Java - LineNumberReader mark(int readAheadLimit) method

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Description

The Java LineNumberReader mark(int readAheadLimit) method marks the current position in the stream so that it can be reset later using reset(). The parameter r specifies the read-ahead limit, which defines how many characters can be read before the mark becomes invalid.

Declaration

Following is the declaration for java.io.LineNumberReader.mark(int readAheadLimit) method −

 public void mark(int readAheadLimit) 

Parameters

readAheadLimit − limit on the number of characters that may be read while still preserving the mark.

Return Value

The method does not return any value.

Exception

IOException − If an I/O error occurs.

Example - Usage of LineNumberReader mark(int readAheadLimit) method

The following example shows the usage of Java LineNumberReader mark(int readAheadLimit) method.

LineNumberReaderDemo.java

 package com.tutorialspoint; import java.io.FileReader; import java.io.IOException; import java.io.LineNumberReader; public class LineNumberReaderDemo { public static void main(String[] args) throws IOException { FileReader fr = null; LineNumberReader lnr = null; int i; try { // create new reader fr = new FileReader("test.txt"); lnr = new LineNumberReader(fr); // reads and prints data from reader System.out.println((char)lnr.read()); System.out.println((char)lnr.read()); // mark invoked at this position lnr.mark(0); System.out.println("mark() invoked"); System.out.println((char)lnr.read()); System.out.println((char)lnr.read()); // if this reader supports mark() an reset() if(lnr.markSupported()) { // reset() repositioned the reader to the mark lnr.reset(); System.out.println("reset() invoked"); System.out.println((char)lnr.read()); System.out.println((char)lnr.read()); } } catch(Exception e) { // if any error occurs e.printStackTrace(); } finally { // closes the stream and releases system resources if(fr!=null) fr.close(); if(lnr!=null) lnr.close(); } } } 

Output(Assuming test.txt contains "ABCDE")

Let us compile and run the above program, this will produce the following result−

 A B mark() invoked C D reset() invoked C D 

Example - Marking and Resetting a Line

The following example shows the usage of Java LineNumberReader mark(int readAheadLimit) method. This example demonstrates marking a position in the stream, reading some lines, and then using reset() to go back to the marked position.

LineNumberReaderDemo.java

 package com.tutorialspoint; import java.io.IOException; import java.io.LineNumberReader; import java.io.StringReader; public class LineNumberReaderDemo { public static void main(String[] args) { String text = "First Line\nSecond Line\nThird Line"; LineNumberReader lnr = new LineNumberReader(new StringReader(text)); try { System.out.println("Reading first line: " + lnr.readLine()); // Mark the current position lnr.mark(50); System.out.println("Marked at Line Number: " + lnr.getLineNumber()); System.out.println("Reading second line: " + lnr.readLine()); System.out.println("Reading third line: " + lnr.readLine()); // Reset to the marked position lnr.reset(); System.out.println("After reset, current line: " + lnr.readLine()); System.out.println("Line Number after reset: " + lnr.getLineNumber()); lnr.close(); } catch (IOException e) { e.printStackTrace(); } } } 

Output(if example.txt contains "Hello")

Let us compile and run the above program, this will produce the following result−

 Reading first line: First Line Marked at Line Number: 1 Reading second line: Second Line Reading third line: Third Line After reset, current line: Second Line Line Number after reset: 2 

Explanation

• Reads the first line (First Line).

• Calls mark(50), saving the position at line 1.

• Reads two more lines (Second Line and Third Line).

• Calls reset(), which returns to the marked position.

• Reads again from the marked line (Second Line), confirming that reset() worked.

Example - Marking and Skipping Lines

The following example shows the usage of Java LineNumberReader mark(int readAheadLimit) method. This example marks a position, skips a few lines, and then resets to the marked position.

LineNumberReaderDemo.java

 package com.tutorialspoint; import java.io.IOException; import java.io.LineNumberReader; import java.io.StringReader; public class LineNumberReaderDemo { public static void main(String[] args) { String text = "Apple\nBanana\nCherry\nDate\nElderberry"; LineNumberReader lnr = new LineNumberReader(new StringReader(text)); try { System.out.println("Reading first line: " + lnr.readLine()); // Mark the position after reading the first line lnr.mark(100); System.out.println("Marked at Line Number: " + lnr.getLineNumber()); // Skip two lines lnr.readLine(); lnr.readLine(); System.out.println("After skipping, Line Number: " + lnr.getLineNumber()); // Reset to the marked position lnr.reset(); System.out.println("After reset, reading again: " + lnr.readLine()); System.out.println("Line Number after reset: " + lnr.getLineNumber()); lnr.close(); } catch (IOException e) { e.printStackTrace(); } } } 

Output

Let us compile and run the above program, this will produce the following result−

 Reading first line: Apple Marked at Line Number: 1 After skipping, Line Number: 3 After reset, reading again: Banana Line Number after reset: 2 

Explanation

• Reads the first line (Apple).

• Calls mark(100), marking the position at line 1.

• Reads two more lines (Banana and Cherry), moving to line 3.

• Calls reset(), returning to line 1.

• Reads again, confirming that resetting works as expected.