Develop Reference

Why is this java code corrupting my pdf while checking access?

String pathSrc = "C:\\Users\\me\\Desktop\\somefile.pdf";
//should just check if file is opened by someone else
try
{
FileWriter fw = new FileWriter(pathSrc );
fw.close();
}
catch (IOException e)
{
System.out.println("File was already opened");
return;
}
This code should just check if pdf file is already opened. Instead after that code pdf file is corrupted. and can no longer be opened. Why is that?

Note that FileWriter starts empty everytime you instantiate a FileWriter with the Filename only, and then starts writing the data to the beginning of the file.
There's a second constructor that takes a boolean append flag that starts at the end of the file, appending data to the current file's contents.
This means that your code erases the whole pdf file and then close()s it, saving an empty PDF file, with zero bytes.
This simple change will fix your issue:
String pathSrc = "C:\\Users\\me\\Desktop\\somefile.pdf";
//should just check if file is opened by someone else
try {
FileWriter fw = new FileWriter(pathSrc, true);
fw.close();
}
catch (IOException e) {
System.out.println("File was already opened");
return;
}

How do I write an object to a file that already exists?

I'm having a problem with an assignment from school. I'm meant to output an arraylist of objects to a file, which I can do. But I'm supposed to check if the file exists, and if it does, then to output to that file, and not create a new one.
I've tried putting the FileOutputStream declaration outside of my if statement, but then the file will always exist.
I've also tried creating a new ObjectOutputStream in my first half of the if statement, but I get an IOException about the headers.
How do I write the objects (of class Employee) to a file that already exists?
public void saveEmployeesToFile() {
try {
File employeeFile = new File("CurrentEmployees.emp");
if (employeeFile.exists()) {
System.out.println("File already exists");
} else {
FileOutputStream employeeFileObject = new FileOutputStream(employeeFile);
ObjectOutputStream output = new ObjectOutputStream(employeeFileObject);
for (Employee thisEmp : getEmployees()) {
output.writeObject(thisEmp);
}
System.out.println("Employees successfully saved to new file");
employeeFileObject.close();
output.close();
}
} catch (IOException ioe) {
System.out.println("Error initializing stream");
System.out.println(ioe.getMessage());
}
}

You can leverage the newer NIO package.
Your question is essentially, "how do I create a FileOuputStream for a file that already exists?"
Path employeeFilePath = Paths.get(employeeFile.toURI());
FileOutputStream employeeFileObject;
if (employeeFile.exists()) {
employeeFileObject = Files.newOutputStream(employeeFilePath, StandardOpenOption.TRUNCATE_EXISTING);
}
else {
employeeFileObject = Files.newOutputStream(employeeFilePath, StandardOpenOption.CREATE_NEW);
}
// Proceed to write data

Get the filename of the file from FileNotFoundException

I want to get the filename of the file that do not exist when a file exception occur in my java application so that i can give a short message to the user.
catch (FileNotFoundException e) {
/* what code to put here to get the filename of the file */
}

This should display the non-existing file path:
try {
//access file
} catch(FileNotFoundException e) {
System.out.println(e.getMessage());
}
Output, for creating a Scanner with new Scanner(new File("C:/filetest")):
C:\filetest (The system cannot find the file specified)

You cannot grap properly the filename unless you parse the stacktrace in your catch block.

You can either store the filename outside of the try block, i.e.:
String filename = ...
try {
// process file
} catch (FileNotFoundException e) {
String message = String.format("The file % could not be found.", filename);
// Show message to user
}
Or you can check whether the file exists before trying to access it:
File file = new File(filename);
if (!file.exists()) {
// Show error to user.
}

Java IO Create / Append File

Im trying to create a file if it doesnt exist, if it does exist append to it.
Is this the best way to do it? Im not sure having two try catches inside one method is good personally?
public static void main(String [] args)
{
String fileLocation = "/temp/";
String name = "Bob";
String timeStamp = "1988-03-15";
Path path = Paths.get(fileLocation+ "info.log");
if(!Files.exists(path)){
try {
Files.createFile(path);
} catch (IOException e) {
e.printStackTrace();
}
}
try (BufferedWriter writer = Files.newBufferedWriter(path, StandardCharsets.UTF_8, StandardOpenOption.APPEND)) {
SimpleDateFormat tTimeFormatter = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss:SSS");
writer.write(tTimeFormatter.format(System.currentTimeMillis()) + " name: " + name + " Timestamp: "+ timeStamp);
writer.newLine();
} catch (IOException e) {
System.out.print(e.getStackTrace());
}
}

You can write to file with StandardOpenOptions: CREATE and APPEND.
Files.write(Paths.get(""), new byte[] {}, StandardOpenOption.CREATE, StandardOpenOption.APPEND);
CREATE - means if file doesn't exists it creates new one otherwise get existing.
APPEND - means append new data to existing content in file.
So, you can do all your operations with a single line.

The File.createNewFile() method creates a new, empty file named by this abstract pathname if and only if a file with this name does not yet exist. This methods return a true value if the file is created successfully and false if the file already exists or the operation failed.
if (myFile.createNewFile()){
System.out.println("File is created!");
}else{
System.out.println("File already exists.");
}

Try using the printWriter class like this
java.io.PrintWriter p = new java.io.PrintWriter(yourFileHere);
// You can use println to print a new line if it allready exists
p.println(yourTextHere);
// Or append to the end of the file
p.append("TEXT HERE!!!!")

java.nio.file.AccessDeniedException when reading a file

I want to read file content using this code:
String content = new String(Files.readAllBytes(Paths.get("/sys/devices/virtual/dmi/id/chassis_serial")));
On some systems this file is not present or it's empty. How I catch this exception? I want to print message "No file" when there is no file and there is no value.

The AccessDeniedException can be thrown only when using the new file API. Use an inputStream to open a stream from the source file so that you could catch that exception.
Try with this code :
try
{
final InputStream in = new Files.newInputStream(Path.get("/sys/devices/virtual/dmi/id/chassis_serial"));
} catch (FileNotFoundException ex) {
System.out.print("File not found");
} catch(AccessDeniedException e) {
System.out.print("File access denied");
}

Try to use filter file.canRead()) to avoid any access exceptions.

Create a File object and check if it exists.
If it does then it's safe to convert that file to a byte array and check that the size is greater then 0. If it is convert it to a String. I added some sample code below.
File myFile = new File("/sys/devices/virtual/dmi/id/chassis_serial");
byte[] fileBytes;
String content = "";
if(myFile.exists()) {
fileBytes = File.readAllBytes(myfile.toPath);
if(fileBytes.length > 0) content = new String(fileBytes);
else System.out.println("No file");
else System.out.println("No file");
I know it's not the one liner you were looking for. Another option is just to do
try {
String content = new String(Files.readAllBytes(Paths.get("/sys/devices/virtual/dmi/id/chassis_serial")));
} catch(Exception e) {
System.out.print("No file exists");
}
Read up on try catch blocks here like MrTux suggested, as well as java Files and java io here.