App seemed to me fairly outdated, so updated all dependecies and converted classes to kotlin

app/build.gradle

@@ -2,7 +2,7 @@ apply plugin: 'com.android.application'
 
 android {
     compileSdkVersion 26
-    buildToolsVersion "26.0.1"
+    buildToolsVersion "26.0.3"
     defaultConfig {
         applicationId "com.nealgosalia.timetable"
         minSdkVersion 15
@@ -19,15 +19,15 @@ android {
 }
 
 dependencies {
-    compile 'com.roughike:bottom-bar:2.0.2'
-    compile 'com.ncapdevi:frag-nav:1.2.5'
-    compile 'com.shawnlin:number-picker:2.4.4'
-    compile fileTree(include: ['*.jar'], dir: 'libs')
-    compile 'com.daimajia.numberprogressbar:library:1.2@aar'
+    implementation 'com.roughike:bottom-bar:2.0.2'
+    implementation 'com.ncapdevi:frag-nav:1.2.5'
+    implementation 'com.shawnlin:number-picker:2.4.4'
+    implementation fileTree(include: ['*.jar'], dir: 'libs')
+    implementation 'com.daimajia.numberprogressbar:library:1.4@aar'
     //noinspection GradleCompatible
-    compile 'com.android.support:appcompat-v7:26.0.+'
-    compile 'com.android.support:design:26.0.+'
+    implementation 'com.android.support:appcompat-v7:26.1.0'
+    implementation 'com.android.support:design:26.1.0'
     testCompile 'junit:junit:4.12'
-    compile 'com.android.support:support-v4:26.0.+'
-    compile 'com.android.support:recyclerview-v7:26.0.+'
+    implementation 'com.android.support:support-v4:26.1.0'
+    implementation 'com.android.support:recyclerview-v7:26.1.0'
 }

app/src/main/java/com/nealgosalia/timetable/MainActivity.kt

@@ -0,0 +1,95 @@
+package com.nealgosalia.timetable
+
+import android.support.v7.app.AppCompatActivity
+import android.support.v4.app.Fragment
+import android.content.Intent
+import android.os.Bundle
+import android.os.Handler
+import android.support.annotation.IdRes
+import android.view.Menu
+import android.view.MenuItem
+import android.widget.Toast
+
+import com.ncapdevi.fragnav.FragNavController
+import com.nealgosalia.timetable.activities.PreferencesActivity
+import com.nealgosalia.timetable.fragments.AttendanceFragment
+import com.nealgosalia.timetable.fragments.SubjectsFragment
+import com.nealgosalia.timetable.fragments.TimetableFragment
+import com.roughike.bottombar.BottomBar
+import com.roughike.bottombar.OnTabReselectListener
+import com.roughike.bottombar.OnTabSelectListener
+
+import java.util.ArrayList
+
+class MainActivity : AppCompatActivity() {
+
+    private val TAB_FIRST = FragNavController.TAB1
+    private val TAB_SECOND = FragNavController.TAB2
+    private val TAB_THIRD = FragNavController.TAB3
+    internal var doubleBackToExitPressedOnce = false
+    private var fragNavController: FragNavController? = null
+
+    @Override
+    protected fun onCreate(savedInstanceState: Bundle) {
+        super.onCreate(savedInstanceState)
+        setContentView(R.layout.activity_main)
+        val fragments = ArrayList(3)
+        fragments.add(TimetableFragment())
+        fragments.add(AttendanceFragment())
+        fragments.add(SubjectsFragment())
+        fragNavController = FragNavController(savedInstanceState, getSupportFragmentManager(), R.id.contentContainer, fragments, TAB_FIRST)
+        val bottomBar = findViewById(R.id.bottomBar) as BottomBar
+        bottomBar.setDefaultTabPosition(1)
+        bottomBar.setOnTabSelectListener(object : OnTabSelectListener() {
+            @Override
+            fun onTabSelected(@IdRes tabId: Int) {
+                when (tabId) {
+                    R.id.tab_timetable -> fragNavController!!.switchTab(TAB_FIRST)
+                    R.id.tab_attendance -> fragNavController!!.switchTab(TAB_SECOND)
+                    R.id.tab_subjects -> fragNavController!!.switchTab(TAB_THIRD)
+                }
+            }
+        })
+
+        bottomBar.setOnTabReselectListener(object : OnTabReselectListener() {
+            @Override
+            fun onTabReSelected(@IdRes tabId: Int) {
+                fragNavController!!.clearStack()
+            }
+        })
+    }
+
+    @Override
+    fun onCreateOptionsMenu(menu: Menu): Boolean {
+        getMenuInflater().inflate(R.menu.menu, menu)
+        return super.onCreateOptionsMenu(menu)
+    }
+
+    @Override
+    fun onBackPressed() {
+        if (doubleBackToExitPressedOnce) {
+            super.onBackPressed()
+            return
+        }
+        this.doubleBackToExitPressedOnce = true
+        Toast.makeText(this, getResources().getString(R.string.press_back), Toast.LENGTH_SHORT).show()
+        Handler().postDelayed(object : Runnable() {
+            @Override
+            fun run() {
+                doubleBackToExitPressedOnce = false
+            }
+        }, 2000)
+    }
+
+    @Override
+    fun onOptionsItemSelected(item: MenuItem): Boolean {
+        when (item.getItemId()) {
+            R.id.settings -> {
+                val intent = Intent(this@MainActivity, PreferencesActivity::class.java)
+                startActivity(intent)
+                return true
+            }
+            else -> return super.onOptionsItemSelected(item)
+        }
+    }
+}