80
4  1000

Is there a way to achieve this (seemingly) easy task? I know you can append to a file, but I am more interested in a random access

Solution

  • I know you can append to a file, but I am more interested in a random access

    You're trying to insert and delete bytes in the middle of a file. You can't do that. File systems simply don't (in general) support that. You can overwrite specific bytes, but you can't insert or delete them.

    You could update specific records with random access if your records were fixed-length (in bytes) but it looks like that's not the case.

    You could either load the whole file into memory, or read from the original file, writing to a new file with either the old data or the new data as appropriate on a per line basis.