MainActivity运行时未启动[重复]

Question

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

<application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme"
        tools:ignore="GoogleAppIndexingWarning">

    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN"/>
            <category android:name="android.intent.category.LAUNCHER"/>
            <category android:name="android.intent.category.DEFAULT"/>
        </intent-filter>
    </activity>

    <activity
            android:name=".LoginActivity">
    </activity>
    <activity
            android:name=".RegisterActivity">
    </activity>


    <service android:name=".MyFirebaseMessagingService">
        <intent-filter>
            <action android:name="com.google.firebase.MESSAGING_EVENT"/>
        </intent-filter>
    </service>
    <service android:name=".MyFirebaseInstanceIDService">
        <intent-filter>
            <action android:name="com.google.firebase.INSTANCE_ID_EVENT"/>
        </intent-filter>
    </service>

    <meta-data
            android:name="android.app.alertbox"
            android:resource="@xml/automotive_app_desc"/>
</application>

</manifest> 

Caused by: kotlin.KotlinNullPointerException
        at com.example.acer.alertbox.MainActivity.onCreate(MainActivity.kt:17)
        at android.app.Activity.performCreate(Activity.java:6910)
        at 
   android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123)
        at 
   android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2746)
        at 
   android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2864) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1567) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:156) 
        at android.app.ActivityThread.main(ActivityThread.java:6523) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at 


 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:941) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:831) 

package com.example.acer.alertbox

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

class MainActivity : AppCompatActivity() {

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

        val btnLogin: Button? = null
        val btnRegister: Button? = null

        btnLogin!!.setOnClickListener {
            val intent = Intent(this@MainActivity, LoginActivity::class.java)
            startActivity(intent)
        }

        btnRegister!!.setOnClickListener {
            val regIntent = Intent(this@MainActivity, RegisterActivity::class.java)
            startActivity(regIntent)
        }

    }
}

*更新：我已经通过删除空指针解决了这些问题。我不知道有任何错误，因为我没有检查，也不知道如何打开日志错误，这就是为什么我不知道这种类型的问题曾经问过。顺便说一下，我是一个新的android开发者。

Answer 1

因此，经过长时间的讨论，我可以看到您将按钮设置为null

val btnLogin: Button? = null
val btnRegister: Button? = null

然后在null上设置onClickListener，这就是它崩溃的原因，所以您必须在声明之后设置按钮的引用，如下所示

btnLogin = findViewById(R.id.buttonid) as Button
btnRegister = findViewById(R.id.buttonid) as Button

您可以从activity_main布局文件中找到按钮id。