Java: File IO

Q: How to handle encoding?

Use InputStreamReader with explicit charset: new InputStreamReader(fis, StandardCharsets.UTF_8) .

File operations are an important program feature. This lesson covers file IO in Java.

1. File Class

The File class is used to operate on files and directories.

(1) Creating File Objects

TEXT 📖 Display only

import java.io.File;

// Method 1: Path string
File file1 = new File("test.txt");

// Method 2: Parent path + filename
File file2 = new File("/home/user", "test.txt");

// Method 3: File object + filename
File dir = new File("/home/user");
File file3 = new File(dir, "test.txt");

(2) Common Methods

Method	Description
`exists()`	Exists
`isFile()`	Is file
`isDirectory()`	Is directory
`getName()`	Get filename
`getPath()`	Get path
`getAbsolutePath()`	Get absolute path
`length()`	File size (bytes)
`createNewFile()`	Create file
`mkdir()`	Create directory
`mkdirs()`	Create nested directories
`delete()`	Delete
`list()`	List directory contents

▶ Example: File Operations

JAVA

import java.io.File;
import java.io.IOException;

public class FileDemo {
    public static void main(String[] args) throws IOException {
        File file = new File("test.txt");
        
        // Create file
        if (!file.exists()) {
            file.createNewFile();
            System.out.println("File created successfully");
        }
        
        // File info
        System.out.println("Filename: " + file.getName());
        System.out.println("Path: " + file.getPath());
        System.out.println("Absolute path: " + file.getAbsolutePath());
        System.out.println("Size: " + file.length() + " bytes");
        System.out.println("Is file: " + file.isFile());
        System.out.println("Is directory: " + file.isDirectory());
        
        // Delete file
        // file.delete();
    }
}

▶ Try it Yourself

▶ Example: Directory Operations

JAVA

import java.io.File;

public class DirectoryDemo {
    public static void main(String[] args) {
        // Create directory
        File dir = new File("testdir/subdir");
        dir.mkdirs();
        System.out.println("Directory created: " + dir.getAbsolutePath());
        
        // List directory contents
        File parent = new File("testdir");
        String[] files = parent.list();
        if (files != null) {
            for (String name : files) {
                System.out.println("  " + name);
            }
        }
        
        // Traverse directory
        File[] fileArray = parent.listFiles();
        if (fileArray != null) {
            for (File f : fileArray) {
                String type = f.isDirectory() ? "[DIR]" : "[FILE]";
                System.out.println(type + " " + f.getName());
            }
        }
    }
}

▶ Try it Yourself

2. File Read/Write

(1) BufferedReader Read File

JAVA

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class ReadFile {
    public static void main(String[] args) {
        // try-with-resources auto-closes resources
        try (BufferedReader reader = new BufferedReader(new FileReader("test.txt"))) {
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            System.out.println("Read failed: " + e.getMessage());
        }
    }
}

(2) BufferedWriter Write File

JAVA

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;

public class WriteFile {
    public static void main(String[] args) {
        // Overwrite
        try (BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt"))) {
            writer.write("Hello, World!");
            writer.newLine();
            writer.write("Java file operations");
            System.out.println("Write successful");
        } catch (IOException e) {
            System.out.println("Write failed: " + e.getMessage());
        }
        
        // Append
        try (BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt", true))) {
            writer.write("Appended content");
            writer.newLine();
            System.out.println("Append successful");
        } catch (IOException e) {
            System.out.println("Append failed: " + e.getMessage());
        }
    }
}

3. try-with-resources

Introduced in Java 7, automatically closes resources that implement AutoCloseable.

(1) Syntax

JAVA

try (ResourceType variable = new Resource()) {
    // Use resource
} catch (ExceptionType e) {
    // Handle exception
}

▶ Example: File Copy

JAVA

import java.io.*;

public class FileCopy {
    public static void copy(String src, String dest) throws IOException {
        try (BufferedReader reader = new BufferedReader(new FileReader(src));
             BufferedWriter writer = new BufferedWriter(new FileWriter(dest))) {
            
            String line;
            while ((line = reader.readLine()) != null) {
                writer.write(line);
                writer.newLine();
            }
        }
        System.out.println("Copy complete");
    }
    
    public static void main(String[] args) {
        try {
            copy("source.txt", "dest.txt");
        } catch (IOException e) {
            System.out.println("Copy failed: " + e.getMessage());
        }
    }
}

▶ Try it Yourself

4. Read All Content

(1) Method 1: Line by Line

JAVA

public static String readAll(String filename) throws IOException {
    StringBuilder sb = new StringBuilder();
    try (BufferedReader reader = new BufferedReader(new FileReader(filename))) {
        String line;
        while ((line = reader.readLine()) != null) {
            sb.append(line).append("\n");
        }
    }
    return sb.toString();
}

(2) Method 2: Files Utility Class (Java 7+)

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

public class FilesDemo {
    public static void main(String[] args) throws IOException {
        // Read all lines
        List<String> lines = Files.readAllLines(Paths.get("test.txt"));
        lines.forEach(System.out::println);
        
        // Read as string
        String content = new String(Files.readAllBytes(Paths.get("test.txt")));
        System.out.println(content);
        
        // Write file
        Files.write(Paths.get("output.txt"), "Hello".getBytes());
    }
}

5. File Traversal

(1) Recursive Directory Listing

JAVA

import java.io.File;

public class ListFiles {
    public static void listFiles(File dir, String indent) {
        File[] files = dir.listFiles();
        if (files == null) return;
        
        for (File file : files) {
            System.out.println(indent + file.getName());
            if (file.isDirectory()) {
                listFiles(file, indent + "  ");
            }
        }
    }
    
    public static void main(String[] args) {
        File dir = new File(".");
        listFiles(dir, "");
    }
}

(2) File Filtering

JAVA

import java.io.File;
import java.io.FilenameFilter;

public class FileFilter {
    public static void main(String[] args) {
        File dir = new File(".");
        
        // List only .txt files
        String[] txtFiles = dir.list((d, name) -> name.endsWith(".txt"));
        if (txtFiles != null) {
            for (String name : txtFiles) {
                System.out.println(name);
            }
        }
        
        // List only directories
        File[] dirs = dir.listFiles(File::isDirectory);
        if (dirs != null) {
            for (File d : dirs) {
                System.out.println("[DIR] " + d.getName());
            }
        }
    }
}

6. Serialization

Convert objects to byte streams for saving to files or network transmission.

(1) Serialization Requirements

Class must implement Serializable interface
All fields must also be serializable

▶ Example: Serialization

JAVA

import java.io.*;

// Serializable class
public class User implements Serializable {
    private static final long serialVersionUID = 1L;
    private String name;
    private int age;
    private transient String password;  // transient excludes from serialization
    
    public User(String name, int age, String password) {
        this.name = name;
        this.age = age;
        this.password = password;
    }
    
    @Override
    public String toString() {
        return "User{name='" + name + "', age=" + age + ", password='" + password + "'}";
    }
}

public class SerializeDemo {
    public static void main(String[] args) {
        // Serialize
        try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("user.dat"))) {
            User user = new User("Alice", 25, "123456");
            oos.writeObject(user);
            System.out.println("Serialization successful");
        } catch (IOException e) {
            System.out.println("Serialization failed: " + e.getMessage());
        }
        
        // Deserialize
        try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream("user.dat"))) {
            User user = (User) ois.readObject();
            System.out.println("Deserialized: " + user);
            // User{name='Alice', age=25, password='null'} (password excluded)
        } catch (IOException | ClassNotFoundException e) {
            System.out.println("Deserialization failed: " + e.getMessage());
        }
    }
}

▶ Try it Yourself

7. ❓ Frequently Asked Questions

Q What's the difference between relative and absolute paths?

A Relative paths are relative to the current working directory. Absolute paths start from the root directory.

Q Why use try-with-resources?

A Automatically closes resources, preventing resource leaks. More concise than manual finally.

Q What does the transient keyword do?

A Marks fields that should not participate in serialization.

❓ FAQ

Q What is try-with-resources?

A It automatically closes resources like files. No need for explicit finally blocks.

Q FileReader vs BufferedReader?

A FileReader reads character by character; BufferedReader reads line by line efficiently.

Q How to handle encoding?

A Use InputStreamReader with explicit charset: new InputStreamReader(fis, StandardCharsets.UTF_8).

📖 Summary

File class operates on files and directories
BufferedReader/BufferedWriter for file read/write
try-with-resources automatically closes resources
Files utility class simplifies file operations
Serialization saves objects to files

📝 Exercises

File statistics: Count lines, words, and characters in a file
File copy: Implement file copy with support for large files
Directory traversal: Recursively list all files in a directory, sorted by size

8. Next Lesson

In the next lesson, we'll learn about Streams and NIO — Java's stream-based IO.

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Java: File IO

1. File Class

(1) Creating File Objects

(2) Common Methods

▶ Example: File Operations

▶ Example: Directory Operations

2. File Read/Write

(1) BufferedReader Read File

(2) BufferedWriter Write File

3. try-with-resources

(1) Syntax

▶ Example: File Copy

4. Read All Content

(1) Method 1: Line by Line

(2) Method 2: Files Utility Class (Java 7+)

5. File Traversal

(1) Recursive Directory Listing

(2) File Filtering

6. Serialization

(1) Serialization Requirements

▶ Example: Serialization

7. ❓ Frequently Asked Questions

❓ FAQ

📖 Summary

📝 Exercises

8. Next Lesson

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