Java Tutorial · Advanced Java · Lesson 35 of 37

Java File Handling

Read and write files with java.nio.file.Files and Path.

Files and Path

The modern file API lives in java.nio.file. Path represents a file location; Files has static methods to read, write, copy and delete:

Example 1

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

public class Main {
    public static void main(String[] args) throws IOException {
        Path file = Files.createTempFile("notes", ".txt");
        Files.writeString(file, "first line\n");
        Files.writeString(file, "second line\n", StandardOpenOption.APPEND);
        Files.write(file, List.of("third line", "fourth line"), StandardOpenOption.APPEND);
        List<String> lines = Files.readAllLines(file);
        System.out.println(lines.size() + " lines, first: " + lines.get(0));
        try (Stream<String> stream = Files.lines(file)) {
            System.out.println(stream.filter(l -> l.startsWith("t")).toList());
        }
        System.out.println(Files.exists(file) + " " + Files.size(file) + " bytes");
        Files.delete(file);
        System.out.println(Files.exists(file));
    }
}

Output

4 lines, first: first line
[third line]
true 46 bytes
false

Run this on your computer — see “Get Started” for setup.

Large files

Files.readAllLines() loads everything into memory. For big files, stream the lines lazily with Files.lines() inside try-with-resources, or use a BufferedReader.

File operations throw the checked IOException — handle it, or declare throws IOException as in this example.

Test yourself

Which checked exception do most file operations throw?

FileError
IOException
NullPointerException
RuntimeException

Show answer

Answer: IOException