Reading & Writing Files

Java provides comprehensive tools to read from and write to files. The java.io package offers classic streams, while java.nio provides modern, efficient file operations. Learning file I/O is essential for any practical application.

40 min•By Priygop Team•Last updated: Feb 2026

File I/O Basics

File I/O (Input/Output) is how programs interact with files on disk. Java provides two main approaches: the classic java.io package (FileReader, BufferedReader, FileWriter) and the modern java.nio.file package (Files, Path). The newer API is generally recommended for its simplicity and performance.

File Reading & Writing

Example

import java.io.*;
import java.nio.file.*;
import java.util.*;

public class FileIODemo {
    public static void main(String[] args) {
        String filename = "demo.txt";
        
        // === Writing to a file ===
        try (FileWriter writer = new FileWriter(filename)) {
            writer.write("Hello, Java File I/O!\n");
            writer.write("Line 2: Learning file operations\n");
            writer.write("Line 3: This is awesome!\n");
            System.out.println("File written successfully!");
        } catch (IOException e) {
            System.out.println("Write error: " + e.getMessage());
        }
        
        // === Reading from a file ===
        try (BufferedReader reader = new BufferedReader(new FileReader(filename))) {
            String line;
            int lineNum = 1;
            System.out.println("\n=== Reading File ===");
            while ((line = reader.readLine()) != null) {
                System.out.println(lineNum + ": " + line);
                lineNum++;
            }
        } catch (IOException e) {
            System.out.println("Read error: " + e.getMessage());
        }

        // === Modern NIO approach ===
        try {
            // Write all lines at once
            List<String> lines = Arrays.asList(
                "Name,Age,City",
                "Alice,25,New York",
                "Bob,30,London",
                "Charlie,28,Tokyo"
            );
            Files.write(Path.of("data.csv"), lines);
            
            // Read all lines at once
            System.out.println("\n=== CSV Data ===");
            List<String> readLines = Files.readAllLines(Path.of("data.csv"));
            for (String csvLine : readLines) {
                System.out.println(csvLine);
            }
        } catch (IOException e) {
            System.out.println("NIO error: " + e.getMessage());
        }
    }
}

Try It Yourself: Log File Manager

Try It Yourself: Log File ManagerJava

Java Editor

✓ ValidTab = 2 spaces

import java.io.*;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class LogManager {
    static void log(String filename, String level, String message) {
        try (FileWriter writer = new FileWriter(filename, true)) {
            String timestamp = LocalDateTime.now().format(
                DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss")
            );
            writer.write("[" + timestamp + "] [" + level + "] " + message + "\n");
        } catch (IOException e) {
            System.out.println("Log error: " + e.getMessage());
        }
    }
    
    static void readLogs(String filename) {
        try (BufferedReader reader = new BufferedReader(new FileReader(filename))) {
            String line;
            System.out.println("=== Log Contents ===");
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (FileNotFoundException e) {
            System.out.println("Log file not found!");
        } catch (IOException e) {
            System.out.println("Read error: " + e.getMessage());
        }
    }
    
    public static void main(String[] args) {
        String logFile = "app.log";
        
        // Write log entries
        log(logFile, "INFO", "Application started");
        log(logFile, "INFO", "User logged in: admin");
        log(logFile, "WARNING", "Memory usage above 80%");
        log(logFile, "ERROR", "Database connection timeout");
        log(logFile, "INFO", "Cache cleared successfully");
        
        // Read and display logs
        readLogs(logFile);
        
        // Count lines
        try (BufferedReader br = new BufferedReader(new FileReader(logFile))) {
            long count = br.lines().count();
            System.out.println("\nTotal log entries: " + count);
        } catch (IOException e) {
            System.out.println("Count error: " + e.getMessage());
        }
    }
}

import java.io.*;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class LogManager {
    static void log(String filename, String level, String message) {
        try (FileWriter writer = new FileWriter(filename, true)) {
            String timestamp = LocalDateTime.now().format(
                DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss")
            );
            writer.write("[" + timestamp + "] [" + level + "] " + message + "\n");
        } catch (IOException e) {
            System.out.println("Log error: " + e.getMessage());
        }
    }
    
    static void readLogs(String filename) {
        try (BufferedReader reader = new BufferedReader(new FileReader(filename))) {
            String line;
            System.out.println("=== Log Contents ===");
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (FileNotFoundException e) {
            System.out.println("Log file not found!");
        } catch (IOException e) {
            System.out.println("Read error: " + e.getMessage());
        }
    }
    
    public static void main(String[] args) {
        String logFile = "app.log";
        
        // Write log entries
        log(logFile, "INFO", "Application started");
        log(logFile, "INFO", "User logged in: admin");
        log(logFile, "WARNING", "Memory usage above 80%");
        log(logFile, "ERROR", "Database connection timeout");
        log(logFile, "INFO", "Cache cleared successfully");
        
        // Read and display logs
        readLogs(logFile);
        
        // Count lines
        try (BufferedReader br = new BufferedReader(new FileReader(logFile))) {
            long count = br.lines().count();
            System.out.println("\nTotal log entries: " + count);
        } catch (IOException e) {
            System.out.println("Count error: " + e.getMessage());
        }
    }
}

Output

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Edit the code on the left, then click Run

Java|52 lines|1957 chars|✓ Valid syntax

UTF-8

Reading & Writing Files

File I/O Basics

File Reading & Writing

Try It Yourself: Log File Manager

Quick Quiz: File I/O