使用 Intent 从 Java Activity 启动 Kotlin Activity 时，应用程序崩溃

Question

This app currently has three activities, a Login activity, Register activity and an activity written in Kotlin called the Bookjamroom activity. The Bookjamroom activity is supposed to be displayed On click of the login button, but whenever i do so the app forcefully shuts down.

Log Cat Error:

 - 2020-10-15 20:17:53.047 12954-12981/com.example.jamsecure
   E/eglCodecCommon: glUtilsParamSize: unknow param 0x000082da
   2020-10-15 20:17:53.047 12954-12981/com.example.jamsecure
   E/eglCodecCommon: glUtilsParamSize: unknow param 0x000082da
   2020-10-15 20:17:53.303 12954-12981/com.example.jamsecure
   D/EGL_emulation: eglMakeCurrent: 0xebb840c0: ver 3 1 (tinfo
   0xebb832e0) 2020-10-15 20:18:03.883 12954-12954/com.example.jamsecure
   D/AndroidRuntime: Shutting down VM 2020-10-15 20:18:03.889
   12954-12954/com.example.jamsecure E/AndroidRuntime: FATAL EXCEPTION:
   main
       Process: com.example.jamsecure, PID: 12954
       java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.jamsecure/com.example.jamsecure.Bookjamroom}:
   java.lang.NullPointerException: Attempt to invoke virtual method
   'android.content.pm.ApplicationInfo
   android.content.Context.getApplicationInfo()' on a null object
   reference
           at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2679)
           at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
           at android.app.ActivityThread.-wrap11(Unknown Source:0)
           at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
           at android.os.Handler.dispatchMessage(Handler.java:106)
           at android.os.Looper.loop(Looper.java:164)
           at android.app.ActivityThread.main(ActivityThread.java:6494)
           at java.lang.reflect.Method.invoke(Native Method)
           at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
        Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo
   android.content.Context.getApplicationInfo()' on a null object
   reference
           at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:152)
           at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:157)
           at android.content.Context.obtainStyledAttributes(Context.java:655)
           at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:839)
           at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:806)
           at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:630)
           at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:223)
           at com.example.jamsecure.Bookjamroom.<init>(Bookjamroom.kt:12)
           at java.lang.Class.newInstance(Native Method)
           at android.app.Instrumentation.newActivity(Instrumentation.java:1174)
           at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2669)
           at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
           at android.app.ActivityThread.-wrap11(Unknown Source:0) 
           at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
           at android.os.Handler.dispatchMessage(Handler.java:106) 
           at android.os.Looper.loop(Looper.java:164) 
           at android.app.ActivityThread.main(ActivityThread.java:6494) 
           at java.lang.reflect.Method.invoke(Native Method) 
           at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

Kotlin .kt class code

    package com.example.jamsecure

import android.os.Bundle
import android.view.View
import android.view.animation.Animation
import android.view.animation.AnimationUtils
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import com.google.android.material.floatingactionbutton.FloatingActionButton

class Bookjamroom : AppCompatActivity() {
    private val bfa = findViewById<FloatingActionButton>(R.id.bfa)
    private val bnj = findViewById<FloatingActionButton>(R.id.bnj)
    private val bp = findViewById<FloatingActionButton>(R.id.bp)
    private val bhb = findViewById<FloatingActionButton>(R.id.bhb)
    private val rotateOpen: Animation by lazy { AnimationUtils.loadAnimation(this, R.anim.rotate_open_anim) }
    private val rotateClose: Animation by lazy { AnimationUtils.loadAnimation(this, R.anim.rotate_close_anim) }
    private val fromBottom: Animation by lazy { AnimationUtils.loadAnimation(this, R.anim.from_bottom) }
    private val toBottom: Animation by lazy { AnimationUtils.loadAnimation(this, R.anim.to_bottom) }
    private var clicked = false
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_book_jam_room)
        bfa?.setOnClickListener {
            onAddButtonClicked()
        }
        bnj?.setOnClickListener {
            Toast.makeText(this, "1button is clicked", Toast.LENGTH_SHORT).show()

        }
        bp?.setOnClickListener {
            Toast.makeText(this, "2button is clicked", Toast.LENGTH_SHORT).show()

        }
        bhb?.setOnClickListener {
            Toast.makeText(this, "3button is clicked", Toast.LENGTH_SHORT).show()

        }

    }

    private fun onAddButtonClicked() {
        setVisibility(clicked)
        setAnimation(clicked)
        clicked = !clicked
    }

    private fun setVisibility(clicked: Boolean) {
        if (!clicked) {
            bnj.visibility = View.VISIBLE
            bp.visibility = View.VISIBLE
            bhb.visibility = View.VISIBLE
        } else {
            bnj.visibility = View.INVISIBLE
            bp.visibility = View.INVISIBLE
            bhb.visibility = View.INVISIBLE
        }
    }

    private fun setAnimation(clicked: Boolean) {
        if (!clicked) {
            bnj.startAnimation(fromBottom)
            bp.startAnimation(fromBottom)
            bhb.startAnimation(fromBottom)
            bfa.startAnimation(rotateOpen)
        } else {
            bnj.startAnimation(toBottom)
            bp.startAnimation(toBottom)
            bhb.startAnimation(toBottom)
            bfa.startAnimation(rotateClose)
        }
    }
}                        

Java Code that calls the Kotlin class

import android.widget.TextView;

import org.jetbrains.annotations.NotNull;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView textView = findViewById(R.id.reg);
        Button lbtn = findViewById(R.id.lb);
       lbtn.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
             
                Intent i = new Intent(MainActivity.this,Bookjamroom.class);
                startActivity(i);
            }
        });
        SpannableString spannableString = new SpannableString("New user? tap here to register!");
        ClickableSpan span = new ClickableSpan() {
            @Override
            public void onClick(@NotNull View v) {
                Intent intent = new Intent(MainActivity.this, Register.class);
                startActivity(intent);
            }
        };
        spannableString.setSpan(span, 0, 31, Spanned.SPAN_EXCLUSIVE_INCLUSIVE);
        textView.setText(spannableString);
        textView.setMovementMethod(LinkMovementMethod.getInstance());
    }
}

XML file of Kotlin Activity's Layout

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
    android:background="@drawable/back3"
    tools:context=".Bookjamroom">

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/bfa"
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:backgroundTint="@color/colorPrimary"
        android:clickable="true"
        android:focusable="true"
        android:tint="@android:color/white"

        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.955"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.976"
        app:srcCompat="@drawable/b_add" />

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/bnj"
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:layout_marginBottom="8dp"
        android:backgroundTint="@color/colorPrimary"
        android:clickable="true"
        android:focusable="true"
        android:tint="@android:color/white"
        android:visibility="invisible"
        app:layout_constraintBottom_toTopOf="@+id/bfa"
        app:layout_constraintEnd_toEndOf="@+id/bfa"
        app:srcCompat="@drawable/b_new_jam" />

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/bp"
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:layout_marginBottom="8dp"
        android:backgroundTint="@color/colorPrimary"
        android:clickable="true"
        android:focusable="true"
        android:tint="@android:color/white"
        android:visibility="invisible"
        app:layout_constraintBottom_toTopOf="@+id/bnj"
        app:layout_constraintEnd_toEndOf="@+id/bnj"
        app:srcCompat="@drawable/b_past" />

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/bhb"
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:layout_marginBottom="7dp"
        android:backgroundTint="@color/colorPrimary"
        android:clickable="true"
        android:focusable="true"
        android:tint="@android:color/white"
        android:visibility="invisible"
        app:layout_constraintBottom_toTopOf="@+id/bp"
        app:layout_constraintEnd_toEndOf="@+id/bp"
        app:srcCompat="@drawable/b_helpbot" />

    <TextView
        android:id="@+id/textView9"
        android:layout_width="157dp"
        android:layout_height="145dp"
        android:layout_marginTop="64dp"
        android:text="@string/welcom"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.46"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

XML of Java Layout

 <?xml version="1.0" encoding="utf-8"?>
    <androidx.constraintlayout.widget.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"
        android:background="@drawable/back3"
        tools:context=".MainActivity">
    
        <Button
            android:id="@+id/lb"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/login"
            android:textSize="24sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.449"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />
    
        <EditText
            android:id="@+id/editTextTextPersonName"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:ems="10"
            android:fontFamily="sans-serif-black"
            android:hint="@string/enter_username"
            android:importantForAutofill="no"
            android:inputType="textPersonName"
            android:textColor="#F6F6F6"
            android:textColorHint="#FFFFFF"
            android:textSize="24sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.496"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.262" />
    
        <EditText
            android:id="@+id/tpw"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:ems="10"
            android:fontFamily="sans-serif-black"
            android:hint="@string/enter_password"
            android:importantForAutofill="no"
            android:inputType="textPassword"
            android:textColor="#F6F6F6"
            android:textColorHint="#FFFDFD"
            android:textSize="24sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.496"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.377" />
    
        <TextView
            android:id="@+id/textView"
            android:layout_width="360dp"
            android:layout_height="105dp"
            android:fontFamily="sans-serif-black"
            android:text="@string/login_sign_up"
            android:textColor="#FFFFFF"
            android:textSize="36sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="1.0"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.065" />
    
        <TextView
            android:id="@+id/reg"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/new_user_tap_here_to_register"
            android:textColor="#FFFFFF"
            android:textColorHint="#FFFEFE"
            android:textSize="24sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.494"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.852" />
    </androidx.constraintlayout.widget.ConstraintLayout>

PS: This is my first post here at stack overflow, please excuse me if I have made any discrepancies, Thank you. Also the classes/activities are declared in the Manifest. And there isn't any issue with the Anim files.

Answer 1

You should instantiate views in onCreate method.

private var bfa: FloatingActionButton? = null

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_book_jam_room)
    bfa = findViewById<FloatingActionButton>(R.id.bfa)
}

or use access directly by id:

import kotlinx.android.synthetic.main.activity_book_jam_room.*

class Bookjamroom : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_book_jam_room)
        bfa?.setOnClickListener {
            onAddButtonClicked()
        }
    }

Answer 2

Try the following change on your kotlin activity.

class Bookjamroom : AppCompatActivity() {
private lateinit var bfa: FloatingActionButton 
private lateinit var bnj: FloatingActionButton 
private lateinit var bp : FloatingActionButton
private lateinit var bhb : FloatingActionButton

then onCreate:

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

        bfa = findViewById<FloatingActionButton>(R.id.bfa)
        bnj = findViewById<FloatingActionButton>(R.id.bnj)
        bp = findViewById<FloatingActionButton>(R.id.bp)
        bhb = findViewById<FloatingActionButton>(R.id.bhb)

        bfa?.setOnClickListener {
            onAddButtonClicked()
        }
        bnj?.setOnClickListener {
            Toast.makeText(this, "1button is clicked", Toast.LENGTH_SHORT).show()

        }
        bp?.setOnClickListener {
            Toast.makeText(this, "2button is clicked", Toast.LENGTH_SHORT).show()

        }
        bhb?.setOnClickListener {
            Toast.makeText(this, "3button is clicked", Toast.LENGTH_SHORT).show()

        }

    }