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Reading files is a fundamental skill in Java programming, allowing you to access and manipulate data stored in files. In this section, we will cover the basics of reading files using different classes and methods provided by the Java I/O (Input/Output) API.

Key Concepts

File Class: Represents a file or directory path.
FileReader Class: Reads character files.
BufferedReader Class: Reads text from a character input stream, buffering characters for efficient reading.
FileInputStream Class: Reads raw byte streams from files.
Scanner Class: Reads formatted input from files.

Using FileReader and BufferedReader

FileReader

The FileReader class is used to read character files. It is a convenient class for reading text files in the default encoding.

BufferedReader

The BufferedReader class reads text from a character input stream and buffers characters for efficient reading.

Example

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

public class FileReadingExample {
    public static void main(String[] args) {
        String filePath = "example.txt";
        
        try (FileReader fileReader = new FileReader(filePath);
             BufferedReader bufferedReader = new BufferedReader(fileReader)) {
            
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Explanation

FileReader: Opens the file specified by filePath.
BufferedReader: Wraps the FileReader to buffer the input for efficient reading.
readLine(): Reads a line of text. Returns null when the end of the stream is reached.
try-with-resources: Ensures that the FileReader and BufferedReader are closed automatically.

Using FileInputStream

The FileInputStream class is used to read raw byte streams from files. It is useful for reading binary data.

Example

import java.io.FileInputStream;
import java.io.IOException;

public class FileInputStreamExample {
    public static void main(String[] args) {
        String filePath = "example.txt";
        
        try (FileInputStream fileInputStream = new FileInputStream(filePath)) {
            int content;
            while ((content = fileInputStream.read()) != -1) {
                System.out.print((char) content);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Explanation

FileInputStream: Opens the file specified by filePath.
read(): Reads a byte of data. Returns -1 when the end of the stream is reached.
try-with-resources: Ensures that the FileInputStream is closed automatically.

Using Scanner

The Scanner class is used to read formatted input from files. It is a versatile class that can parse primitive types and strings using regular expressions.

Example

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class ScannerExample {
    public static void main(String[] args) {
        String filePath = "example.txt";
        
        try (Scanner scanner = new Scanner(new File(filePath))) {
            while (scanner.hasNextLine()) {
                String line = scanner.nextLine();
                System.out.println(line);
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}

Explanation

Scanner: Opens the file specified by filePath.
hasNextLine(): Checks if there is another line in the input.
nextLine(): Reads the next line of text.
try-with-resources: Ensures that the Scanner is closed automatically.

Practical Exercise

Task

Write a Java program that reads a file named data.txt and prints each line to the console. Use BufferedReader for reading the file.

Solution

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

public class ReadDataFile {
    public static void main(String[] args) {
        String filePath = "data.txt";
        
        try (FileReader fileReader = new FileReader(filePath);
             BufferedReader bufferedReader = new BufferedReader(fileReader)) {
            
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Explanation

The program reads the file data.txt using FileReader and BufferedReader.
It prints each line of the file to the console.

Common Mistakes and Tips

File Not Found: Ensure the file path is correct. Use absolute paths if necessary.
Resource Management: Always close file resources to avoid memory leaks. Use try-with-resources for automatic management.
Exception Handling: Handle IOException properly to understand the cause of any issues.

Conclusion

In this section, we covered the basics of reading files in Java using different classes such as FileReader, BufferedReader, FileInputStream, and Scanner. Understanding these classes and their methods is essential for efficient file handling in Java. In the next section, we will explore writing files, which complements the skills learned here.

Reading Files

Key Concepts

Using FileReader and BufferedReader

FileReader

BufferedReader

Example

Explanation

Using FileInputStream

Example

Explanation

Using Scanner

Example

Explanation

Practical Exercise

Task

Solution

Explanation

Common Mistakes and Tips

Conclusion

Java Programming Course

Module 1: Introduction to Java

Module 2: Control Flow

Module 3: Object-Oriented Programming

Module 4: Advanced Object-Oriented Programming

Module 5: Data Structures and Collections

Module 6: Exception Handling

Module 7: File I/O

Module 8: Multithreading and Concurrency

Module 9: Networking

Module 10: Advanced Topics

Module 11: Java Frameworks and Libraries

Module 12: Building Real-World Applications