Replace line

Question

Hi there,
I am new to java. I want to find a specified line of string in the file, and then replace it with a new string. i would like to use RandomAccessFile. Could anyone here kindly give me an idea or show me a sample?
Thank you in advance,
bob

Answer 1

Just to give some details. First, the File object only encapsulates a file name and path, so it cannot be used to read or write data to the disk. To read / write, I recommend that you use a
// the following creates an object to read from a File
//  given a File object or a String with the path
FileInputStream fileIn = new FileInputStream(File file or String path);
// The following creates a FileOutputStream for writing
//  to the file, the wraps it with a PrintStream that
//  adds some useful methods
PrintStream fileOut = new PrintStream( new FileOutputStream(File file or String path) );In this way, you can use fileIn and fileOut exactly in the same way that yopu would be using System.in and System.out, which I assume you already used quite a lot ;)
Another approach, as you said, would be to use the RandomAccessFile, that keeps a FilePointer and allows both reading and writing. look at
http://java.sun.com/docs/books/tutorial/essential/io/rafs.html
for a tutorial about that.
Anyway, I expect that won't fit your needs, since it is not possible to "insert" or "cut" data from a file in this way. It would be only useful if the line you want to modify won't change its lenght... Wow, It looks like I can't explain myself properly, doesn't it?
What I mean is that a RandomAccessFile with the following file:
First Line - 123
Second Line - 456
Third Line - 789might allow you to change the "456" part to "654", since that is still 3 characters long. Yet you might not change "Second" to "2nd", since the number of character changes. If you did, the file would finish like this:
First Line - 123
2ndond Line - 456
Third Line - 789To do this, I'd say you did what someone else already said: Read the file line by line, change what you want, and save each line to a new File. Then, when you finished, delete the old file and rename the new one. If the file is small enough, you can do that on memory - just remember to use a StringBuffer instead of a String since it is much more efficient when it comes to 'growing' strings.
Hope this helps...