Kotlin Android Web Service - Volley Library Registration, Log-in, and Log-out Tutorial

In this tutorial, we will create the basic user registration and log-in module using Volley library and JSON. Volley is an HTTP Library which provides the facilities for network connectivity for our app.

The web API of registration and login is created using PHP with MySQL. Before implementing the client side Android application code, first look at the server side API code at https://www.javatpoint.com/android-volley-library-registration-login-logout.

In Android application, we will create the three activity class for user registration, user log-in and display the user detail in main activity (as a profile).

Create an activity_main.xml in layout and add the following code. This activity is used to display the detail of the user profile.

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="example.javatpoint.com.kotlinvolleyregistrationloginsystem.MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="fill_parent"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:orientation="vertical"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.0">

        <TextView
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:gravity="center"
            android:text="Welcome to Profile"
            android:textAppearance="@style/Base.TextAppearance.AppCompat.Large" />

        <TableLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="50dp">


            <TableRow>

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:padding="10dp"
                    android:text="Id"
                    android:textAppearance="@style/Base.TextAppearance.AppCompat.Large" />

                <TextView
                    android:id="@+id/textViewId"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:padding="10dp"
                    android:text="id"
                    android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium" />

            </TableRow>

            <TableRow>

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:padding="10dp"
                    android:text="Username"
                    android:textAppearance="@style/Base.TextAppearance.AppCompat.Large" />

                <TextView
                    android:id="@+id/textViewUsername"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:padding="10dp"
                    android:text="username"
                    android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium" />

            </TableRow>

            <TableRow>

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:padding="10dp"
                    android:text="Email"
                    android:textAppearance="@style/Base.TextAppearance.AppCompat.Large" />

                <TextView
                    android:id="@+id/textViewEmail"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:padding="10dp"
                    android:text="useremail"
                    android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium" />

            </TableRow>

            <TableRow>

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:padding="10dp"
                    android:text="Gender"
                    android:textAppearance="@style/Base.TextAppearance.AppCompat.Large" />

                <TextView
                    android:id="@+id/textViewGender"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:padding="10dp"
                    android:text="gender"
                    android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium" />

            </TableRow>


        </TableLayout>

        <Button
            android:id="@+id/buttonLogout"
            android:layout_width="210dp"
            android:layout_height="50dp"
            android:layout_marginTop="150dp"
            android:layout_marginLeft="75dp"
            android:text="Logout" />
    </LinearLayout>

</android.support.constraint.ConstraintLayout>

Now, create an activity_login.xml file in the layout directory with following code. This activity is used for user log-in UI.

activity_login.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="example.javatpoint.com.kotlinvolleyregistrationloginsystem.LoginActivity">

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="16dp"
        android:gravity="center"
        android:text="Login"
        android:textAppearance="@style/Base.TextAppearance.AppCompat.Large"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/etUserName"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:inputType="textPersonName"
        android:hint="user name"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.144" />

    <EditText
        android:id="@+id/etUserPassword"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:hint="password"
        android:ems="10"
        android:inputType="textPassword"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/etUserName" />

    <Button
        android:id="@+id/btnLogin"
        android:layout_width="210dp"
        android:layout_height="50dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="Login"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/etUserName"
        app:layout_constraintVertical_bias="0.754" />

    <TextView
        android:id="@+id/tvRegister"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:gravity="center"
        android:text="Create New Account\n Register Here"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/btnLogin"
        app:layout_constraintVertical_bias="0.405" />

    <ProgressBar
        android:id="@+id/progressBar"
        android:visibility="gone"
        style="?android:attr/progressBarStyle"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.456" />

</android.support.constraint.ConstraintLayout>

Create an activity_register.xml file in the layout directory with following code. This activity is used for user registration UI.

activity_register.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="example.javatpoint.com.kotlinvolleyregistrationloginsystem.RegisterActivity">


    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="519dp"
        android:layout_centerVertical="true"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:orientation="vertical"
        android:padding="10dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.0">

        <TextView
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:text="Register"
            android:gravity="center"
            android:textAppearance="@style/Base.TextAppearance.AppCompat.Large" />

        <EditText
            android:id="@+id/editTextUsername"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="8dp"
            android:layout_marginTop="20dp"
            android:hint="Username"
            android:inputType="text" />

        <EditText
            android:id="@+id/editTextEmail"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="8dp"
            android:layout_marginTop="8dp"
            android:hint="Email"
            android:inputType="textEmailAddress" />

        <EditText
            android:id="@+id/editTextPassword"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="8dp"
            android:layout_marginTop="8dp"
            android:fontFamily="sans-serif"
            android:hint="Password"
            android:inputType="textPassword" />

        <RadioGroup
            android:id="@+id/radioGender"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="8dp"
            android:layout_marginTop="8dp"
            android:orientation="horizontal">

            <RadioButton
                android:id="@+id/radioButtonMale"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:checked="true"
                android:text="Male" />


            <RadioButton
                android:id="@+id/radioButtonFemale"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Female" />

        </RadioGroup>

        <Button
            android:id="@+id/buttonRegister"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="8dp"
            android:layout_marginTop="90dp"
            android:text="Register" />

        <TextView
            android:id="@+id/textViewLogin"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="8dp"
            android:layout_marginTop="8dp"
            android:text="Already Registered?\nLogin Here"
            android:textAlignment="center"
            android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium" />
    </LinearLayout>

    <ProgressBar
        android:visibility="gone"
        android:id="@+id/progressBar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true" />

</android.support.constraint.ConstraintLayout>

Add the volley library dependency in build.gradle file.

build.gradle

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'com.android.support:appcompat-v7:26.1.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    testImplementation 'junit:junit:4.12'
    implementation 'com.android.volley:volley:1.0.0'
}

Create a data model class named as User.kt with the following code.

User.kt

package example.javatpoint.com.kotlinvolleyregistrationloginsystem

class User(var id: Int, var name: String?, var email: String?, var gender: String?)

We need to define our URL that calls the API of server-side.

Create an URLs.kt class and define the URL.

URLs.kt

package example.javatpoint.com.kotlinvolleyregistrationloginsystem

object URLs {
    private val ROOT_URL = "http://192.168.1.35/androidphpmysql/registrationapi.php?apicall="
    val URL_REGISTER = ROOT_URL + "signup"
    val URL_LOGIN = ROOT_URL + "login"
}

VolleySingleton.kt

package example.javatpoint.com.kotlinvolleyregistrationloginsystem

import android.content.Context
import com.android.volley.Request
import com.android.volley.RequestQueue
import com.android.volley.toolbox.Volley

class VolleySingleton private constructor(context: Context) {
    private var mRequestQueue: RequestQueue

    // applicationContext is key, it keeps you from leaking the
    // Activity or BroadcastReceiver if someone passes one in.
    val requestQueue: RequestQueue
        get() {
            if (mRequestQueue == null) {
                mRequestQueue = Volley.newRequestQueue(mCtx?.applicationContext)
            }
            return mRequestQueue
        }

    init {
        mCtx = context
        mRequestQueue = requestQueue
    }

    fun <T> addToRequestQueue(req: Request<T>) {
        requestQueue.add(req)
    }

    companion object {
        private var mInstance: VolleySingleton? = null
        private var mCtx: Context? = null

        @Synchronized
        fun getInstance(context: Context): VolleySingleton {
            if (mInstance == null) {
                mInstance = VolleySingleton(context)
            }
            return mInstance as VolleySingleton
        }
    }
}

Create a class named as SharedPreferences.kt. In this class, we use the SharedPreferences class to store the user detail. The SharedPreferences class contains four methods with the following functionalities:

userLogin(): This function is used to store the user information in SharedPreferences after log-in.
isLoggedIn: This method checks whether the user is already log-in or not.
user: User get(): This method gets the user information if log-in.
logout(): This function clears the SharedPreferences data and makes user log-out.

SharedPrefManager.kt

package example.javatpoint.com.kotlinvolleyregistrationloginsystem

import android.content.Context
import android.content.Intent

class SharedPrefManager private constructor(context: Context) {

    //this method will checker whether user is already logged in or not
    val isLoggedIn: Boolean
        get() {
            val sharedPreferences = ctx?.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE)
            return sharedPreferences?.getString(KEY_USERNAME, null) != null
        }

    //this method will give the logged in user
    val user: User
        get() {
            val sharedPreferences = ctx?.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE)
            return User(
                    sharedPreferences!!.getInt(KEY_ID, -1),
                    sharedPreferences.getString(KEY_USERNAME, null),
                    sharedPreferences.getString(KEY_EMAIL, null),
                    sharedPreferences.getString(KEY_GENDER, null)
            )
        }

    init {
        ctx = context
    }

    //this method will store the user data in shared preferences
    fun userLogin(user: User) {
        val sharedPreferences = ctx?.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE)
        val editor = sharedPreferences?.edit()
        editor?.putInt(KEY_ID, user.id)
        editor?.putString(KEY_USERNAME, user.name)
        editor?.putString(KEY_EMAIL, user.email)
        editor?.putString(KEY_GENDER, user.gender)
        editor?.apply()
    }

    //this method will logout the user
    fun logout() {
        val sharedPreferences = ctx?.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE)
        val editor = sharedPreferences?.edit()
        editor?.clear()
        editor?.apply()
        ctx?.startActivity(Intent(ctx, LoginActivity::class.java))
    }

    companion object {

        private val SHARED_PREF_NAME = "volleyregisterlogin"
        private val KEY_USERNAME = "keyusername"
        private val KEY_EMAIL = "keyemail"
        private val KEY_GENDER = "keygender"
        private val KEY_ID = "keyid"
        private var mInstance: SharedPrefManager? = null
        private var ctx: Context? = null
        @Synchronized
        fun getInstance(context: Context): SharedPrefManager {
            if (mInstance == null) {
                mInstance = SharedPrefManager(context)
            }
            return mInstance as SharedPrefManager
        }
    }
}

Now, in the MainActivity.kt class, we will display the user information if the user is logged-in otherwise, it redirects to LoginActivity.kt class. The onClick() method is used to log-out the user when clicking on the button.

MainActivity.kt

package example.javatpoint.com.kotlinvolleyregistrationloginsystem

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.content.Intent
import android.view.View
import android.widget.Button
import android.widget.TextView

class MainActivity : AppCompatActivity(), View.OnClickListener {

    internal lateinit var id: TextView
    internal lateinit var userName: TextView
    internal lateinit var userEmail: TextView
    internal lateinit var gender: TextView
    internal lateinit var btnLogout: Button
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        if (SharedPrefManager.getInstance(this).isLoggedIn) {
            id = findViewById(R.id.textViewId)
            userName = findViewById(R.id.textViewUsername)
            userEmail = findViewById(R.id.textViewEmail)
            gender = findViewById(R.id.textViewGender)
            btnLogout = findViewById(R.id.buttonLogout)

            val user = SharedPrefManager.getInstance(this).user

            id.text = user.id.toString()
            userEmail.text = user.email
            gender.text = user.gender
            userName.text = user.name

            btnLogout.setOnClickListener(this)

        } else {
            val intent = Intent(this@MainActivity, LoginActivity::class.java)
            startActivity(intent)
            finish()
        }
    }

    override fun onClick(view: View) {
        if (view == btnLogout) {
            SharedPrefManager.getInstance(applicationContext).logout()
        }
    }
}

In the LoginActivity.kt class, we check that the user is already logged-in or not, if true then redirect to MainActivity.kt class otherwise, allow a user to log-in.

StringRequest class of Volley library is used for network module. The object of StringRequest class takes the parameters of the type request method, URL, and the response.

LoginActivity.kt

package example.javatpoint.com.kotlinvolleyregistrationloginsystem

import android.support.v7.app.AppCompatActivity
import android.os.Bundle

import android.content.Intent
import android.text.TextUtils
import android.view.View
import android.widget.EditText
import android.widget.ProgressBar
import android.widget.Toast
import com.android.volley.AuthFailureError
import com.android.volley.Request
import com.android.volley.Response
import com.android.volley.toolbox.StringRequest
import kotlinx.android.synthetic.main.activity_login.*
import org.json.JSONException
import org.json.JSONObject
import java.util.HashMap

class LoginActivity : AppCompatActivity() {
    internal lateinit var etName: EditText
    internal lateinit var etPassword: EditText
    internal lateinit var progressBar: ProgressBar
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_login)

        if (SharedPrefManager.getInstance(this).isLoggedIn) {
            finish()
            startActivity(Intent(this, MainActivity::class.java))
        }

        progressBar = findViewById(R.id.progressBar)
        etName = findViewById(R.id.etUserName)
        etPassword = findViewById(R.id.etUserPassword)

        //calling the method userLogin() for login the user
         btnLogin.setOnClickListener(View.OnClickListener {
            userLogin()
        })

        //if user presses on textview it call the activity RegisterActivity
        tvRegister.setOnClickListener(View.OnClickListener {
            finish()
            startActivity(Intent(applicationContext, RegisterActivity::class.java))
        })
    }

    private fun userLogin() {
        //first getting the values
        val username = etName.text.toString()
        val password = etPassword.text.toString()
        //validating inputs
        if (TextUtils.isEmpty(username)) {
            etName.error = "Please enter your username"
            etName.requestFocus()
            return
        }

        if (TextUtils.isEmpty(password)) {
            etPassword.error = "Please enter your password"
            etPassword.requestFocus()
            return
        }

        //if everything is fine
        val stringRequest = object : StringRequest(Request.Method.POST, URLs.URL_LOGIN,
                Response.Listener { response ->
                    progressBar.visibility = View.GONE

                    try {
                        //converting response to json object
                        val obj = JSONObject(response)

                        //if no error in response
                        if (!obj.getBoolean("error")) {
                            Toast.makeText(applicationContext, obj.getString("message"), Toast.LENGTH_SHORT).show()

                            //getting the user from the response
                            val userJson = obj.getJSONObject("user")

                            //creating a new user object
                            val user = User(
                                    userJson.getInt("id"),
                                    userJson.getString("username"),
                                    userJson.getString("email"),
                                    userJson.getString("gender")
                            )

                            //storing the user in shared preferences
                            SharedPrefManager.getInstance(applicationContext).userLogin(user)
                            //starting the MainActivity
                            finish()
                            startActivity(Intent(applicationContext, MainActivity::class.java))
                        } else {
                            Toast.makeText(applicationContext, obj.getString("message"), Toast.LENGTH_SHORT).show()
                        }
                    } catch (e: JSONException) {
                        e.printStackTrace()
                    }
                },
                Response.ErrorListener { error -> Toast.makeText(applicationContext, error.message, Toast.LENGTH_SHORT).show() }) {
            @Throws(AuthFailureError::class)
            override fun getParams(): Map<String, String> {
                val params = HashMap<String, String>()
                params["username"] = username
                params["password"] = password
                return params
            }
        }

        VolleySingleton.getInstance(this).addToRequestQueue(stringRequest)
    }
}

The RegisterActivity.kt class is used to register the user. This class initially checks the user log-in, if true then redirect to MainActivity.kt class otherwise, allow user for registration.

Similar to LoginActivity.kt class, we use the StringRequest class of Volley library for network connection and pass the parameters of the type request method, URL, and the response. The Response.Listener handles the response generated by the server.

RegisterActivity.kt

package example.javatpoint.com.kotlinvolleyregistrationloginsystem

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.content.Intent
import android.text.TextUtils
import android.view.View
import android.widget.EditText
import android.widget.ProgressBar
import android.widget.RadioButton
import android.widget.RadioGroup
import android.widget.Toast
import com.android.volley.AuthFailureError
import com.android.volley.Request
import com.android.volley.Response
import com.android.volley.toolbox.StringRequest
import kotlinx.android.synthetic.main.activity_register.*
import org.json.JSONException
import org.json.JSONObject
import java.util.HashMap

class RegisterActivity : AppCompatActivity() {
    internal lateinit var editTextUsername: EditText
    internal lateinit var editTextEmail: EditText
    internal lateinit var editTextPassword: EditText
    internal lateinit var radioGroupGender: RadioGroup
    internal lateinit var progressBar: ProgressBar
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_register)
        progressBar = findViewById(R.id.progressBar)

        //if the user is already logged in we will directly start the MainActivity (profile) activity
        if (SharedPrefManager.getInstance(this).isLoggedIn) {
            finish()
            startActivity(Intent(this, MainActivity::class.java))
            return
        }

        editTextUsername = findViewById(R.id.editTextUsername)
        editTextEmail = findViewById(R.id.editTextEmail)
        editTextPassword = findViewById(R.id.editTextPassword)
        radioGroupGender = findViewById(R.id.radioGender)

        buttonRegister.setOnClickListener(View.OnClickListener {
            //if user pressed on button register
            //here we will register the user to server
            registerUser()
        })

        textViewLogin.setOnClickListener(View.OnClickListener {
            finish()
            startActivity(Intent(this@RegisterActivity, LoginActivity::class.java))
        })
    }

    private fun registerUser() {
        val username = editTextUsername.text.toString().trim { it <= ' ' }
        val email = editTextEmail.text.toString().trim { it <= ' ' }
        val password = editTextPassword.text.toString().trim { it <= ' ' }

        val gender = (findViewById<View>(radioGroupGender.checkedRadioButtonId) as RadioButton).text.toString()

        //first we will do the validations
        if (TextUtils.isEmpty(username)) {
            editTextUsername.error = "Please enter username"
            editTextUsername.requestFocus()
            return
        }

        if (TextUtils.isEmpty(email)) {
            editTextEmail.error = "Please enter your email"
            editTextEmail.requestFocus()
            return
        }

        if (!android.util.Patterns.EMAIL_ADDRESS.matcher(email).matches()) {
            editTextEmail.error = "Enter a valid email"
            editTextEmail.requestFocus()
            return
        }

        if (TextUtils.isEmpty(password)) {
            editTextPassword.error = "Enter a password"
            editTextPassword.requestFocus()
            return
        }

        val stringRequest = object : StringRequest(Request.Method.POST, URLs.URL_REGISTER,
                Response.Listener { response ->
                    progressBar.visibility = View.GONE

                    try {
                        //converting response to json object
                        val obj = JSONObject(response)
                        //if no error in response
                        if (!obj.getBoolean("error")) {
                            Toast.makeText(applicationContext, obj.getString("message"), Toast.LENGTH_SHORT).show()

                            //getting the user from the response
                            val userJson = obj.getJSONObject("user")

                            //creating a new user object
                            val user = User(
                                    userJson.getInt("id"),
                                    userJson.getString("username"),
                                    userJson.getString("email"),
                                    userJson.getString("gender")
                            )

                            //storing the user in shared preferences
                            SharedPrefManager.getInstance(applicationContext).userLogin(user)

                            //starting the MainActivity activity
                            finish()
                            startActivity(Intent(applicationContext, MainActivity::class.java))
                        } else {
                            Toast.makeText(applicationContext, obj.getString("message"), Toast.LENGTH_SHORT).show()
                        }
                    } catch (e: JSONException) {
                        e.printStackTrace()
                    }
                },
                Response.ErrorListener { error -> Toast.makeText(applicationContext, error.message, Toast.LENGTH_SHORT).show() }) {
            @Throws(AuthFailureError::class)
            override fun getParams(): Map<String, String> {
                val params = HashMap<String, String>()
                params["username"] = username
                params["email"] = email
                params["password"] = password
                params["gender"] = gender
                return params
            }
        }

        VolleySingleton.getInstance(this).addToRequestQueue(stringRequest)
    }
}

AndroidManifest.xml

Add the following permission in AndroidManifest.xml file

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.INTERNET" />

Note: The server side code of registration and login are implemented in PHP with MySQL at https://www.javatpoint.com/android-volley-library-registration-login-logout.

Output:

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