We Keep Coding

How to get File name from cmd argument to create a copy?

I'm working on a program that reads from a file with a custom extension I made. The idea is that an error report is created every time a file is read. The error report must be in whatever folder the source file was called from. The error file is a copy of the source file, but it has a line number at the beginning of each line and indicates at the end of the line if an error occurred at that line.
(I'm not trying to set up the numbering on this question, this question is just about creating the copy)
So for example, when I call my program from the command prompt:
C:\MyLocation>java =jar myJavaProgram.jar myFileToRead.CustomExtension
Asides from reading the file, it should also create a copy at the same location called myFileToRead-ErrorReport.txt
Additionally: If the source file has no extension, I have to assume that it's still the correct extension, so there won't always be a '.myCustomExtension' segment to replace into .txt
The problem is that I don't know how to grab the file name, because it's coming from the args list of the main method. I am using the following to read the file
public static void main(String[] args) throws FileNotFoundException {
try{
File inputFile = new File(args[0]);
Scanner sc = new Scanner(inputFile);
while(sc.hasNext()){
System.out.println(sc.nextLine());
}
} catch (FileNotFoundException ex) {
System.out.println(ex.getMessage());
System.out.println("File not found.");
}
}
So how can I get that file name to make something like
File errorReport = new File("./" + inputFileName + ".txt"); ?

First the code. The explanations appear after the code.
public static void main(String[] args) {
if (args.length < 1) {
System.out.println("ERROR: Missing filename argument.");
}
else {
String filename = args[0];
if (filename.length() == 0) {
System.out.println("ERROR: Empty filename argument.");
}
else {
if (!filename.endsWith(".CustomExtension")) {
filename += ".CustomExtension";
}
String name = filename.substring(0, filename.indexOf(".CustomExtension"));
name += "-ErrorReport.txt";
File inputFile = new File(filename);
File directory = inputFile.getParentFile();
File errorReport = new File(directory, name);
System.out.println(errorReport.getAbsolutePath());
}
}
}
I make it a habit of checking the parameters. Hence I first check that the file name was supplied. If it was, then I check that it is not an empty string. Note that I have omitted some checks, for example checking whether the named file exists and is readable.
You wrote in your question that the file name argument may or may not include the .CustomExtension. Hence I check whether the supplied name ends with the required extension and append it if necessary. Now, since I know what the file name ends with, that means that the required part of the name is everything up to the extension and that's what the call to substring() gives me.
Once I have the required name, I just append the part that you want to append, i.e. -ErrorReport.txt.
Method getParentFile() in class java.io.File returns the directory that the file is located in. Hence I have the directory that the input file is in. Finally I can create the error report file in the same directory as the input file and with the desired file name. For that I use the constructor of class java.io.File that takes two parameters. Read the javadoc for details.
Note that creating a File object does not create the file. Creating an object to write to the file does, for example FileWriter or OutputStreamWriter.

Here is the code example to create a file, with filename passed from cmd line as argument and to get the same file name :
Class Demo{
public static void main(String[]args){
String path ="<path of file>"
String name= args[0];
File f = new File(path+name+".txt");
f.createNewFile(); //create file
System.out.println(f.getName()); // will give you the file name
}
}
cmd line : java -cp . Demo.java <filename>
Note : '.' used in the cmd if your class file is present in current dir
You can refer the code and modify to suit your requirement.
Hope this is what you are looking for.

Where to insert file path in this Java Program?

I am new to Java , I am currently analyzing a file compare tool with java that compare two files from this link:
http://www.java2s.com/Code/Java/File-Input-Output/Difftextfiledifferenceutility.htm
But no where in the file, the file path is mentioned. Where should I insert the file path? I searched google and checked Java Filestram and buffer input output stream. But did not found any useful information.
I also searched stackoverflow but it seems no such question exists.
Usually, the file path should be updated in main file, right?
But it seems that is missing in main file.
public static void main(String argstrings[])
{
if ( argstrings.length != 2 ) {
System.err.println("Usage: diff oldfile newfile" );
System.exit(1);
}
Diff d = new Diff();
d.doDiff(argstrings[0], argstrings[1]);
return;
}

Your program takes the file names as the parameter. So while giving the command line input you can give the full file paths. Something like this:
java yourClassName volume1:\dir1\filename1 volume2:\dir2\filename2

You can certainly do the way juned told you but if you want to the program to be more user friendly try to manipulate the main method like this
public static void main(String[] args) throws ParseException {
try{
Scanner in = new Scanner(System.in);
System.out.println("Enter the path of old file");
String oldFile = in.nextLine();
System.out.println("Enter the path of new file");
String newFile = in.nextLine();
Diff d = new Diff();
if(!oldFile.equals("") && !newFile.equals("")) {
d.doDiff(oldFile, newFile);
}
}
catch (Exception e){
e.printStackTrace();
}
}

Why is my program catching / throwing a FileNotFoundException when the file exists?

Java newbie here!
I'm writing a program to practice reading input and writing output to files. I've finished coding the program, but when I run it, the program just catches and proceeds with a FileNotFoundException.
The file is in the source folder for the program, and I've even tried placing it in every folder related to the program. I've tried:
Declaring the exceptions in the method header
Surrounding the section-in-question with a try/catch block.
Both of the above together.
Here's the relevant code that is causing problems. Is there something that sticks out that I'm missing?
public static void main(String[] args) throws FileNotFoundException {
Scanner keyboard = new Scanner(System.in);
String playerHighestScore = "", playerLowestScore = "";
int numPlayers = 0, scoreHighest = 0, scoreLowest = 0;
System.out.println("Enter an input file name: ");
String inputFileName = keyboard.nextLine();
String outputFileName = getOutputFileName(keyboard, inputFileName);
File inputFile = new File(inputFileName);
try {
Scanner reader = new Scanner(inputFile);
reader.close();
}
catch (FileNotFoundException exception) {
System.out.println("There was a problem reading from the file.");
System.exit(0);
}
Scanner reader = new Scanner(inputFile);
PrintWriter writer = new PrintWriter(outputFileName);

The answer is simple. If you get a FilENotFoundException, obviously the reason is File Not Found in the given path.
If you use an IDE, path for the working directory is different from the source directory.
For example, if you are using NetBeans, your source files are inside /src. But your working directory (.) is the project directory.
In the other hand, the problem may be the thing that #Don mentioned. If you are going for a cross platform approach, you can use "/" in paths. It works irrespective to the OS.
Example : String fileName = "C:/Directory/File.txt";
And these paths are case sensitive. So make sure you use the correct case. (It won't be a problem in Windows, until you package the program.)

File contents are wiped when trying to read from Scanner

So I'm trying to read a file using the Scanner, however, all the contents of the file are wiped, and then it reads nothing. Here are the methods I've ran in succession, in my Main method:
private static Scanner x;
private static Formatter y;
public void openMainFile(String name){
try{
x = new Scanner(new File("main.mcmm");
y = new Formatter("main.mcmm");
}catch(Exception e){
GUI.error(2);
}
}
This method runs perfectly fine
public void readModMainFile(){
while(x.hasNext()){
Main.name = x.next();
Main.ver = x.nextFloat();
Main.base = x.nextBoolean();
Main.dev = x.next();
Main.date = x.next();
}
}
After this method runs, the file is empty, and the 'Main.-' variables don't have any values.

Don't open the same file for reading and writing at the same time. Write into a temporary file first, then rename it. Alternatively, you can read the whole file first, store everything, close the Scanner and then you can overwrite the file.

Your Formatter is truncating the output file every time. From your comments in this post, you indicate that the number of variables will remain constant. You could use a temporary file to achieve this (+1 to #biziclop):
File inputFile = new File("main.mcmm");
Scanner scanner = new Scanner(inputFile);
File tempFile = File.createTempFile("main.mcmm",".temp");
Formatter y = new Formatter(tempFile);
y.format("%s", name);
// more reading & formatting, etc.
y.close();
scanner.close();
inputFile.delete();
tempFile.renameTo(inputFile);
Remember to close both the Scanner and Formatter so that the input & output files can be deleted & renamed respectively.

My java code is flawed, but i dont understand why

I am very new at java and my be missing something very basic. When i run my code i am trying to add value to accounts created in the code. When i try to run the code i recieve an error that a file cannot be found, but i thought that the file was created inside the code.
import java.util.Scanner;
import java.io.File;
import java.io.IOException;
class DoPayroll
{
public static void main(String args[])
throws
IOException
{
Scanner diskScanner =
new Scanner(new File("EmployeeInfo.txt"));
for (int empNum = 1; empNum <= 3; empNum++)
{
payOneEmployee(diskScanner);
}
}
static void payOneEmployee(Scanner aScanner)
{
Employee anEmployee = new Employee();
anEmployee.setName(aScanner.nextLine());
anEmployee.setJobTitle(aScanner.nextLine());
anEmployee.cutCheck(aScanner.nextDouble());
aScanner.nextLine();
}
}
once run i recieve the following error
Exception in thread "main" java.io.FileNotFoundException: EmployeeInfo.txt (No such file or directory)
at java.io.FileInputStream.open(Native Method)
at java.io.FileInputStream.<init>(FileInputStream.java:106)
at java.util.Scanner.<init>(Scanner.java:636)
at DoPayroll.main(jobexe.java:11)
i thought that in the above code using new Scanner(new File("EmployeeInfo.txt") would create the new file once i input a value. Please give me a simple solution and an explanation.

It will create a new file when you write to it. However to read from it, it must already exist. You might like to check it exists with
File file = new File("EmployeeInfo.txt");
if (file.exists()) {
Scanner diskScanner = new Scanner(file);
for (int empNum = 1; empNum <= 3; empNum++)
payOneEmployee(diskScanner);
}

The File object can't find the filename you've passed. You either need to pass the full path of EmployeeInfo.txt to new File(...) or make sure current working directory is the directory that contains this file.

The File constructor does not create a file. Rather, it creates the information in Java needed to access a file on disk. You'd have to actually do file IO in Java using the created File for a new file to be created.
The Scanner constructor requires an existing File. So you need a full path to the real, valid location of EmployeeInfo.txt or to create that file using File I/O first. This tutorial on I/O in Java will help.

You are mistaking instantiating an instance of class File with actually writing a temp file to Disk. Take this line
Scanner diskScanner =
new Scanner(new File("EmployeeInfo.txt"));
And replace it with this
File newFile = File.createTempFile("EmployeeInfo", ".txt");
Scanner diskScanner = new Scanner(newFile);
Edit: Peter makes a good point. I'm face palming right now.

You thought wrong :D A Scanner needs a existing file, which seems quite logical as it reads values and without a existing file its difficult to read. The documentation also states that:
Throws:
FileNotFoundException - if source is not found
So, in short: You must provide a readable, existing file to a scanner.

As the other answer explain, the file is not created just by using new File("EmployeeInfo.txt").
You can check is the file exists using
File file = new File("EmployeeInfo.txt");
if(file.exists()) {
//it exists
}
or you can create the file (if it doesn't exists yet) using
file.createNewFile();
that method returns true if the file was created and false if it already existed.