Text and Binary Files
Random-access files
Read and write
Two interfaces implemented by the RandomAccessFile class
DataOutput DataInput
Methods of the RandomAccessFile class used for input and output
| Method | Throws | Description |
|---|---|---|
| seek(long) | IOException | Sets the pointer to the specified number of bytes from the beginning of the file. If set beyond the end of the file, the pointer will be moved to the end of the file. |
| length() | IOException | Returns a long for the number of bytes in the file. |
| setLength(long) | IOException | Sets the length of the file to the specified number of bytes. |
| long getFilePointer() | IOException | Returns the current offset, in bytes, from the beginning of the file to where the next read or write occurs. |
| final void writeChar(int v) | IOException | Writes a character to the file as a two-byte Unicode, with the high byte written first. |
| final void writeChars(String s) | IOException | Writes a string to the file as a sequence of characters. |
| close() | IOException | Closes the file. |
Code that writes data to a file
RandomAccessFile productsFile =
new RandomAccessFile(file, "rw");
// write 3 records with codes and prices to the file
String[] codes = {"java", "jsps", "txtp"};
double[] prices = {49.5, 49.5, 20.0};
for (int i = 0; i < codes.length; i++)
{
productsFile.writeChars(codes[i]);
productsFile.writeDouble(prices[i]);
}
productsFile.close();
Code that reads data from a file
final int RECORD_LENGTH = 16; // 4 chars @ 2 bytes per char
// + 1 double @ 8 bytes
RandomAccessFile productsFile =
new RandomAccessFile(file, "r");
// move the pointer to the third record
int recordNumber = 3;
productsFile.seek((recordNumber - 1) * RECORD_LENGTH);
// read the third record
String code = "";
for (int i = 0; i < 4; i++)
code += productsFile.readChar();
double price = productsFile.readDouble();
productsFile.close();