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"?
Related
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 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.
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.
This question already has answers here:
Print java output to a file
(5 answers)
Closed 9 years ago.
My question is related to this one. However, I am looking for a way to append the text file over several runs. is there a way to write console output to a text file without erasing the old runs information? I am working on 30+ classes and it would be tedious to change System.out.println statements so I prefer sticking with the System.setOut solution.
I have the following code based on #Mac answer
PrintStream out = new PrintStream(new FileOutputStream("aa.txt"),true);
System.setOut(out);
but the file aa.txtdoes not append the results, am I missing something here?
When you create a FileWriter or FileOutputStream for your file, pass true as a second argument to the constructor of the FileWriter or FileOutputStream.
FileWriter(File file, boolean append)
FileWriter and FileOutputStream provide a constructor with an append flag. Just modify the referenced code accordingly
You should use:
PrintStream out = new PrintStream(new FileOutputStream("output.txt"),true);
System.setOut(out);
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How to append text to an existing file in Java
I ave a file already made in C:\myfile.txt and it has some data in it.. Now when I try to use File files = new File("C:\myfile.txt"); , It overwrites th orignal data and inserts the new data in it.. Is there a mechanism to aviod overwriting of old data?...
You could use append mode in one of the file writer classes:
FileWriter writer = new FileWriter("myfile.txt", true);
use the method new FileOutputStream(File,true) to append to an existing file.
Provide true in the constructor to append the file as told above
For more detailed control, use RandomAccessFile