Opening and Closing a File in Java

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Opening and Closing a File
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Here s the function prototype for fopen(), found in the file <stdio.h>:
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FILE *fopen( const char *name, const char *mode );
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The const keyword marks a variable or parameter as read-only. In other words, fopen() is not allowed to modify the array of characters pointed at by name or mode. Here s another example:
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const int kMyInt = 27;
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This declaration creates an int named kMyInt and assigns it a value of 27 (we ll talk about definitions that also initialize in 11). More importantly, the value of kMyInt is now permanently set. kMyInt is now read-only. As long as kMyInt remains in scope, you can t change its value. I start all my const variables with the letter k. The first parameter to fopen(), name, tells fopen() which file you want to open. The second parameter, mode, tells fopen() how you ll be accessing the file. The three basic file modes are "r", "w", and "a", which stand for read, write, and append, respectively.
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CHAPTER 10: Working with Files
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CAUTION
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The mode parameter is char *, not char. In other words, mode is a zero-terminated string, so use "r", not 'r'.
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"r" tells fopen() that you want to read data from the file and that you won t be writing to
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the file at all. The file must already exist in order to use this mode. In other words, you can t use the mode "r" to create a file. The mode "w" tells fopen() that you want to write to the specified file. If the file doesn t exist yet, a new file with the specified name is created. If the file does exist, fopen() deletes it and creates a new empty file for you to write into.
CAUTION
This point bears repeating: calling fopen() with a mode of "w" will delete a file (along with the file s contents!) if the file already exists, essentially starting you over from the beginning of the file. Be careful!
The mode "a" is similar to "w". It tells fopen() that you want to write to the specified file and to create the file if it doesn t exist. If the file does exist, however, the data you write to the file is appended to the end of the file. If fopen() successfully opens the specified file, it allocates a struct of type FILE and returns a pointer to the FILE struct. The FILE struct contains information about the open file, including the current mode (e.g., "r", "w", or "a") as well as the current file position. The file position is a pointer into the file that acts like a bookmark in a book. When you open a file for reading, for example, the file position points to the first byte in the file. When you read the first byte, the file position moves to the next byte. It s not really important to know the details of the FILE struct. All you need to do is keep track of the FILE pointer returned by fopen(). By passing the pointer to a Standard Library function that reads or writes, you ll be sure the read or write takes place in the right file and at the right file position. You ll see how all this works as we go through this chapter s sample code. Here s a sample fopen() call:
FILE *fp;
if ( (fp = fopen( "My Data File", "r")) == NULL ) { printf( "File doesn't exist!!!\n" ); exit(1); }
CHAPTER 10: Working with Files
This code first calls fopen(), attempting to open the file named "My Data File" for reading. If fopen() cannot open the file for some reason perhaps you ve asked it to open a file that doesn t exist or you ve already opened the maximum number of files it returns NULL. In that case, we ll print an error message and exit.
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