Java FileInputStream ObjectInputStream reaches end of file EOF

java file-io fileinputstream objectinputstream
user69514 picture user69514 · Apr 13, 2010 · Viewed 60.4k times · Source

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

erickson picture erickson · Apr 13, 2010

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.

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