In java, how do I edit 1 line of a text file?

Question

Ok, so I know the value of the line, I do not have a line number, how would I edit only one line?

Its configuration file, i.e. x = y I need a command to edit x = y for x = y, z.

or even x = z.

Answer 1

In Java, you can use the Properties class:
App.config file:

x=y 

Java:

  public void writeConfig() throws Exception { Properties tempProp = new Properties(); tempProp.load(new FileInputStream("app.config")); tempProp.setProperty("x", "y,z"); tempProp.store(new FileOutputStream("app.config"), null); } 

Answer 2

If you use this configuration format, you can use

java.util.Properties

to read / write in this file.

But if you just want to edit it manually, you can just read the file line by line and map the variable you want to change.

Answer 3

One way to do this:

• Read the file in memory; for example, as an array of strings representing the lines of a file.
• Find the line / line you want to change.
• Use regex (or other) to change line / line
• Write the new version of the file from the version in memory.

There are many variations to this. You also need to take care when you write a new version of the file to protect against the loss of everything if something goes wrong during recording. (Usually, you write the new version to a temporary file, rename the old version in the path (for example, as a backup) and rename the new version instead of the old one.)

Unfortunately, there is no way to add or remove characters in the middle of a regular text file without overwriting a large part of the file. This "problem" is not specific to Java. It is crucial that text files are modeled / presented on most major operating systems.

Answer 4

If the new line is the same length as the old one, best

• Open temporary output file
• Read the configuration file line by line
• Key Search
• If you cannot find it, just write the line you just read into the output file
• If you can find it, write the new value in a temporary file instead
• Until you hit EOF
• Delete old file
• Rename the new file to the old file

If your configuration file is small, you can also perform the entire parsing / changing step in memory, and then write the final result back to the configuration file, thereby skipping the temporary file (although the temporary file is a good way to prevent corruption if something breaks when you write a file).

If this is not what you are looking for, you should change your question to be much clearer. I just guess what you're asking for.

Answer 5

If your data is all key and value pairs, for example ...

key1 = value1

key2 = value2

... then load them into the Properties object. At the top of my head, you'll need a FileInputStream to load properties, change using myProperties.put (key, value), and then save the properties using FileOutputStream.

Hope this helps!

rel