To use Files.lines() to process a text file line by line in Java, you should follow the pattern of returning a Stream<String> within a try-with-resources block. This approach is memory-efficient because it reads the file lazily, meaning it doesn’t load the entire file into memory at once.

Basic Implementation

package org.kodejava.nio;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.stream.Stream;

public class FileStreamExample {
    public static void main(String[] args) {
        Path path = Paths.get("example.txt");

        // Use try-with-resources to ensure the stream (and underlying file) is closed
        try (Stream<String> lines = Files.lines(path)) {
            lines.forEach(System.out::println);
        } catch (IOException e) {
            System.err.println("Error reading file: " + e.getMessage());
        }
    }
}

Advanced Processing (Filtering and Mapping)

The power of Files.lines() comes from the Stream API, allowing you to filter, transform, or search through the file content easily:

try (Stream<String> lines = Files.lines(path)) {
    long count = lines
        .filter(line -> line.contains("ERROR")) // Only keep lines with "ERROR"
        .map(String::trim)                       // Remove whitespace
        .peek(System.out::println)               // Print each matching line
        .count();                                // Count the occurrences

    System.out.println("Total errors found: " + count);
} catch (IOException e) {
    e.printStackTrace();
}

Key Considerations:

1. Try-with-Resources is Mandatory: Unlike most streams, the stream returned by Files.lines() holds an open resource (the file handle). If you don’t close it, you may run into a “too many open files” error.
2. Character Encoding: By default, Files.lines() uses UTF-8. If your file uses a different encoding (like ISO-8859-1), you can specify it as a second argument:

   Files.lines(path, StandardCharsets.ISO_8859_1)
   

3. Performance: For very large files, Files.lines() is significantly more efficient than Files.readAllLines(), which would attempt to store every line in a List<String>, potentially causing an OutOfMemoryError.