Java 8 Stream Read a file line by line

摘要: In Java 8, you can use Files.lines to read file as Stream.

In Java 8, you can use Files.lines to read file as Stream.

c://lines.txt – A simple text file for testing
line1
line2
line3
line4
line5

1. Java 8 Read File + Stream

TestReadFile.java
package com.mkyong.java8;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.stream.Stream;
public class TestReadFile {
	public static void main(String args[]) {
		String fileName = "c://lines.txt";
		//read file into stream, try-with-resources
		try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
			stream.forEach(System.out::println);
		} catch (IOException e) {
			e.printStackTrace();

Output

line1
line2
line3
line4
line5

2. Java 8 Read File + Stream + Extra

This example shows you how to use Stream to filter content, convert the entire content to upper case and return it as a List.

TestReadFile2.java
package com.mkyong.java8;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class TestReadFile2 {
	public static void main(String args[]) {
		String fileName = "c://lines.txt";
		List<String> list = new ArrayList<>();
		try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
			//1. filter line 3
			//2. convert all content to upper case
			//3. convert it into a List
			list = stream
					.filter(line -> !line.startsWith("line3"))
					.map(String::toUpperCase)
					.collect(Collectors.toList());
		} catch (IOException e) {
			e.printStackTrace();
		list.forEach(System.out::println);

Output

LINE1
LINE2
LINE4
LINE5

3. BufferedReader + Stream

A new method lines() has been added since 1.8, it lets BufferedReader returns content as Stream.

TestReadFile3.java
package com.mkyong.java8;
import java.io.BufferedReader;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
public class TestReadFile3{
	public static void main(String args[]) {
		String fileName = "c://lines.txt";
		List<String> list = new ArrayList<>();
		try (BufferedReader br = Files.newBufferedReader(Paths.get(fileName))) {
			//br returns as stream and convert it into a List
			list = br.lines().collect(Collectors.toList());
		} catch (IOException e) {
			e.printStackTrace();
		list.forEach(System.out::println);

Output

line1
line2
line3
line4
line5

4. Classic BufferedReader And Scanner

Enough of Java 8 and Stream, let revisit the classic BufferedReader (JDK1.1) and Scanner (JDK1.5) examples to read a file line by line, it is working still, just developers are moving toward Stream.

4.1 BufferedReader + try-with-resources example.

TestReadFile4.java
package com.mkyong.core;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
public class TestReadFile4{
	public static void main(String args[]) {
		String fileName = "c://lines.txt";
		try (BufferedReader br = new BufferedReader(new FileReader(fileName))) {
			String line;
			while ((line = br.readLine()) != null) {
				System.out.println(line);
		} catch (IOException e) {
			e.printStackTrace();

4.2 Scanner + try-with-resources example.

TestReadFile5.java
package com.mkyong.core;
import java.io.File;
import java.io.IOException;
import java.util.Scanner;
public class TestReadFile5 {
	public static void main(String args[]) {
		String fileName = "c://lines.txt";
		try (Scanner scanner = new Scanner(new File(fileName))) {
			while (scanner.hasNext()){
				System.out.println(scanner.nextLine());
		} catch (IOException e) {
			e.printStackTrace();

References

  1. Java 8 File.lines()
  2. Java 8 Stream
  3. Java BufferedReader

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