Files.readString() / Files.readAllLines()

// Reads the entire file as a string (Java 11+).
String content = Files.readString(Path.of("file-path"));

// Reads all lines of the file as a list.
List<String> lines = Files.readAllLines(Path.of("file-path"));

// Reads all lines of the file as a stream (Java 8+).
Stream<String> lineStream = Files.lines(Path.of("file-path"));

// Creates a path object (Java 11+).
Path path = Path.of("file-path");

FilesReadstring.java

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

class FilesReadstring {
    public static void main(String[] args) throws IOException {
        Path path = Path.of("sample.txt");
        Files.writeString(path, "// first line\nHello, World!\n// last line");

        String content = Files.readString(path);
        System.out.println(content);

        List<String> lines = Files.readAllLines(path);
        System.out.println("Line count: " + lines.size());
        for (String line : lines) {
            System.out.println(line);
        }

        try (var stream = Files.lines(path)) {
            stream
                .filter(line -> line.startsWith("//"))
                .forEach(System.out::println);
        }
    }
}

The command looks like this:

javac FilesReadstring.java
java FilesReadstring
// first line
Hello, World!
// last line
Line count: 3
// first line
Hello, World!
// last line
// first line
// last line

Common Mistake 1: Specifying a non-existent file path causes NoSuchFileException

Okabe Rintaro tried to read a non-existent file with readString(), resulting in a NoSuchFileException.

ReadStringNg.java

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

class ReadStringNg {
    public static void main(String[] args) throws IOException {
        String content = Files.readString(Path.of("missing.txt"));
        System.out.println(content);
    }
}

The command looks like this:

javac ReadStringNg.java
java ReadStringNg
Exception in thread "main" java.nio.file.NoSuchFileException: missing.txt
	at java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:92)
	at ReadStringNg.main(ReadStringNg.java:7)

As Makise Kurisu fixed it, check with Files.exists() first, or wrap the call in a try-catch block to handle the exception gracefully.

ReadStringOk.java

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

class ReadStringOk {
    public static void main(String[] args) {
        Path path = Path.of("missing.txt");
        try {
            String content = Files.readString(path);
            System.out.println(content);
        } catch (IOException e) {
            System.out.println("File not found: " + path);
        }
    }
}

The command looks like this:

javac ReadStringOk.java
java ReadStringOk
File not found: missing.txt

Common Mistake 2: Loading a large file with readString() all at once causes OutOfMemoryError

Shiina Mayuri tried to load a multi-gigabyte log file all at once with readString(), causing an OutOfMemoryError.

ReadStringOomNg.java

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

class ReadStringOomNg {
    public static void main(String[] args) throws IOException {
        String content = Files.readString(Path.of("huge_log.txt"));
        System.out.println(content.length());
    }
}

The command looks like this:

javac ReadStringOomNg.java
java ReadStringOomNg
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

As Hashida Itaru fixed it, processing a large file line by line with BufferedReader keeps memory usage low.

ReadStringOomOk.java

import java.io.BufferedReader;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

class ReadStringOomOk {
    public static void main(String[] args) throws IOException {
        int count = 0;
        try (BufferedReader reader = Files.newBufferedReader(Path.of("huge_log.txt"))) {
            String line;
            while ((line = reader.readLine()) != null) {
                count++;
            }
        }
        System.out.println("Line count: " + count);
    }
}

The command looks like this:

javac ReadStringOomOk.java
java ReadStringOomOk
Line count: 1000000

Both readString() and readAllLines() load the entire file into memory, so reading very large files (several hundred MB or more) may cause an out-of-memory error. For large files, use Files.lines() or BufferedReader to process the file one line at a time.

Because Files.lines() returns a Stream, use it inside a try-with-resources block. If you do not close the stream, the file handle will not be released. The default encoding is UTF-8; if your file uses a different encoding, pass a Charset as the second argument (e.g., Charset.forName("Shift_JIS")).

For writing to files, see Files.writeString() / Files.write().