How can i return the address of a line in a Random Access file?

i'm trying to create a Random Access file in java. I write something in a new line.

How can i return the address of that line in Java?

Also, I'm a bit confused with RAFs.

For example i have a file that consists of the following entries in alphabetical manner

George 10 10 8

Mary 9 10 10

Nick 8 8 8

Nickolas 10 10 9

I would like to return the grades of Nickolas. How can i declare that in a RAF?

Is there any method that can "read("Nickolas")" and return to me the line?

Thanks, in advance

Solution

Random access files usually contain binary data rather than ascii (e.g. plain text) data. The example you are showing is ascii.

Since the data is ascii, this means it's not as easy to seek to various places in the file. In fact, generally the approach to get the grades for Nickolas would be to read the file line by line and parse each line into columns. Then, compare the first column for Nickolas.

For example,


BufferedReader in = new BufferedReader(new FileReader("grades.txt"));
String line = in.readLine();
while(null != line) {
  String [] columns = line.split(" ");
  if( columns[0].equals("Nickolas") )
    System.out.println("I found the line! " + line);
  line = in.readLine();
}

EDIT:

There are a number of ways to speed this up. Here are three:

Storing all data in a HashMap

If you don't have too many records, or if each record doesn't take much space, you could read them all into RAM. You can also use a HashMap to map the name of the student to their record. For example:


HashMap<String, Student> grades = new HashMap<String, Student>();
BufferedReader in = new BufferedReader(new FileReader("grades.txt"));
String line = in.readLine();
while(null != line) {
  String [] columns = line.split(" ");
  grades.put( column[0],
    new Student( /* create student class instance from columns */ );
  line = in.readLine();
}

Now, lookups will be extremely fast.

Using a Binary Search

If you have too many records to fit in RAM, you can write all of the student data to a random access (binary) file. Here, you have a couple of options: you can either make each record vary in length, or you can make each record have a fixed length. Fixed length records are easier for some kinds of searching, like binary searches.

For example, if you know each record is 100 bytes, then you know how to get to the n'th record in the binary file storing the records. Basically, read 99*n bytes. Then the next 100 bytes are the 100th record.

Thus, if the records are sorted by student name, you can very easily use a binary search to find a specific student. This approach will still be fast, albeit not as fast as the RAM-based data structure.

Using a HashMap as an index

Yet another option is to combine the two approaches I mentioned above. Write the data to a binary file, and store the byte offsets of the records in a hash map. The hash map can use the student name as the key as before, but then stores a long integer offset to the record in the random access file. Thus, to look up a specific student, you find the byte offset using the hash map, and then "seek" to the record in the file and then read it. This last approach works even if the records vary in length.