MediaPlayer class in Android

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/play_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Play"/>

    <Button
        android:id="@+id/pause_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Pause"/>

    <Button
        android:id="@+id/middle_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Skip to middle"/>

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content">
        <Button
            android:id="@+id/lower_volume_button"
            android:layout_width="48dp"
            android:layout_height="48dp"
            android:text="-"/>

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:paddingLeft="8dp"
            android:paddingRight="8dp"
            android:textSize="16sp"
            android:text="Volume"/>

        <Button
            android:id="@+id/higher_volume_button"
            android:layout_width="48dp"
            android:layout_height="48dp"
            android:text="+"/>
    </LinearLayout>

</LinearLayout>

MainActivity.kt

package com.example.android.musicplayer

import android.media.MediaPlayer
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.widget.Toast
import kotlinx.android.synthetic.main.activity_main.*


class MainActivity : AppCompatActivity() {

    private lateinit var mediaPlayer: MediaPlayer

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        mediaPlayer = MediaPlayer.create(this, R.raw.dog)
        var volume = 0.5f
        mediaPlayer.setVolume(volume, volume)

        play_button.setOnClickListener{
            mediaPlayer.start()
        }

        pause_button.setOnClickListener{
            mediaPlayer.pause()
        }

        middle_button.setOnClickListener{
            if (mediaPlayer.isPlaying) {
                val duration = mediaPlayer.duration
                val middle = duration / 2
                mediaPlayer.seekTo(middle)
            } else {
                Toast.makeText(this, "Please click Play button.", Toast.LENGTH_SHORT).show()
            }
        }

        lower_volume_button.setOnClickListener {
            if (volume > 0f) {
                volume -= 0.1f
                mediaPlayer.setVolume(volume, volume)
            }
        }

        higher_volume_button.setOnClickListener {
            if (volume < 1.0f) {
                volume += 0.1f
                mediaPlayer.setVolume(volume, volume)
            }
        }
    }
}