Java 8 Stream – Read a file line by line

最新推荐文章于 2022-07-29 16:11:30 发布

转载最新推荐文章于 2022-07-29 16:11:30 发布 · 2.3k 阅读

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本文介绍使用 Java 8 的 Stream API 和 Files 类进行文件读取的方法，包括按行读取文件、过滤内容、转换为大写及收集到 List 中等操作。同时展示了如何利用 BufferedReader 和 Scanner 进行传统文件读取。

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();
		}

	}

}