Java - LineNumberReader Class

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Introduction

The Java LineNumberReader class is a buffered character-input stream that keeps track of line numbers.A line is considered to be terminated by any one of a line feed ('\n'), a carriage return ('\r'), or a carriage return followed immediately by a linefeed.

Class declaration

Following is the declaration for Java.io.LineNumberReader class −

 public class LineNumberReader extends BufferedReader 

Field

Following are the fields for Java.io.LineNumberReader class −

• protected Object lock − This is the object used to synchronize operations on this stream.

Class constructors

Class methods

Methods inherited

This class inherits methods from the following classes −

• Java.io.BufferedReader
• Java.io.Reader
• Java.io.Object

Example - Reading a File with Line Numbers

The following example shows the usage of Java LineNumberReader getLineNumber() method. This example reads a text file line by line and prints the current line number.

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) { try { // Create a LineNumberReader from a file LineNumberReader lnr = new LineNumberReader(new FileReader("example.txt")); String line; while ((line = lnr.readLine()) != null) { System.out.println("Line " + lnr.getLineNumber() + ": " + line); } lnr.close(); } catch (IOException e) { e.printStackTrace(); } } } 

Output(if example.txt contains multiple lines.)

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

 Line 1: Hello Line 2: World Line 3: Java Programming 

Explanation

• LineNumberReader reads lines from "example.txt".

• readLine() reads a line, and getLineNumber() returns the current line number.

• The line number starts at 1 and increments automatically.

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 - Reading Characters One by One

The following example shows the usage of Java LineNumberReader read() method. This example reads characters from a LineNumberReader one at a time and prints their ASCII values along with the current line number.

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 = "Hello\nWorld\nJava"; LineNumberReader lnr = new LineNumberReader(new StringReader(text)); try { int character; while ((character = lnr.read()) != -1) { // Read one character at a time System.out.println("Character: " + (char) character + ", Line Number: " + lnr.getLineNumber()); } lnr.close(); } catch (IOException e) { e.printStackTrace(); } } } 

Output

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

 Character: H, Line Number: 0 Character: e, Line Number: 0 Character: l, Line Number: 0 Character: l, Line Number: 0 Character: o, Line Number: 0 Character: , Line Number: 1 Character: W, Line Number: 1 Character: o, Line Number: 1 Character: r, Line Number: 1 Character: l, Line Number: 1 Character: d, Line Number: 1 Character: , Line Number: 2 Character: J, Line Number: 2 Character: a, Line Number: 2 Character: v, Line Number: 2 Character: a, Line Number: 2 

Explanation

• The read() method reads characters one at a time.

• Line number starts at 0 and increments when a newline (\n) is encountered.

• The program prints the character and its corresponding line number.