I am new to Java. I programmed in C++. I am trying to work with files but my code fails when I try create a file, that is, when the program tests if the file exists, it fails, but I have already created the file.
public Schedule(String name, String event)
{
String filename= name+event+".txt";
File TimeTable=new File(filename);
if (TimeTable.exists()&&TimeTable.isFile()){
writeToFile(TimeTable,name,event,filename);
System.out.println("In constructor");
}//fails here
}
I have already created the file
If the following line is the reason you are saying this, then you're wrong.
File TimeTable=new File(filename);
This is making an abstract representation of file/directory pathnames but will not make the file if it doesn't exist.
File TimeTable=new File(filename);
TimeTable.createNewFile();
The createNewFile method will make the physical file if it doesn't already exist.
Related
I am creating a file like this (I am sending arg[0] as the name of the file to be created).
No file is created I searched through the source of the project and found nothing, why?
import java.io.File;
public class Test {
public static void main (String [] args)
{
File f=new File(args[0]);
}
}
Try with
File f=new File(args[0]);
f.createNewFile();
File is just a representation of the path. You need to actually open an output stream with that file and write to that for a file to be created.
This is normal.
A File is an abstract object. It may, or may not, refer to an existing resource on the filesystem.
But since this is 2015, drop File, use java.nio.file instead:
final Path path = Paths.get(args[0]);
Files.createFile(path);
But really, you shouldn't use File in 2015. Seriously. Yes, .createNewFile() exists on File but... Well, read the page. In short: returns a boolean, need to check the return value, if false, SOL, you can't even diagnose.
Edit: a page to learn how to use java.nio.file: here
(shameless self-advertising for both links, sorry for that)
Just creating file object does not create physical file on disk.Actual file is created with f.createNewFile() as explained in below demo
When yo do File file=new File(args[0]); file just represents the java object not the file object on file system
Here is demo for basic file create and delete operations
public class FileDemo {
public static void main(String[] args) {
File f = null;
try{
// create new file
f = new File("test.txt");
// tries to create new file in the system
f.createNewFile();
// deletes file from the system
f.delete();
}catch(Exception e){
e.printStackTrace();
}
}
}
Executing new File(...) does not create a file in the file system. A File object is just a way of representing the path for a file system object that may or may not existing.
The typical way to create a file is to open a FileOutputStream or FileWriter for the file. The file is created even if you don't write anything. Other alternatives are to call File.createNewFile() or File.createTempFile(...).
I would like to write to a .txt file that is inside a package. I can get it to read from the exact location the .txt file is stored but not from inside the package. I'm assuming it is using class loaders but I cannot seem to get it to work.
Here is what I have so far.
public void writeFile(String fileLocation) {
Writer output = null;
File file = new File(fileLocation);
try {
output = new BufferedWriter(new FileWriter(file));
output.append("WRITING TEST");
output.close();
} catch (IOException ex) {
System.out.println("Couldn't write to file.");
}
}
Then I use this in another class to write.
WriteFile writeFile = new WriteFile();
writeFile.writeFile("src/com/game/scores.txt");
I understand that if using class loaders you remove "src/" because that will no longer exist when the program is compiled in a .jar.
It is not possible to write or update a file inside jar. Since jar itself is a file.
Please refer this link.
Write To File Method In JAR
You could use a class in that package to give you the location of the folder.
Try something like
public URL getPackageLocation() {
return getClass().getResource(".");
}
This should give you the location of the folder from which this method is being called from.
From the comments you already know that you cann't write to a file, which resides in a JAR file. At best what you can do, is creating your file, relative to the path where the JAR is located like bellow:
mylocation
|-- my-jar.jar
|-- com
|--game
|--myfile.txt
I would like to write to it to update the scores in my game as the
user goes through the levels.
While it might be possible to write to the JAR file, I don't recommend it for this use case. Just write it somewhere at:
Path userdir = Paths.get(System.getProperty("user.home"), ".myApp", "<my app version>");
You can't write into Jar file. Writing into Jar file is not recommendable. You can write outside the jar file.
Please refer this and this stack overflow question for more details.
I'm sure I'm missing something basic here.
I'm trying to create a new file on my drive, but I'm getting an error:
Exception in thread "main" java.io.FileNotFoundException: C:\ProgramData\msena\test.txt (The system cannot find the file specified)
at java.io.FileInputStream.open(Native Method)
at java.io.FileInputStream.<init>(FileInputStream.java:120)
at java.io.FileReader.<init>(FileReader.java:55)
at net.meosoft.relatetoit.core.HibernateSessionFactory.main(HibernateSessionFactory.java:89)
My code at the moment is:
final File file = new File("C:\\ProgramData\\uname2\\test.txt");
final BufferedReader in = new BufferedReader(new FileReader(file));
while(in.ready()) {
System.out.println(in.readLine());
}
in.close();
What's wrong at the moment? I want to just read, even if it's there (so file should be made).
Java doesn't automatically check that File() exists, nor will it automatically create it if you ask it.
You'll need to do one of the following:
Add in a check for the file's existence: if(file.exists()) { ... }.
Add in a check, similar to above, but then if it doesn't exist, call: file.createNewFile();. This will make a new file on the file system for you to use.
If that still doesn't work, I'd check you have write permissions to that directory. :)
The File class represents the path to a file, not the file itself. If the file does not exist (!File.exists()), an exception will be thrown when you try to access it. Make sure the path to the file is correct and that you have permission to read from that location.
If you want to create the file, you can use File.createNewFile().
this is the method to create file.
Formatter output;//pointer to an object that will write to a file
public void createFile(){
try{
output = new Formatter("C:\\ProgramData\\uname2\\test.txt");
//test.txt is the name of the file to be created
//create file in the same folder called test.txt
//if existed overwrite it
}
catch(FileNotFoundException e){
System.err.println("Error creating file"+e.getMessage());
}
call createFile() in the main
CreateTextFile file = new CreateTextFile();
file.createFile();
Check your file name. It should not contain any colon.
In an Android application I'm working on, the user should be able to create a new CSV file on the SD card, named using text they input in an EditText.
The problem is that after instantiating the File using the directory and filename, file.exists() returns false, even when the file does indeed exist at that location. I have browsed to SD card using an Android file browser and through Windows Explorer, and the file does exist.
Is this the correct way to check if the file already exists, and if so, what am I missing so that it returns true when it exists?
String csvname = edittext.getText().toString() + ".csv";
File sdCard = Environment.getExternalStorageDirectory(); //path returns "/mnt/sdcard"
File dir = new File (sdCard.getAbsolutePath() + "/Android/data/" + getPackageName() + "/files/"); // path returns "/mnt/sdcard/Android/data/com.phx.license/files"
dir.mkdirs();
File file = new File(dir, csvname); //path returns "/mnt/sdcard/Android/data/com.phx.license/files/Test.csv"
if(!file.exists()) //WHY DOES IT SAY IT DOESN'T EXIST WHEN IT DOES?
{
...
}
If you use createNewFile it will only create a file if it does not already exist.
Java Files Documentation
public boolean createNewFile()
throws IOException
Atomically creates a new, empty file named by this abstract pathname if and only if a file with this name does not yet exist. The check for the existence of the file and the creation of the file if it does not exist are a single operation that is atomic with respect to all other filesystem activities that might affect the file.
Note: this method should not be used for file-locking, as the resulting protocol cannot be made to work reliably. The FileLock facility should be used instead.
Returns:
true if the named file does not exist and was successfully created; false if the named file already exists
Throws:
IOException - If an I/O error occurred
SecurityException - If a security manager exists and its SecurityManager.checkWrite(java.lang.String) method denies write access to the file
Since:
1.2
Creating a new file object like so new File(dir, csvname); does not create a new file in the file system.
You need to write data to it first.
I had the exact same issue but with yarn on hadoop where a spark job was trying to execute a command.
It was a file permission issue. I troubleshooted it by code like below which is in scala. exists and notExists both return false, which means the jvm is not able to tell if the file exists or not.
import java.nio.file.Path
import java.nio.file.Paths
val path = Paths.get(fileLocation);
println(":"+ Files.exists(path)+ ":" + Files.notExists(path))
Im trying to write a program to read a text file through args but when i run it, it always says the file can't be found even though i placed it inside the same folder as the main.java that im running.
Does anyone know the solution to my problem or a better way of reading a text file?
Do not use relative paths in java.io.File.
It will become relative to the current working directory which is dependent on the way how you run the application which in turn is not controllable from inside your application. It will only lead to portability trouble. If you run it from inside Eclipse, the path will be relative to /path/to/eclipse/workspace/projectname. If you run it from inside command console, it will be relative to currently opened folder (even though when you run the code by absolute path!). If you run it by doubleclicking the JAR, it will be relative to the root folder of the JAR. If you run it in a webserver, it will be relative to the /path/to/webserver/binaries. Etcetera.
Always use absolute paths in java.io.File, no excuses.
For best portability and less headache with absolute paths, just place the file in a path covered by the runtime classpath (or add its path to the runtime classpath). This way you can get the file by Class#getResource() or its content by Class#getResourceAsStream(). If it's in the same folder (package) as your current class, then it's already in the classpath. To access it, just do:
public MyClass() {
URL url = getClass().getResource("filename.txt");
File file = new File(url.getPath());
InputStream input = new FileInputStream(file);
// ...
}
or
public MyClass() {
InputStream input = getClass().getResourceAsStream("filename.txt");
// ...
}
Try giving an absolute path to the filename.
Also, post the code so that we can see what exactly you're trying.
When you are opening a file with a relative file name in Java (and in general) it opens it relative to the working directory.
you can find the current working directory of your process using
String workindDir = new File(".").getAbsoultePath()
Make sure you are running your program from the correct directory (or change the file name so that it will be relative to where you are running it from).
If you're using Eclipse (or a similar IDE), the problem arises from the fact that your program is run from a few directories above where the actual source is located. Try moving your file up a level or two in the project tree.
Check out this question for more detail.
The simplest solution is to create a new file, then see where the output file is. That is the correct place to put your input file into.
If you put the file and the class working with it under same package can you use this:
Class A {
void readFile (String fileName) {
Url tmp = A.class.getResource (fileName);
// Or Url tmp = this.getClass().getResource (fileName);
File tmpFile = File (tmp);
if (tmpFile.exists())
System.out.print("I found the file.")
}
}
It will help if you read about classloaders.
say I have a text file input.txt which is located on the desktop
and input.txt has the following content
i came
i saw
i left
and below is the java code for reading that text file
public class ReadInputFromTextFile {
public static void main(String[] args) throws Exception
{
File file = new File(
"/Users/viveksingh/desktop/input.txt");
BufferedReader br
= new BufferedReader(new FileReader(file));
String st;
while ((st = br.readLine()) != null)
System.out.println(st);
}
}
output on the console:
i came
i saw
i left