Mark Current Position in Input Stream in Java

In this article, we will learn to mark and reset the current position in a file input stream in Java. Marking the current position in an input stream is a crucial functionality provided by Java's InputStream class. It enables developers to reset the stream to a previously marked position, facilitating efficient data handling, especially in scenarios like re-reading or backtracking within a file.

Key Concepts

Following are the key concepts we need to know to mark the current position in this input stream ?

• InputStream Class: The InputStream class is an abstract class that represents an input stream of bytes. Subclasses like FileInputStream provide concrete implementations for reading data from files.
• mark() Method: The mark() method is used to mark the current position in the input stream. Once marked, you can return to this position later using the reset() method.
• reset() Method: The reset() method repositions the stream to the previously marked location. It works only if mark() was called earlier and the stream supports marking.
• markSupported() Method: This method checks whether the input stream supports marking. Not all streams support this functionality.

Marking and Resetting a Position in a File Input Stream

The method java.io.InputStream.mark() is used to mark the current position in this input stream. This method requires a single parameter i.e. the bytes that can be read before the position marked is invalidated.

Opening the file input stream ?

i = new FileInputStream("C://JavaProgram//data.txt");

Marking the current position ?

i.mark(0);

 Resetting to the marked position ?

i.reset();

Example

A program that demonstrates this is given as follows ?

import java.io.FileInputStream;
import java.io.InputStream;
public class Demo {
   public static void main(String[] args) throws Exception {
      InputStream i = null;
      try {
         i = new FileInputStream("C://JavaProgram//data.txt");
         System.out.println("Char : "+(char)i.read());
         System.out.println("Char : "+(char)i.read());
         System.out.println("Char : "+(char)i.read());
         i.mark(0);
         System.out.println("Char : "+(char)i.read());
         System.out.println("Char : "+(char)i.read());
         if(i.markSupported()){
            i.reset();
            System.out.println("Char : "+(char)i.read());
            System.out.println("Char : "+(char)i.read());
         }
      } catch(Exception e) {
         e.printStackTrace();
      }
   }
}

Output

Char : D
Char : A
Char : T
Char : A

Time Complexity: The overall time complexity is O(n), where n is the total number of characters read.

Space Complexity: The overall space complexity is O(1) (constant space) since no additional data structures are used that depend on the size of the file.