AP Computer Science Exam Question:

Explain the concept of file classes in Java and provide an example that demonstrates how to read data from a file using the FileInputStream and Scanner classes. Include step-by-step detailed explanation of the code.

Answer:

File classes in Java provide a convenient way to interact with files and directories on the file system. These classes can be used to perform various operations such as creating, reading, writing, and deleting files.

Here is an example that demonstrates how to read data from a file using the FileInputStream and Scanner classes:

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

public class FileReaderExample {

    public static void main(String[] args) {
        // Step 1: Define the file path
        String filePath = "data.txt";

        try {
            // Step 2: Create a File object using the file path
            File file = new File(filePath);

            // Step 3: Create a FileInputStream object to read data from the file
            FileInputStream fileInputStream = new FileInputStream(file);

            // Step 4: Create a Scanner object to read data from the FileInputStream
            Scanner scanner = new Scanner(fileInputStream);

            // Step 5: Read and process the data from the file
            while (scanner.hasNext()) {
                String data = scanner.nextLine();
                System.out.println(data);
            }

            // Step 6: Close the FileInputStream and Scanner objects
            fileInputStream.close();
            scanner.close();

        } catch (FileNotFoundException e) {
            System.out.println("File not found: " + e.getMessage());
        } catch (Exception e) {
            System.out.println("Error reading from file: " + e.getMessage());
        }
    }
}

Explanation:

1. Step 1: Define the file path - In this example, the file path is set to "data.txt". Ensure that the file exists in the specified path.

2. Step 2: Create a File object using the file path - The File class represents a file or directory. It is used to create a File object that represents the specified file.

3. Step 3: Create a FileInputStream object to read data from the file - The FileInputStream class is used to read data from a file. It takes the File object as a parameter and creates a FileInputStream object.

4. Step 4: Create a Scanner object to read data from the FileInputStream - The Scanner class is used to read data from various input sources. In this case, we create a Scanner object that reads data from the FileInputStream object.

5. Step 5: Read and process the data from the file - We use a while loop along with the scanner.hasNext() method to iterate through the file and read each line of data using scanner.nextLine(). In this example, we simply print each line of data, but you can perform any required processing.

6. Step 6: Close the FileInputStream and Scanner objects - It is important to close the file input stream and scanner objects to release system resources. This is done using the close() method.

Note: The code includes exception handling using try-catch blocks to handle possible exceptions like file not found (FileNotFoundException) or general exceptions that may occur during file reading.

Overall, this code demonstrates how to read data from a file using the FileInputStream and Scanner classes in Java. Remember to adjust the file path and perform the necessary data processing based on your requirements.