Guide to Java LineNumberReader

For reading a file line by line, the LineNumberReader class could be a perfect choice. It is a buffered character-input stream that keeps track of line numbers.

By default, line numbering begins at 0. A line is considered to be terminated by any one of a line feed (‘\n’), a carriage return (‘\r’), combination of both (‘\r\n’)  or any of the previous terminators followed by the end of stream, or end of stream not preceded by another terminator.

1. How Does LineNumberReader Works?

LineNumberReader is a subclass of the BufferedReader class and allows us to keep track of which line we are currently processing.

Line numbering begins at 0 (similar to array indices). Whenever the LineNumberReader encounters a line terminator by the wrapped Reader, the line number is incremented. At the end of the stream, the line number is incremented last time.

The LineNumberReader provides the following important methods:

• getLineNumber() : gets the current line number where the reader is reading.
• setLineNumber(int lineNumber) : sets the current line number. Note that this method does not change the current position of reader in the stream; it only changes the value that will be returned by getLineNumber().
• readLine() : reads the current line, not including any line termination characters, or null if the end of the stream has been reached.
• reset() : resets the stream to the most recent mark created by the mark() method.
• mark(n) : marks the present position in the stream. Calling reset() will attempt to reposition the stream to this point, and will also reset the line number appropriately.

LineNumberReader can be useful if we are parsing a text file that can contain errors. When reporting the error to the user, it is easier to correct the error if the error message includes the line number.

2. Using LineNumberReader

Let’s build a quick example to show the capabilities of LineNumberReader. This is the file content that I will use to read using LineNumberReader in the below example.

firstName=Lokesh
lastName=Gupta
blog=howtodoinjava
technology=java

In the given example, we are iterating over the lines using the method lineNumberReader.readLine() until it returns null. A null value means that all the lines in the file have been read.

final String fileName = "app.properties";

try(LineNumberReader lineNumberReader 
	= new LineNumberReader(new FileReader(filename))) {

	//Print initial line number
	System.out.println("Line " + lineNumberReader.getLineNumber());

	//Setting initial line number
	lineNumberReader.setLineNumber(5);

	//Get current line number
	System.out.println("Line " + lineNumberReader.getLineNumber());

	//Read all lines now; Every read increase the line number by 1
	String line = null;
	while ((line = lineNumberReader.readLine()) != null)
	{
		System.out.println("Line " + lineNumberReader.getLineNumber() + ": " + line);
	}
}
catch (Exception ex)
{
 	ex.printStackTrace();
} 

Program Output:

Line 0
Line 5
Line 6: firstName=Lokesh
Line 7: lastName=Gupta
Line 8: blog=howtodoinjava
Line 9: technology=java

Happy Learning !!

Source Code on Github