How do I read data from a Random Access File?

**ineedahero** · December 19th, 2013, 02:27 PM

Let's say I have a random access file that holds the names of 1000 students.

Somewhere in there is a student with the first name "Eli". (assume only one student has the first name Eli).

Maybe he's the 400th person in the file.

How do I get to him???

This code might work...but random access files don't have "hasNext()" function.

So how am I supposed to do it?

And if I also want Eli's last name, and I know it comes right after his first name, how would I get to that?

                        String fileName = "";
                        boolean nameFound = false;
 
                        while (raf.hasNext() && !nameFound)
                        {
                                fileName = raf.readUTF();
 
                                if (fileName.equals(name))
                                {
                                        nameFound = true;
                                }
                        }

**andbin** · December 19th, 2013, 04:48 PM

Originally Posted by ineedahero

Let's say I have a random access file that holds the names of 1000 students.

In general, a "random access file" simply means a file in which you can quickly "jump" back and forth at any location at any moment. Nothing more.

If you have a file that contains N information records, and you can jump at any record using a random access, then this should mean that the content is, at least, in one of the following formats:
1) All "records" take a well known, fixed, number of bytes (eg. 100 bytes for each record).
2) There's some sort of header or footer (or other more complex structure) that can "index" all the records. For example, given the record numbered 34, obtain its absolute position in the file. In this scenario, records can be at fixed or variable length.

Do you have one of these two scenarios? If not, please, clarify.

**Waker3210** · December 19th, 2013, 05:02 PM

Well, i am by no means an expert on Random Access Files, but after looking up a few things I have an idea of how you can navigate through the file.

The RandomAccessFile has a method that returns the files length. So you could use a variable that increments as you check each byte and stops after it is equal to the length.

Example:

long curPos = file.getFilePointer();
 
while(curPos < file.length()){
     String fname = file.readUTF();
     curPos = file.getFilePointer();
}

note: the variable file represents the RandomAccessFile and the readUTF() method reads in the next string from the file.

**ineedahero** · December 19th, 2013, 05:03 PM

Naw, I don't have any of that.

I thought Random Access Files took care of everything automatically.

I'm not even going to both with this stuff then. Thanks for the clarification.

**andbin** · December 19th, 2013, 05:16 PM

Originally Posted by ineedahero

Naw, I don't have any of that.

I thought Random Access Files took care of everything automatically.

No. Technically every file can be random accessed, if you have the appropriate API (and in Java there is java.io.RandomAccessFile). But this doesn't mean that all file types are appropriate for random access!
For a "pure" text file it has no sense the random access, you can't say "move at line 1234", because there's no "indexing" of lines, you don't know the absolute offset of line 1234. You have to read sequentially all lines until the end of file or until you find a line that you want.

So, if you have a "text" file, you can read on "lines" at least in two ways:
1) java.util.Scanner, with hasNextLine() / nextLine()
2) java.io.BufferedReader, with readLine(). Note: BufferedReader cannot be used alone for a file, you need a FileReader or a FileInputStream+InputStreamReader.

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