How can I make Java's ProcessBuilder append the data in the output file? what it does now is that each time it writes in the output file, it deletes whatever is in it, then writes.
The key is how you create your FileWriter -- you must use the constructor that has a second boolean parameter. If the boolean, which stands for "append", is true, the file is appended to and not overwritten.
The FileWriter API.
Related
If you work with FileOutputStream methods, each time you write your file through this methods you've been lost your old data. Is it possible to write file without losing your old data via FileOutputStream?
Use the constructor that takes a File and a boolean
FileOutputStream(File file, boolean append)
and set the boolean to true. That way, the data you write will be appended to the end of the file, rather than overwriting what was already there.
Use the constructor for appending material to the file:
FileOutputStream(File file, boolean append)
Creates a file output stream to write to the file represented by the specified File object.
So to append to a file say "abc.txt" use
FileOutputStream fos=new FileOutputStream(new File("abc.txt"),true);
I am generating random numbers and putting that in a file and from that file I m reading the values. Now while reading it is reading all the values which has been put to the file again rather than reading the last added value. So what I want is to delete the contents of the file before writing any random number to it again so that when it reads from it, it will only read the last added value.
If you're doing this all at the same time via RandomAccessFile, just call RandomAccessFile.setLength(0).
If you're doing it via a FileOutputStream or FileWriter, just create a new one without the append parameter, or with that parameter set to `false.
Write an empty string into the file like below
PrintWriter writer = new PrintWriter(file);
writer.print("");
writer.close();
How can i prevent prinwriter in java from overwriting what's inside of that particular file?
Ex. I have a student.txt file. I already have few names there. After running and modifying this How do I create a file and write to it in Java? whats inside of that file will be overwritten. I just want to add it to the new line.
Also, how can i possibly perform search?
PrintWriter out = new PrintWriter(new FileWriter("student.txt", true));
The true is the append parameter - which indicates whether the FileWriter should append to the file. If it was false it would overwrite existing data in the file.
What do you mean by "how can i possibly perform search"?
My program is currently using
FileOutputStream output = new FileOutputStream("output", true);
A while loop creates the output file if it is not yet created and appends some data to this file for every iteration of the while loop using
output.write(data).
This is fine and is what I want.
If I run the program again the file just doubles in size as it appends the exact information to the end of the file. This is not what I want. I would like to overwrite the file if I run the program again.
This documentation suggests that the parameter you're passing is the append parameter.
the signature looks like the following
FileOutputStream(File file, boolean append)
You should set that parameter to false since you don't want to append.
FileOutputStream output = new FileOutputStream("output", false);
I would like to overwrite the file if I run the program again.
Pass false as 2nd argument, to set append to false:
FileOutputStream output = new FileOutputStream("output", false);
Check out the constructor documentation:
If the second argument is true, then bytes will be written to the end of the file rather than the beginning.
Or you could just delete the file if it's there, and then always create it and job done.
I am using BufferedWriter to write text with specific encoding to file, I want to count the file size in bytes before I close the file.
bw = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(file),encoding),bsize);
bw.write(string);
My plan was to use string.getBytes() but this method doesn't allow to provide specific encoding (and I can't override the default encoding property).
Use String#getBytes(encoding) instead
If you're looking for a method that works regardless of whether you're appending or not and does not duplicate data just for counting, store the reference to the FileOutputStream in a variable and access the FileChannel through getFileChannel(). You can call long position() before and after the data has been written, and the difference of the values will give you the number of bytes that have been written.