How is actually End of File detected in java?

I never thought about it.

But if you read a file you can use for example this code.

FileReader fileReader = new FileReader("c:\\data\\input-text.txt");

int data = fileReader.read();
while(data != -1) {
  data = fileReader.read();
}

But how is actually recognised that the file ends. Is this because operating system know size of the file. Or is there a special character . I think java will call some C/C++ function from operating system and this function will return -1 , so java knows end of file is reached. But how does operating system know that file end is reached. Which special character is used for this.

> How is actually End of File detected in java?

Java doesn't detect it. The operating system does.

The meaning of end-of-file depends on the nature of the "file" that you are reading.

• If the file is a regular file in a file system, then the operating system knows or can find out what the actual file size is. It is part of the file's metadata.

• If the file is a Socket stream, then end-of-file means that all available data has been consumed, and the OS knows that there cannot be any more. Typically, the socket has been closed or half closed.

• If the file is a Pipe, then end-of-file means that the other end of the Pipe has closed it, and there will be no maore data.

• If the file is a Linux/UNIX device file, then the precise end-of-file meaning will be device dependent. For example, if the device is a "tty" device on Linux/UNIX, it could mean:

  • the modem attached to the serial line has dropped out
  • the tty was in "cooked" mode and received the character that denotes EOF
  • and possibly other things.

It is common for a command shell to provide a way to signal an "end of file". Depending on the implementation, it may implement this itself, or it may be implemented at the device driver level. In either case, Java is not involved in the recognition.

> I think java will call some C/C++ function from operating system and this function will return -1 , so java knows end of file is reached.

On Linux / UNIX / MacOS, the Java runtime calls the read(fd, buffer, count) native library method. That will return -1 if the fd is at the end-of-file position.

I see the chances of most popular file systems like ext and NTFS using a delimiter/ special char to mark the end of data as very slim. This is because files often have to store binary information too rather than text data and if the delimiter is present within its data, it can easily confuse the OS. In Linux, VFS (Virtual Filesystem Layer) offloads these details to implementations themselves and most of them construct a unique iNode (sort of like metadata) for every file that's resident in the filesystem. iNodes tend to have information on the blocks where the data is stored and also the exact size of the file among other things. Detecting EOF becomes is trivial when you have those.