Sql

import java.io.IOException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/login")
public class LoginServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String username = request.getParameter("username");
String password = request.getParameter("password");

    try {
        Class.forName("org.apache.derby.jdbc.EmbeddedDriver");
        Connection conn = DriverManager.getConnection("jdbc:derby://localhost:1527/UserDB;create=true", "user", "password");

        String sql = "SELECT * FROM Users WHERE username=? AND password=?";
        PreparedStatement ps = conn.prepareStatement(sql);
        ps.setString(1, username);
        ps.setString(2, password);
        ResultSet rs = ps.executeQuery();

        if (rs.next()) {
            request.setAttribute("message", "Login successful! Welcome " + username);
            request.getRequestDispatcher("success.jsp").forward(request, response);
        } else {
            response.getWriter().println("Invalid username or password!");
        }
    } catch (Exception e) {
        e.printStackTrace();
        response.getWriter().println("Error: " + e.getMessage());
    }
}

}

code for sum of digits
data types
primitive non primitive [string is non primivite its class it has methods]

exception
error
there diff

arraylist code
add remove sort[Collection.sort()] search[contains] update[set]
iterator to print the list
diff bet
arraylist list
set list

which searching or sorthing algo u know
binary search code
hierarchy inheritance code
abstraction
abstract method
interface

sql
crud
left join [incomplete]
diff bet truncate drop

[basics,collections,some algo, oop,sql]
remark work on collections
that list is an interface and ArrayList is a standard Collection class

diff bet sql and mysql