Java FileInputStream ObjectInputStream reaches end of file EOF

Question

I am trying to read the number of line in a binary file using readObject, but I get IOException EOF. Am I doing this the right way?

    FileInputStream istream = new FileInputStream(fileName);
    ObjectInputStream ois = new ObjectInputStream(istream);

    /** calculate number of items **/
    int line_count = 0;
    while( (String)ois.readObject() != null){            
        line_count++;
    }

Answer 1

readObject() doesn't return null at EOF. You could catch the EOFException and interpret it as EOF, but this would fail to detect distinguish a normal EOF from a file that has been truncated.

A better approach would be to use some meta-data. That is, rather than asking the ObjectInput how many objects are in the stream, you should store the count somewhere. For example, you could create a meta-data class that records the count and other meta-data and store an instance as the first object in each file. Or you could create a special EOF marker class and store an instance as the last object in each file.

Answer 2

No, you need to know how many objects there is in the binary file. You could write the number of objects at the beginning of the file (using writeInt for example) and read it while loading it.

Another option is to call ois.available() and loop until it returns 0. However, I am not sure if this is 100% sure.

Answer 3

It looks like the problem is with the data that you wrote out. Assuming the data is written as expected by this code, there shouldn't be a problem.

(I see you are reading Strings. This ObectInputStream isn't for reading text files. Use InputStreamReader and BufferedReader.readLine for that. Similarly if you have written the file with DataOutputSteam.writeUTF, read it with DataInputStream.readUTF)

Answer 4

No. Catch EOFException and use that to terminate the loop.