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FAQs and Interesting Questions

Reading from a File Efficiently

Q8. How to move the file pointer to skip reading unwanted input data?

A8. It is possible that when a program reads a large file, it wants to skip reading the first N lines of data. Skipping saves time. Just to think if the program had to read all N lines of data just to get to one particular line of a large input file, it would be very inefficient. In Java, under the FileInputStream class (that we have already discussed), there is a method called skip:

java.io
Class FileInputStream

java.lang.Object
 java.io.InputStream
 java.io.FileInputStream

Method Summary
long	skip(long n)           Skips over and discards n bytes of data from the input stream.

There is also another class called DataInputStream (that we have already discussed), it has several methods to read specific data types from an input file, in addition to a method that skips. These methods are overloaded. One overload allows the programmer to specify an offset:

java.io
Class DataInputStream

java.lang.Object
 java.io.InputStream
 java.io.FilterInputStream
 java.io.DataInputStream

Method Summary
int	read(byte[] b, int off, int len)           Reads up to len bytes of data from the contained input stream into an array of bytes.
void	readFully(byte[] b, int off, int len)           See the general contract of the readFully method of DataInput.
int	skipBytes(int n)           See the general contract of the skipBytes method of DataInput.

But what about moving the file pointer to a specific location?

Q9. How to move the file pointer to a specific location?

A9. There is one way. We use the methods “mark()” and “reset()” that are readily available (inherited) from the java.io.FilterInputStream class (for the DataInputStream class) and the java.io.InputStream class (for the FileInputStream class). Basically, you could “mark” a position in the input stream, and then “reset” to get back to that position.

mark

 public void mark(int readlimit)

Marks the current position in this input stream. A subsequent call to the reset method repositions this stream at the last marked position so that subsequent reads re-read the same bytes. The readlimit argument tells this input stream to allow that many bytes to be read before the mark position gets invalidated.

reset

 public void reset()
 throws IOException

Repositions this stream to the position at the time the mark method was last called on this input stream. Stream marks are intended to be used in situations where you need to read ahead a little to see what's in the stream. Often this is most easily done by invoking some general parser. If the stream is of the type handled by the parse, it just chugs along happily. If the stream is not of that type, the parser should toss an exception when it fails. If this happens within readlimit bytes, it allows the outer code to reset the stream and try another parser.

So, in short, there is not an easy way to move the file pointer around in a file, as easy as one could do in ‘C’ or ‘C++’.