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File manipulation (changing lines in a File) in java

I'm trying to read in a file and change some lines.
The instruction reads "invoking java Exercise12_11 John filename removes the string John from the specified file."
Here is the code I've written so far
import java.util.Scanner;
import java.io.*;
public class Exercise12_11 {
public static void main(String[] args) throws Exception{
System.out.println("Enter a String and the file name.");
if(args.length != 2) {
System.out.println("Input invalid. Example: John filename");
System.exit(1);
}
//check if file exists, if it doesn't exit program
File file = new File(args[1]);
if(!file.exists()) {
System.out.println("The file " + args[1] + " does not exist");
System.exit(2);
}
/*okay so, I need to remove all instances of the string from the file.
* replacing with "" would technically remove the string
*/
try (//read in the file
Scanner in = new Scanner(file);) {
while(in.hasNext()) {
String newLine = in.nextLine();
newLine = newLine.replaceAll(args[0], "");
}
}
}
}
I don't quite know if I'm headed in the correct direction because I'm having some issue getting the command line to work with me. I only want to know if this is heading in the correct direction.
Is this actually changing the lines in the current file, or will I need different file to make alterations? Can I just wrap this in a PrintWriter to output?
Edit: Took out some unnecessary information to focus the question. Someone commented that the file wouldn't be getting edited. Does that mean I need to use PrintWriter. Can I just create a file to do so? Meaning I don't take a file from user?

Your code is only reading file and save lines into memory. You will need to store all modified contents and then re-write it back to the file.
Also, if you need to keep newline character \n to maintain format when re-write back to the file, make sure to include it.
There are many ways to solve this, and this is one of them. It's not perfect, but it works for your problem. You can get some ideas or directions out of it.
List<String> lines = new ArrayList<>();
try {
Scanner in = new Scanner(file);
while(in.hasNext()) {
String newLine = in.nextLine();
lines.add(newLine.replaceAll(args[0], "") + "\n"); // <-- save new-line character
}
in.close();
// save all new lines to input file
FileWriter fileWriter = new FileWriter(args[1]);
PrintWriter printWriter = new PrintWriter(fileWriter);
lines.forEach(printWriter::print);
printWriter.close();
} catch (IOException ioEx) {
System.err.println("Error: " + ioEx.getMessage());
}

Read and Write to a file

I am having to make a gui project for my CSIS class and I am having trouble with the read and Write I am using. I am making a game where you battle stuff and after you beat five of them it shows a message saying "YOU WIN". Every time you win a battle, I have it write the number of wins to a file so if you were to close the game you can continue when it is opened again. Here is the code that i have Written - this is my read method.
private static int read()
{
int returnValue = 0;
try(Scanner reader = new Scanner("wins.txt"))
{
while(reader.hasNextLine())
{
String read = reader.nextLine();
returnValue = Integer.parseInt(read);
}
}
catch(NullPointerException e)
{
System.out.println("No such File! Please Try Again! " + e.getMessage());
}
return returnValue;
and this is my Write method.
private static void write(int wins)
{
try(Formatter writer = new Formatter("wins.txt");)
{
writer.format("%d", wins);
}
catch (FileNotFoundException e)
{
System.out.println("File not Found!!");
}
}
the only thing that is in the wins.txt file is the number that the Write method writes into it. so i win once then the file will have "1" and if i win twice then it will have "2"
Whenever I run the program, it throws a NumberFormatException. I am not sure why it is doing this because I am parseing the String that that reader reads into an int.

The problem is that this code...
Scanner reader = new Scanner("wins.txt")
... constructs a Scanner with the literal text "wins.txt", not the contents of the file "wins.txt".
To read a file with a Scanner, the easiest way for you is probably to construct it using a File object...
Scanner reader = new Scanner(new File("wins.txt"))
There are some other changes you will need to make to your code to get it to work from this point, but this should cover the major issue.

Cannot get PrintWriter to replace text in file

I am trying to complete a simple program that uses the command line to replace a specified String in a file. Command line entry would be java ReplaceText textToReplace filename
The code completes, but the file does not replace the specified string. I have Googled similar situations but I cannot figure out why my code is not working.
import java.io.*;
import java.util.*;
public class ReplaceText{
public static void main(String[] args)throws IOException{
if(args.length != 2){
System.out.println("Incorrect format. Use java ClassName textToReplace filename");
System.exit(1);
}
File source = new File(args[1]);
if(!source.exists()){
System.out.println("Source file " + args[1] + " does not exist.");
System.exit(2);
}
File temp = new File("temp.txt");
try(
Scanner input = new Scanner(source);
PrintWriter output = new PrintWriter(temp);
){
while(input.hasNext()){
String s1 = input.nextLine();
String s2 = s1.replace(args[0], "a");
output.println(s2);
}
temp.renameTo(source);
source.delete();
}
}
}
Edit: edited the code so I am not reading and writing to the file at the same time, but it still does not work.

First of all you have a problem with your logic. You are renaming your temporary file then immediately deleting it. Delete the old one first, then rename the temporary file.
Another problem is that you are attempting to do perform the delete and rename within your try block:
try(
Scanner input = new Scanner(source);
PrintWriter output = new PrintWriter(temp);
){
...
temp.renameTo(source);
source.delete();
}
Your streams are not automatically closed until the try block ends. You will not be able to rename or delete while the stream is open. Both delete and renameTo return a boolean to indicate whether they were successful so it may be prudent to check those values.
Correct code may look something like:
try(
Scanner input = new Scanner(source);
PrintWriter output = new PrintWriter(temp);
){
while(...)
{
...
}
}
// Try block finished, resources now auto-closed
if (!source.delete())
{
throw new RuntimeException("Couldn't delete file!");
}
if (!temp.renameTo(source))
{
throw new RuntimeException("Couldn't rename file!");
}

You can't replace strings a file in general. You need to read the input line by line, replace each line as necessary, and write each line to a new file. Then delete the old file and rename the new one.

Getting FileNotFoundException when trying to run a java program through CMD using input and output files

When I try to run this code through eclipse, it works fine, but when I try to run it through CMD using "java MainClass >result.txt" I get a FileNotFoundException.
This is the code in question:
import java.io.;
import java.util.;
public class MainClass
{
static int cellNumber;
static int freeSpace;
static int randomResult;
static int chosen;
static int choiceSize;
public static void main(String[] args)
{
Scanner in = null;
try
{
in = new Scanner(new FileReader("C:\\users\\Alon\\workspace\\ex2temp\\bin\\input.txt"));
FileWriter fw = new FileWriter("C:\\users\\Alon\\workspace\\ex2temp\\bin\\result.txt");
PrintWriter pw = new PrintWriter(fw);
chosen = getRandomInt();
pw.printf("Choice=%d", chosen);
pw.println();
while (in.hasNext())
{
cellNumber = in.nextInt();
freeSpace = in.nextInt();
if (sizeOfChosen(chosen) <= freeSpace)
{
pw.printf("%d", cellNumber);
pw.println();
break;
}
}
if (!in.hasNext())
{
pw.println("Cannot allocate memory");
pw.println();
}
pw.close();
fw.close();
in.close();
}
catch (IOException e)
{
e.printStackTrace();
}
}
Can anyone help please? Thanks :)

You're using the files from the ".../ext2temp/bin/.." which I assume it's Eclipse's output folder. Use the path where you have the original files.

In order to create a continuous writing on the same file, I had to append text into the existing file. I have added ", true" when creating the FileWriter. Then, I created a new BufferedWriter which received the FileWriter object, and finally, changed the PrintWriter receiving object to the bufferredWriter. This way, every time I run the program, 2 new lines of data are formed below the old ones and I get a "log file".

Java - Scanner not scanning after a certain number of lines

I'm doing some relatively simple I/O in Java. I have a .txt files that I'm reading from using a Scanner and a .txt file I'm writing to using a BufferedWriter. Another Scanner then reads that file and another BufferedWriter then creates another .txt file. I've provided the code below just in case, but I don't know if it will help too much, as I don't think the code is the issue here. The code compiles without any errors, but it's not doing what I expect it to. For some reason, charReader will only read about half of its file, then hasNext() will return false, even though the end of the file hasn't been reached. These aren't big text files - statsReader's file is 34 KB and charReader's file is 29 KB, which is even weirder, because statsReader reads its entire file fine, and it's bigger! Also, I do have that code surrounded in a try/catch, I just didn't include it.
From what I've looked up online, this may happen with very large files, but these are quite small, so I'm pretty lost.
My OS is Windows 7 64-bit.
Scanner statsReader = new Scanner(statsFile);
BufferedWriter statsWriter = new BufferedWriter(new FileWriter(outputFile));
while (statsReader.hasNext()) {
statsWriter.write(statsReader.next());
name = statsReader.nextLine();
temp = statsReader.nextLine();
if (temp.contains("form")) {
name += " " + temp;
temp = statsReader.next();
}
statsWriter.write(name);
statsWriter.newLine();
statsWriter.write(temp);
if (! (temp = statsReader.next()).equals("-"))
statsWriter.write("/" + temp);
statsWriter.write("\t");
statsWriter.write(statsReader.nextInt() + "\t");
statsWriter.write(statsReader.nextInt() + "\t");
statsWriter.write(statsReader.nextInt() + "\t");
statsWriter.write(statsReader.nextInt() + "\t");
statsWriter.write(statsReader.nextInt() + "\t");
statsWriter.write(statsReader.nextInt() + "");
statsWriter.newLine();
statsReader.nextInt();
}
Scanner charReader = new Scanner(charFile);
BufferedWriter codeWriter = new BufferedWriter(new FileWriter(codeFile));
while (charReader.hasNext()) {
color = charReader.next();
name = charReader.nextLine();
name = name.replaceAll("\t", "");
typing = pokeReader.next();
place = charReader.nextInt();
area = charReader.nextInt();
def = charReader.nextInt();
shape = charReader.nextInt();
size = charReader.nextInt();
spe = charReader.nextInt();
index = typing.indexOf('/');
if (index == -1) {
typeOne = determineType(typing);
typeTwo = '0';
}
else {
typeOne = determineType(typing.substring(0, index));
typeTwo = determineType(typing.substring(index+1, typing.length()));
}
}
SSCCE:
public class Tester {
public static void main(String[] args) {
File statsFile = new File("stats.txt");
File testFile = new File("test.txt");
try {
Scanner statsReader = new Scanner(statsFile);
BufferedWriter statsWriter = new BufferedWriter(new FileWriter(testFile));
while (statsReader.hasNext()) {
statsWriter.write(statsReader.nextLine());
statsWriter.newLine();
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}

This is a classic problem: You need to flush and close the output stream (in this case statsWriter) before reading the file.
Being buffered, it doesn't actually write to the file with ever call to write. Calling flush forces it to complete any pending write operations.
Here's the javadoc for OutputStream.flush():
Flushes this output stream and forces any buffered output bytes to be written out. The general contract of flush is that calling it is an indication that, if any bytes previously written have been buffered by the implementation of the output stream, such bytes should immediately be written to their intended destination.

After you have written your file with your statsWriter, you need to call:
statsWriter.flush();
statsWriter.close();
or simply:
statsWriter.close(); // this will call flush();
This is becuase your are using a Buffered Writer, it does not write everything out to the file as you call the write functions, but rather in buffered chunks. When you call flush() and close(), it empties all the content it still has in it's buffer out to the file, and closes the stream.
You will need to do the same for your second writer.