Ok so I know the value of the line, I dont have the line number, how would I edit only 1 line?
Its a config file, i.e
x=y
I want a command to edit x=y to x=y,z.
or even x=z.
In Java you can use `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);
}
If you are using that configuration format, you might want to use
java.util.Properties
component to read/write on that file.
But if you just want to edit it by hand, you can just read the file line by line and match the variable you want to change.
One way to do it is to:
Read the file into memory; e.g. as an array of Strings representing the lines of the file.
Locate the String/line you want to change.
Use a regex (or whatever) to modify the String/line
Write a new version of the file from the in memory version.
There are many variations on this. You also need to take care when you write the new version of the file to guard against losing everything if something goes wrong during the write. (Typically you write the new version to a temporary file, rename the old version out of the way (e.g. as a backup) and rename the new version in place of the old one.)
Unfortunately, there is no way to add or remove characters in the middle of a regular text file without rewriting a large part of the file. This "problem" is not specific to Java. It is fundamental to the way that text files are modelled / represented on most mainstream operating systems.
Unless the new line has the exact same length as the old one, your best bet is to
Open a temporary output file
Read the config file, line by line
Search for your key
If you can't find it, just write the line you just read to the output file
If you can find it, write the new value to the temporary file instead
Until you hit EOF
Delete old file
Rename new file to the old file
IF your config file is small, you can also do the whole parsing/modification step in memory and then write the final result back to the config file, that way you skip the temporary file (although a temporary file is a good way to prevent corruption if something breaks while you write the file).
If this is not what you're looking for, you should edit your question to be a lot more clear. I'm just guessing what you're asking for.
If your data is all key and value pairs, for example ...
key1=value1
key2=value2
... then load them into a Properties object. Off the top of my head, you'll need a FileInputStream to load the properties, modify with myProperties.put(key, value) and then save the properties with the use of a FileOutputStream.
Hope this helps!
rh
Related
From java I want to read .properties file and if property is present I want to re-set the property again. lets say .properties file has entry password=123 now I want to check If password entry is there replace 123 with 567. but need to keep all content as it is. how to do that? please help
One solution I can think of rightaway is
Load all the properties from the base .properties file.
Create a new temp properties file and loop through the entries from original file to write the same in to new file. In the loop you can change the value of any property while writing it to a new file.
After the loop delete the original file and rename the temp file as original file.
This approach has couple of limitations,
Do not use it if you are really concerned about the file last modified/created date, since we are creating a whole new file here.
If the original file is too big, this approach may cause memory problems.
Hope this helps!
I was writing a program that implements a dictionary.
Actually what I did is just to write a java applet to show the words which is defined in a .xml file. And I did that with the org.w3c.dom package.
Now, I want to add a new feature that users can modify a word in the dictionary in the the program then the modification will be saved to the original .xml file.
Here is my question: what should I do to save the changes? Note that users can only modify one word a time so I don't want to load the whole file and modify the certain part and re-write the whole file to the disk. Is there a novel way to do that?
An XML file is a sequential text file. This means that there is no formula or other convenient way to locate the n-th word in a dictionary stored in XML. Elements need to be written one after the other, character by character (and one character may or may not result in a byte). Thus, what is called a random update, is out.
Look at JAXB for a most convenient way to read and write XML, and invest some work so that a user cannot update in memory and terminate the program without saving.
Reading and writing files in specific formats is a little bit trickier that what you portray.
Seen with "XML eyes" you are only changing a portion of the file - but to do that on the file level you need to seek to the position of change and write new bytes from there. The problem with that is that the content after that position won't adjust according to the new portion you write.
TL;DR - no - you need to read+write the complete XML file when making changes.
I have a file which contains lot of zeros and as per the requirement the zeros in the file are invalid. I am using RandomAccessFile api to locate data in the file. Is there way so that all the zeros can be removed from the file using the same api.
You'll have to stream through the file and write out the content, minus the zeros, to a separate temporary file. You can then close and delete the original and rename the new file to the old file name. That's your best alternative for this particular use case.
You can use RandomAccessFile to read the files' data, and when you reach a point where you need to change the data you can overwrite the existing number of bytes with equal number of bytes. It's iff the new value is exactly the same length as the old value.
With RandomAccessFile its difficult and equally complex when the size of two, the one being changed and the new value are different. It involves a lot of seeks, reads and writes to move data back
Try to read the whole file, change the bits you have to change and write a new file. You might process one line at a time or read the whole file into memory, modify it and write it all back out again. It is a good idea to perform the edit in the following manner:
Read file
Write to Temporary File [just to back-up]
Rename original to back-up
Work on Temporary file.
Remove Backup if you were successful.
I'm writing an application that is writing to a file. I'm wondering if it's possible to write to a folder, without specifying a file name. The way I have it set up now, my program will overwrite the previous saved file. I'm looking to have it add to the folder rather than replace.
Here's the line in question:
File testFile = new File("C:/TargetFolder/testFile.png");
There is no way to write to a file without assigning a file name. However, if you don't want to chose a file name you can have your system generate a random one. For example look at these options: What is the best way to generate a unique and short file name in Java.
Another option would be to add numbers to your file name like: test01.png, test02.png and so on.
If you don't want to do the unique file in the folder logic yourself and you don't care much about the exact name, you might use:
java.io.File.createTempFile("testFile", ".png", new File("C:/TargetFolder"));
I want to have an internal File like stock features with these properties:
File has a Maximum of LineNumber i.e.: 400.
New line appends to file until this limitation is reached; then, firstly removes the First Line and then appends New Line.
To remove a line it shouldn't be necessary to read and rewrite the whole file.
Is it possible to do it so?
Many thanks and best regards
You need a circular file buffer. There is nothing built into Android, but you can use This implementation which I have used in the past and works well for me.