I am trying to clear the contents of a file I made in java. The file is created by a PrintWriter call. I read here that one can use RandomAccessFile to do so, and read somewhere else that this is in fact better to use than calling a new PrintWriter and immediately closing it to overwrite the file with a blank one.
However, using the RandomAccessFile is not working, and I don't understand why. Here is the basic outline of my code.
PrintWriter writer = new PrintWriter("temp","UTF-8");
while (condition) {
writer.println("Example text");
if (clearCondition) {
new RandomAccessFile("temp","rw").setLength(0);
// Although the solution in the link above did not include ',"rw"'
// My compiler would not accept without a second parameter
writer.println("Text to be written onto the first line of temp file");
}
}
writer.close();
Running the equivalent of the above code is giving my temp file the contents:(Lets imagine that the program looped twice before clearCondition was met)
Example Text
Example Text
Text to be written onto the first line of temp file
NOTE: writer needs to be able to write "Example Text" to the file again after the file is cleared. The clearCondition does not mean that the while loop gets broken.
You want to either flush the PrintWriter to make sure the changes in its buffer are written out first, before you set the RandomAccessFile's length to 0, or close it and re-open a new PrintWriter to write the last line (Text to be written...). Preferably the former:
if (clearCondition) {
writer.flush();
new RandomAccessFile("temp","rw").setLength(0);
You'll be lucky if opening the file twice at the same time works. It isn't specified to work by Java.
What you should do is close the PrintWriter and open a new one without the 'append' parameter, or with 'append' set to 'false'.
Related
I have a.txt list trying to move the first line to the last line in Java
I've found scripts to do the following
Find "text" from input file and output to a temp file. (I could set
"text" to a string buffRead.readLine ??) and then...
delete the orig file and rename the new file to the orig?
Please for give me I am new to Java but I have done a lot of research and can't find a solution for what I thought would be a simple script.
Because this is Java and concerns file IO, this is a non-trivial setup. The algorithm itself is simple, but the symbols required to do so are not immediately evident.
BufferedReader reader = new BufferedReader(new FileReader("fileName"));
This gives you an easy way to read the contents of the file fileName.
PrintWriter writer = new PrintWriter(new FileWriter("fileName"));
This gives you a simple way to write to the file. The API to do so is the exact same as System.out when you use a PrintWriter, thus my choice to use one here.
At this point its a simple matter of reading the file and echoing it back in the correct order.
String text = reader.readLine();
This saves the first line of the file to text.
while (reader.ready()) {
writer.println(reader.readLine());
}
While reader has text remaining in it, print the lines into the writer.
writer.println(text);
Print the line that you saved at the start.
Note that if your program does anything else (and it's just a good habit anyway), you want to close your IO streams to avoid leaking resources.
reader.close();
writer.close();
Alternatively, you could also wrap the entire thing in a try-with-resources to perform the same cleanup automatically.
Scanner fileScanner = new Scanner(myFile);
fileScanner.nextLine();
This will return the first line of text from the file and discard it because you don't store it anywhere.
To overwrite your existing file:
FileWriter fileStream = new FileWriter("my/path/for/file.txt");
BufferedWriter out = new BufferedWriter(fileStream);
while(fileScanner.hasNextLine()) {
String next = fileScanner.nextLine();
if(next.equals("\n") out.newLine();
else out.write(next);
out.newLine();
}
out.close();
Note that you will have to be catching and handling some IOExceptions this way. Also, the if()... else()... statement is necessary in the while() loop to keep any line breaks present in your text file.
Add the same line to the last line of this file have a look into this link https://stackoverflow.com/a/37674446/6160431
I want to clear the content of a file witch have a specific extension file.tctl, i don't want to change any thing about the file neither deleting it. The file is generated from a specific model checker so that i have just to delete the content and write my own. I tried to print an empty string like that:
PrintWriter writer = new PrintWriter(file.tctl);
writer.print("");
writer.close();
but the file doesn't work any more. So if there's another method to clear the content of the file.
Just remove the print altogether from your code. You've already truncated the file with the new FileOutputStream/PrintWriter/ whatever you use to open it. No I/O or truncate() necessary. Don't use append mode.
Most easy way I guess
new RandomAccessFile("filename.ext", "rw").setLength(0);
Call your write() method like this:
.write((new String()).getBytes());
This will make your file empty. If that doesn't works, try with this:
FileOutputStream erasor = new FileOutputStream("filename.ext");
erasor.write((new String()).toByteArray());
erasor.close();
Or just try to overwrite the file
//open file in override mode
FileOutputStream out = new FileOutputStream("filename.ext");
//now anything that we write here will remove the old one so just write space ("") here
You have to use a FileOutputStream and then you have the truncate() method:
File f = new File("path-of-the-file.here");
FileChannel channel = new FileOutputStream(f, true).getChannel();
channel.truncate(0);
channel.close();
I have a program that is only meant to be terminated by pressing Ctrl + C. In this program I write to an external file using:
File logFile = new File("output.txt");
PrintWriter log_file_writer = new PrintWriter(logFile);
log_file_writer.println("TEXT");
However because I don't know when the program will be terminated, I can't close the file using:
log_file_writer.close();
I think this is resulting in no text appearing in the output file.
Would anyone have a solution for this?
Thank you for your help.
log_file_writer.flush();
will push the content to disk
As the javadoc says:
PrintWriter(File file) Creates a new PrintWriter, without automatic line flushing, with the specified file.
Therefore, you need to flush the data you want to print that is actually buffered:
log_file_writer.flush();
You did not flush the content, I always use the autoFlush argument, but it is not available with File:
PrintWriter log_file_writer = new PrintWriter(new FileOutputStream("output.txt"),true);
but you can also use log_file_writer.flush(); after each write.
OK, I'm having some trouble writing multiple lines to a text file.
the program runs, but it won't use new lines each time
when I want it run 4 times, the text file should look like:
a
b
c
d
instead, it looks like:
d
who knows how to fix this problem? all imports are correctly imported.
source(it's been slightly edited, assume everything is properly defined):
import java.io.*;
public class Compiler {
public static void main (String args[]) throws IOException
{
//there's lots of code here
BufferedWriter outStream= new BufferedWriter(new FileWriter("output.txt"));
outStream.newLine();
outStream.write(output);
outStream.close();
}
}
Make sure that when you create an instance of a FileWriter, that you are appending to the end of it. This can be done by using this specific FileWriter constructor which takes an additional boolean as a second parameter. This boolean tells the FileWriter to append to the end of the file, rather than overwriting the file.
BufferedWriter outStream= new BufferedWriter(new FileWriter("encoded.txt", true));
By default FileWriter will overwrite the file. What you might want to do is define the reader in the following manner:
new FileWriter("encoded.txt", true)
This way the file will be appended to instead of being overwritten.
Hope this helps!
I'm not sure what this code is supposed to do. It throws an error if your input string is more than one character long, because you close your FileWriter inside the loop, then try to write to it again.
I'm interpreting your question the following way: you're wondering why only the most recent output is in the file. In that case, it's because you didn't create your FileWriter in append mode. Look at the different constructors available for FileWriter, and use the one that allows you to append to the file.
I want create text file but if the file already exists it should not create new file but should append the text to the content (at the end) of the existing file. How can I do it in Java?
for every one second I'm reading data from inputstream when i stop reading and again i start reading data at that time i should write to same file if file already exist
does I have to check the condition:
if(file.exists){
} else{
new File();
}
What I have to do?
You can use the following code to append to a file which already exists -
try {
BufferedWriter out = new BufferedWriter(new FileWriter("filename", true));
out.write("data");
out.close();
} catch (IOException e) { }
If the second argument in FileWriter's constructor is true, then bytes will be written to the end of the file rather than the beginning.
Quoting Stephen's comment:
...passing true as the 2nd
argument causes the file to be created
if it doesn't exist and to be opened
for appending if it does exist.
do i have to check the condition
if(file.exists){ }else{ new File(); }
No you don't have to do that: see other answers for the solution.
Actually, it would be a bad idea to do something like that, as it creates a potential race condition that might make your application occasionally die ... or clobber a file!
Suppose that the operating system preempted your application immediately after the file.exists() call returns false, and gave control to some other application. Then suppose that the other application created the file. Now when your application is resumed by the operating system it will not realise that the file has been created, and try to create it itself. Depending on the circumstance, this might clobber the existing file, or it might cause this application to throw an IOException due to a file locking conflict.
Incidentally, new File() does not actually cause any file system objects to be created. That only happens when you 'open' the file; e.g. by calling new FileOutputStream(file);
If you wish to append to the file if it already exists there's no need to check for its existence at all using exists(). You merely need to create a FileWriter with the append flag set to true; e.g.
PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("foo.txt", true)));
pw.println("Hello, World");
pw.close();
This will create the file if it does not exist or else append to the end of it otherwise.