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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());
}

How to read and append text file based on certain conditions

What the program does: A user loads a file and it saves the file name, creation date and a file number to a text file. I want the program to check the file to see if the filename is already stored. If it is, to then check the creation date to see if it matches the one the user is currently trying to save. If it differs, to save over the currently saved creation date with the new one.
An example of Info.txt:
CreationDate: 140319, FileName: example1.txt, FileNumber: 1
CreationDate: 110219, FileName: example2.txt, FileNumber: 6
CreationDate: 100319, FileName: example3.txt, FileNumber: 14
How I create and write to the file:
public void fileCreation() throws IOException {
String fileinformation = File.creationdate + ", " + File.name + ", " + "Number: " + File.Number;
FileWriter wr = new FileWriter("Info.txt", true);
PrintWriter output = new PrintWriter(wr);
output.println(fileinformation);
output.close();
}
How I'm currently checking the file, at the moment it only prints the file contents, so if a User goes to save example2.txt with a different creation date, then the new one should overwrite the current one
public void scanFile() throws IOException {
BufferedReader br = new BufferedReader(new FileReader("Info.txt"));
String readfile;
while ((readfile = br.readLine()) != null) {
System.out.println(readfile);
}
System.out.println("file not found");
}
Creation date is a long number and File.creationdate is a string such as "CreationDate: 140319", it would also be good to put a check to see if the FileNumber matches, but this is not necessary.

You should use split on your line with ',' as a separator.
Then you can go through every line and for each line check if first the name is the same as the file name, if it is, go and check the date and then finally check the FileNumber.
But why do you need to check it one by one ? Better to write it in with &&, like this
public boolean scanFile() throws IOException {
BufferedReader br = new BufferedReader(new FileReader("Info.txt"));
String readfile;
Boolean isAlreadyIn = false;
while ((readfile = br.readLine()) != null) {
String[] parts = readFile.split(',');
if(parts[1].replace("FileName: ","").equals(File.name) &&parts[0].replace("CreationDate: ","").equals(File.creationdate) &&parts[2].replace("FileNumber: ","").equals(File.number){
isAlreadyIn = true; // you can also write directly in this fonction
}
return isAlreadyIn;
}
}
if you really need to check it one by one, just make a if(if( if()) with the condition in it

I cannot write to my .CSV file

My guess is, the code I've written doesn't work with .CSV files, but only .txt.
The purpose of my code is to take the user input from field1, and check against my .CSV file to see if there is an instance of the user input located within the file. If there is, then it will be replaced by the user input from field2.
This works with my .txt file, but not with my .CSV file.
Here's the code that is activated at the push of a button (save button):
try{
// Input the file location into Path variable 'p'
//Cannot write to CSV files
//Path p = Paths.get("C:\\Users\\myname\\Documents\\Stock Take Program\\tiger.csv");
Path p = Paths.get("C:\\Users\\myname\\Desktop\\test.txt");
//Read the whole file to a ArrayList
List<String> fileContent = new ArrayList<>(Files.readAllLines(p));
//Converting user input from editSerialField to a string
String strSerial = editSerialField.getText();
//Converting user input from editLocationField to a string
String strLocation = editLocationField.getText();
//This structure looks for a duplicate in the text file, if so, replaces it with the user input from editLocationField.
for (int i = 0; i < fileContent.size(); i++)
{
if (fileContent.get(i).equals(strSerial))
{
fileContent.set(i, strLocation);
}
break;
}
// write the new String with the replaced line OVER the same file
Files.write(p, fileContent);
}catch(IOException e)
{
e.printStackTrace();
}
My question is, how can I update my code to work with updating and replacing the contents of a .CSV file with the user input, the same way as it works for my .txt files.
When writing to a text file, it replaces only the first line, but when writing to a .CSV file, it does not replace anything.
Is there anyway I should be writing my code differently to replace text within a .CSV file.
Any help is greatly appreciated, thanks.

I'm an idiot. My '.CSV' file is actually titled tiger.csv as a text file. I've now saved an actual CSV version and it is now working.
Haha, thanks for the help guys.

Probably should be in another question, but the problem relating to the change only working on the first line is due to the break being called and shortcutting the loop on the first way round. Put it within the if block.
for (int i = 0; i < fileContent.size(); i++)
{
if (fileContent.get(i).equals(strSerial))
{
fileContent.set(i, strLocation);
break;
}
}
Or leave it off completely if you want it to be able to update multiple lines.

HTH,
public static void main(String[] args) throws FileNotFoundException {
String UserEntredValu="Karnataka";
String csvFile = "C:/Users/GOOGLE/Desktop/sample/temp.csv";
String line = "";
String cvsSplitBy = ",";
PrintWriter pw = null;
pw = new PrintWriter(new File(csvFile));
try (BufferedReader br = new BufferedReader(new FileReader(csvFile))) {
while ((line = br.readLine()) != null) {
// use comma as separator
String[] country = line.split(cvsSplitBy);
for( int i = 0; i < country.length - 1; i++)
{
String element = country[i];
if(element.contains(UserEntredValu)){
String newEle=element.replace(element, "NEW INDIA");
pw.write(newEle);
System.out.println("done!");
pw.close();
}
}
}
} catch (IOException e) {
e.printStackTrace();
}
}

Read one line of a csv file in Java

I have a csv file that currently has 20 lines of data.
The data contains employee info and is in the following format:
first name, last name, Employee ID
So one line would like this: Emma, Nolan, 2
I know how to write to the file in java and have all 20 lines print to the console, but what I'm not sure how to do is how to get Java to print one specific line to the console.
I also want to take the last employee id number in the last entry and have java add 1 to it one I add new employees. I thinking this needs to be done with a counter just not sure how.

You can do something like this:
BufferedReader reader = new BufferedReader(new FileReader(<<your file>>));
List<String> lines = new ArrayList<>();
String line = null;
while ((line = reader.readLine()) != null) {
lines.add(line);
}
System.out.println(lines.get(0));
With BufferedReader you are able to read lines directly. This example reads the file line by line and stores the lines in an array list. You can access the lines after that by using lines.get(lineNumber).

You can read text from a file one line at a time and then do whatever you want to with that line, print it, compare it, etc...
// Construct a BufferedReader object from the input file
BufferedReader r = new BufferedReader(new FileReader("employeeData.txt"));
int i = 1;
try {
// "Prime" the while loop
String line = r.readLine();
while (line != null) {
// Print a single line of input file to console
System.out.print("Line "+i+": "+line);
// Prepare for next loop iteration
line = r.readLine();
i++;
}
} finally {
// Free up file descriptor resources
r.close();
}
// Remember the next available employee number in a one-up scheme
int nextEmployeeId = i;

BufferedReader reader =new BufferedReader(new FileReader("yourfile.csv"));
String line = "";
while((line=reader.readLine())!=null){
String [] employee =line.trim().split(",");
// if you want to check either it contains some name
//index 0 is first name, index 1 is last name, index 2 is ID
}

Alternatively, If you want more control over read CSV files then u can think about CsvBeanReader that will give you more access over files contents..

Here is an algorithm which I use for reading csv files. The most effective way is to read all the data in the csv file into a 2D array first. It just makes it a lot more flexible to manipulate the data.
That way you can specify which line of the file to print to the console by specifying it in the index of the array and using a for. I.e: System.out.println(employee_Data[1][y]); for record 1. y is the index variable for fields. You would need to use a For Loop of course, to print every element for each line.
By the way, if you want to use the employee data in a larger program, in which it may for example store the data in a database or write to another file, I'd recommend encapsulating this entire code block into a function named Read_CSV_File(), which will return a 2D String array.
My Code
// The return type of this function is a String.
// The CSVFile_path can be for example "employeeData.csv".
public static String[][] Read_CSV_File(String CSVFile_path){
String employee_Data[][];
int x;
int y;
int noofFields;
try{
String line;
BufferedReader in = new BufferedReader(new FileReader(CSVFile_path));
// reading files in specified directory
// This assigns the data to the 2D array
// The program keeps looping through until the line read in by the console contains no data in it i.e. the end of the file.
while ( (( line = in.readLine()) != null ){
String[] current_Record = line.split(",");
if(x == 0) {
// Counts the number of fields in the csv file.
noofFields = current_Record.length();
}
for (String str : values) {
employee_Data[x][y] = str;
System.out.print(", "+employee_Data[x][y]);
// The field index variable, y is incremented in every loop.
y = y + 1;
}
// The record index variable, x is incremented in every loop.
x = x + 1;
}
// This frees up the BufferedReader file descriptor resources
in.close();
/* If an error occurs, it is caught by the catch statement and an error message
* is generated and displayed to the user.
*/
}catch( IOException ioException ) {
System.out.println("Exception: "+ioException);
}
// This prints to console the specific line of your choice
System.out.println(("Employee 1:);
for(y = 0; y < noofFields ; y++){
// Prints out all fields of record 1
System.out.print(employee_Data[1][y]+", ");
}
return employee_Data;
}

For reading large file,
log.debug("****************Start Reading CSV File*******");
copyFile(inputCSVFile);
StringBuilder stringBuilder = new StringBuilder();
String line= "";
BufferedReader brOldFile = null;
try {
String inputfile = inputCSVFile;
log.info("inputfile:" + inputfile);
brOldFile = new BufferedReader(new FileReader(inputfile));
while ((line = brOldFile.readLine()) != null) {
//line = replaceSpecialChar(line);
/*do your stuff here*/
stringBuilder.append(line);
stringBuilder.append("\n");
}
log.debug("****************End reading CSV File**************");
} catch (Exception e) {
log.error(" exception in readStaffInfoCSVFile ", e);
}finally {
if(null != brOldFile) {
try {
brOldFile.close();
} catch (IOException e) {
}
}
}
return stringBuilder.toString();

How can I read text appended to the end of a file I already read?

I want to read a text file in Java. After I finish, some text will be appended by another application, and then I want to read that. Lets say there are ten lines. When the other app appends one more line, I dont want to read the whole file again; just the new line. How can I do this?

Something like this could work:
BufferedReader reader = .. // create a reader on the input file without locking it
while(otherAppWritesToFile) {
String line = reader.readLine();
while(line != null) {
processLine(line);
line = reader.readLine();
}
Thread.sleep(100);
}
Exception handling has been left out for the sake of simplicity.

Once you get an EOF indication, wait a little bit and then try reading again.
Edit: Here is teh codez to support this solution. You can try it and then change the control flow mechanisms as needed.
public static void main(final String[] args) throws IOException {
final Scanner keyboard = new Scanner(System.in);
final BufferedReader input = new BufferedReader(new FileReader("input.txt"));
boolean cont = true;
while (cont) {
String line = input.readLine();
while (line != null) {
System.out.println(line);
line = input.readLine();
}
System.out.println("EOF reached, add more input and type 'y' to continue.");
final String in = keyboard.nextLine();
cont = in.equalsIgnoreCase("y");
}
}
EDIT: Thanks for adding some code Tim. Personally, I would just do a sleep instead of waiting for user input. That would more closely match the users' requirements.

You could try using a RandomAccessFile.
Open the file and then invoke the length() to get the length of the file. Then you can use the readLine() method to get your data. Then the next time you open the file you can use the seek() method to position yourself to the previous end of the file. Then read the lines and save the new length of the file.