用Java代码实现下面的需求：Create an application called DBTest.java that emulates a small database program that stores and retrieves records relating to products. Use the RandomAccessFile class and a flat file. Records in the database should consist of string names and integer quantities. Your program should allow the user to display a record, update a record, and add a new record.

以下是实现上述需求的Java代码，注释中有详细说明。

import java.io.*;

public class DBTest {

    // The size of each record in bytes
    private static final int RECORD_SIZE = 64;

    public static void main(String[] args) {
        try {
            // Create a RandomAccessFile object with the name "products.db"
            RandomAccessFile file = new RandomAccessFile("products.db", "rw");

            // Display a menu to the user
            System.out.println("1. Display a record");
            System.out.println("2. Update a record");
            System.out.println("3. Add a new record");
            System.out.print("Enter your choice: ");

            // Read the user's choice
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            int choice = Integer.parseInt(br.readLine());

            switch (choice) {
                case 1:
                    // Display a record
                    System.out.print("Enter the record number to display: ");
                    int recordNumber = Integer.parseInt(br.readLine());
                    displayRecord(file, recordNumber);
                    break;
                case 2:
                    // Update a record
                    System.out.print("Enter the record number to update: ");
                    recordNumber = Integer.parseInt(br.readLine());
                    updateRecord(file, recordNumber);
                    break;
                case 3:
                    // Add a new record
                    addRecord(file);
                    break;
                default:
                    System.out.println("Invalid choice");
                    break;
            }

            // Close the file
            file.close();

        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    // Method to display a record
    private static void displayRecord(RandomAccessFile file, int recordNumber) throws IOException {
        // Seek to the start of the record
        file.seek((recordNumber - 1) * RECORD_SIZE);

        // Read the name and quantity fields from the record
        String name = file.readUTF();
        int quantity = file.readInt();

        // Display the record to the user
        System.out.println("Name: " + name);
        System.out.println("Quantity: " + quantity);
    }

    // Method to update a record
    private static void updateRecord(RandomAccessFile file, int recordNumber) throws IOException {
        // Seek to the start of the record
        file.seek((recordNumber - 1) * RECORD_SIZE);

        // Read the name and quantity fields from the record
        String name = file.readUTF();
        int quantity = file.readInt();

        // Display the current values to the user
        System.out.println("Current name: " + name);
        System.out.println("Current quantity: " + quantity);

        // Prompt the user for new values
        System.out.print("Enter new name: ");
        name = br.readLine();
        System.out.print("Enter new quantity: ");
        quantity = Integer.parseInt(br.readLine());

        // Write the updated values back to the record
        file.seek((recordNumber - 1) * RECORD_SIZE);
        file.writeUTF(name);
        file.writeInt(quantity);

        System.out.println("Record updated");
    }

    // Method to add a new record
    private static void addRecord(RandomAccessFile file) throws IOException {
        // Prompt the user for the new record's name and quantity
        System.out.print("Enter name of new record: ");
        String name = br.readLine();
        System.out.print("Enter quantity of new record: ");
        int quantity = Integer.parseInt(br.readLine());

        // Seek to the end of the file
        file.seek(file.length());

        // Write the new record to the file
        file.writeUTF(name);
        file.writeInt(quantity);

        System.out.println("Record added");
    }
}