DatabaseConnection.kt

import java.sql.Connection
import java.sql.DriverManager
import java.sql.PreparedStatement
import java.sql.ResultSet

class DatabaseConnection {
    fun validateUser(username: String, password: String): Boolean {
        var connection: Connection? = null
        var preparedStatement: PreparedStatement? = null
        var result: ResultSet? = null
        var isValid = false
        val url = "jdbc:mysql://x.x.x.x:3306/IOT"
        val uname = "****"
        val pass = "****"

        try {
            connection = DriverManager.getConnection(url, uname, pass)
            val query = "SELECT * FROM user WHERE username = ? AND password = ?"
            preparedStatement = connection.prepareStatement(query)
            preparedStatement.setString(1, username)
            preparedStatement.setString(2, password)

            result = preparedStatement.executeQuery()

            if (result.next()) {
                isValid = true
            } else {
                println("Query result: User not found") // Debug statement
            }
        } catch (e: Exception) {
            println("error in establishing connection: $e")
        }
        return isValid
    }
}

main.kt

import com.example.iot1.DatabaseConnection

fun main() {
    val data = DatabaseConnection()
    val uname = "lokesh"
    val pass = "1234"
    val database = data.validateUser(uname, pass)

    println(database)
}

MainActivity.kt

import android.os.Bundle
import android.os.StrictMode
import android.util.Log
import android.widget.Button
import android.widget.EditText
import android.widget.Toast
import androidx.activity.enableEdgeToEdge
import androidx.appcompat.app.AppCompatActivity
import main
import java.sql.Connection
import java.sql.DriverManager
import java.sql.PreparedStatement
import java.sql.ResultSet

class MainActivity : AppCompatActivity() {
    lateinit var button: Button
    lateinit var etUsername: EditText
    lateinit var etPassword: EditText

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        enableEdgeToEdge()

        try {
            val policy = StrictMode.ThreadPolicy.Builder().permitAll().build()
            StrictMode.setThreadPolicy(policy)
        } catch (e: Exception) {
            Log.e("MainActivity", "Error setting StrictMode policy", e)
        }

        setContentView(R.layout.activity_main)

        button = findViewById(R.id.btn_submitted)
        etUsername = findViewById(R.id.etUserId)
        etPassword = findViewById(R.id.etPassword)

        button.setOnClickListener {
            val username = etUsername.text.toString()
            val password = etPassword.text.toString()
            println("username: $username and password: $password")
            val database = DatabaseConnection().validateUser(username, password)
            Log.d("xyzabc", "message: $database")
            if (database) {
                Toast.makeText(this, "Valid", Toast.LENGTH_LONG).show()
            } else {
                Toast.makeText(this, "Invalid", Toast.LENGTH_LONG).show()
            }
        }
    }
}

I connected to MySQL server using a function ValidateUser from DatabaseConnection.kt class. When I called this function into seperate main.kt file the output is giving true but when I tried to call the same function into MainActivity using the above code the app is crashing in one scenario or getting the toast as invalid, which is because the return from DatabaseConnection is false.