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Java Forum / First Aid / March 2008Deleting contents of file
Hi all, I am currently devloping a java application where the application when it is launched reads in a CSV file and then populates that data into a JList. I would like the user to be able to select an item from the JList and then click a button which deletes the row that item is in within the CSV file. How would I go about doing this. Any help in this matter would be highly appreciated. Thank you > I am currently devloping a java application where the application > when it is launched reads in a CSV file and then populates that data [quoted text clipped - 3 lines] > > Any help in this matter would be highly appreciated. Files are much like large arrays of bytes. You can't delete a line without rewriting the entire file contents from the start of the deletion to the end of the file. You might as well just rewrite the entire file except the deleted entry. When the user deletes the item from the JList, write the remaining items to a new (temporary) file, delete the old file, then rename the temporary file. By doing it this way you can avoid deleting the original file if any errors occur while writing the temporary one. /gordon -- > When the user deletes the item from the JList, write the remaining > items to a new (temporary) file, delete the old file, then rename the > temporary file. By doing it this way you can avoid deleting the > original file if any errors occur while writing the temporary one. Rather than deleting the old file, consider moving to a back-up file. Like filename + ".bak" or "~"+filename. That way, if you fugger it up, you still have the backup. ;) > > When the user deletes the item from the JList, write the remaining > > items to a new (temporary) file, delete the old file, then rename the [quoted text clipped - 4 lines] > Like filename + ".bak" or "~"+filename. That way, if you fugger it up, > you still have the backup. ;) Excellent advice. Also consider using one of the createTempFile() methods in java.io.File. They're platform agnostic and generate a unique name with each invocation.  SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews On Thu, 28 Feb 2008 04:12:26 -0800 (PST), christopher_board@yahoo.co.uk wrote, quoted or indirectly quoted someone who said : >I am currently devloping a java application where the application when >it is launched reads in a CSV file and then populates that data into a >JList. I would like the user to be able to select an item from the >JList and then click a button which deletes the row that item is in >within the CSV file. How would I go about doing this. the only way to delete a string in a file is to copy the file up to the slice, skip over the slice, and write the rest. You can also chop a file, cutting off the tail. See http://mindprod.com/jgloss/file.html -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com > the only way to delete a string in a file is to copy the file up to > the slice, skip over the slice, and write the rest. It isn't necessary to copy the first part of the file. It's only necessary to rewrite from the cut to the end. /gordon -- >> the only way to delete a string in a file is to copy the file up to >> the slice, skip over the slice, and write the rest. > > It isn't necessary to copy the first part of the file. It's only > necessary to rewrite from the cut to the end. If the file is of any size, this can be very time-consuming, as you'll be jittering the access arm one record at a time. Making a new file is usually much faster, in practice, as the physical I/O can be done in bursts. It is also recoverable, in the event of a crash. Other solutions: Use a database table, and delete the row. Use some kind of indexed file, and delete the record. Read the entire file into memory, manipulate it there, and rewrite. SignatureJohn W. Kennedy "Sweet, was Christ crucified to create this chat?" -- Charles Williams. "Judgement at Chelmsford" On Fri, 29 Feb 2008 14:05:59 -0500, "John W. Kennedy" <jwkenne@attglobal.net> wrote, quoted or indirectly quoted someone who said : >If the file is of any size, this can be very time-consuming, as you'll >be jittering the access arm one record at a time. Making a new file is >usually much faster, in practice, as the physical I/O can be done in >bursts. with the same size chunk buffer, it would be the same speed, unless you had two different physical disks. You have the same arm jerking back and forth. The optimal size of the buffer is probably a lot bigger than the 1-8K typically used in the olden days. I wish there were a way choosing that size were automated, and customised to the particular machine. It is silly for a machine with a 20 gigs of ram to fiddle around with 4K buffers. -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com >It isn't necessary to copy the first part of the file. It's only >necessary to rewrite from the cut to the end. I lived on an island for a number of years where power would go out very frequently. The battery backup did not always rescue you. So my tendency is to avoid doing anything that if interrupted in mid flight would destroy your file, like inplace file updates. I like to create the new file, and only once it is ready, do the delete/rename. That leaves only a tiny window of vulnerability. Back in DOS days, when disks were tiny, but hairy-chested, I did a lot of in place updating, including creating holes, and deleting. You have to be careful how you do your slides, working bottom up or top down to avoid overwriting. The same applies to in-ram sliding. -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com On Thu, 28 Feb 2008 04:12:26 -0800 (PST), christopher_board@yahoo.co.uk wrote, quoted or indirectly quoted someone who said : >I am currently devloping a java application where the application when >it is launched reads in a CSV file and then populates that data into a >JList. I would like the user to be able to select an item from the >JList and then click a button which deletes the row that item is in >within the CSV file. How would I go about doing this. Another approach is to read the whole file into RAM as a String. Fiddle it as you please. Write out the entire file to a temp. When it succeeds, delete the original and rename the temp to the original. For sample code see http://mindprod.com/products1.html#COMPACTOR -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com Free MagazinesGet these publications absolutely FREE for up to 12 months. 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