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Using Random Access Files

The RandomAccessFile class implements both the DataInput and DataOutput interfaces and therefore can be used for both reading and writing. RandomAccessFile is similar to FileInputStream and FileOutputStream in that you specify a file on the native file system to open when you create it. You can do this with a file name or a File object. When you create a RandomAccessFile, you must indicate whether you will be just reading the file or also writing to it. (You have to be able to read a file in order to write it.) The following code creates a RandomAccessFile to read the file named farrago.txt:

new RandomAccessFile("farrago.txt", "r");

And this one opens the same file for both reading and writing:

new RandomAccessFile("farrago.txt", "rw");

After the file has been opened, you can use the common read or write methods defined in the DataInput and DataOutput interfaces to perform I/O on the file.

RandomAccessFile supports the notion of a file pointer. The file pointer indicates the current location in the file. When the file is first created, the file pointer is set to 0, indicating the beginning of the file. Calls to the read and write methods adjust the file pointer by the number of bytes read or written.

A ZIP file has the notion of a current file pointer.

In addition to the normal file I/O methods that implicitly move the file pointer when the operation occurs, RandomAccessFile contains three methods for explicitly manipulating the file pointer.

• int skipBytes(int) — Moves the file pointer forward the specified number of bytes
• void seek(long) — Positions the file pointer just before the specified byte
• long getFilePointer() — Returns the current byte location of the file pointer

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