java.lang.NullPointerException: mainRvRef 不能为 null ，其中 mainRvRef 引用 reyclerView

Question

我使用 findViewById 引用了 mainRvRef， 但我仍然收到错误，因为它不能为空。我不认为这是一个正常的 NullPointerException ，请帮帮我！

这是我的 HomeActivity 课程

package com.example.loginkt


import android.content.Intent
import android.os.Bundle
import android.util.Log
import androidx.appcompat.app.AppCompatActivity
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import com.google.android.material.floatingactionbutton.FloatingActionButton

class HomeActivity:AppCompatActivity() {
    var questions = ArrayList<QuestionFB>()
    init {
        for(i:Int in 1..10){
            questions.add(QuestionFB(-1,"cse $i","krishna $i","$i. multiply $i * ${i*i}","maths $i", listOf("s","Sas")))
        }
    } // for testing
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_home)
        val fabR = findViewById<FloatingActionButton>(R.id.addQnFab)
        val mainRvRef = findViewById<RecyclerView>(R.id.mainRv)
        Log.v("Question", questions[0].solution[0])
        mainRvRef.layoutManager = LinearLayoutManager(this, LinearLayoutManager.VERTICAL,false)

        // ABOVE LINE MAKE IT'S SHOWING "mainRvRef can't be null"

        mainRvRef.adapter = MainAdapters(questions,this)
        fabR.setOnClickListener{
            val intent = Intent(this, AskActivity::class.java)
            startActivity(intent)
        }
    }
}

这是我的 activity_home.xml：

<?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">

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/addQnFab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="16dp"
        android:layout_marginBottom="16dp"
        android:clickable="true"
        android:backgroundTint="@color/pBlue_2"
        android:focusable="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:src="@drawable/add_icons"
        app:maxImageSize="50dp"
        android:contentDescription="@string/add_question" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/mainRv"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

这是我的堆栈跟踪

V/Question: s
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.loginkt, PID: 7231
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.loginkt/com.example.loginkt.HomeActivity}: java.lang.NullPointerException: mainRvRef must not be null
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: mainRvRef must not be null
        at com.example.loginkt.HomeActivity.onCreate(HomeActivity.kt:25)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 
I/Process: Sending signal. PID: 7231 SIG: 9

我不知道为什么在我像往常一样初始化mainRvRef 时会出现这个错误， 请忽略我糟糕的英语！

Answer 1

我建议你在 kotlin 中使用 viewbinding。这将使您的 id 参考变得容易。如果您使用 viewbinding，代码将如下所示

class HomeActivity:AppCompatActivity() {

    private lateinit var binding: ActivityHomeBinding
    var questions = ArrayList<QuestionFB>()
    init {
        for(i:Int in 1..10){
            questions.add(QuestionFB(-1,"cse $i","krishna $i","$i. multiply $i * ${i*i}","maths $i", listOf("s","Sas")))
        }
    } // for testing
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityHomeBinding.inflate(layoutInflater)
        setContentView(binding.root)
        Log.v("Question", questions[0].solution[0])
        binding.mainRvRef.layoutManager = LinearLayoutManager(this, LinearLayoutManager.VERTICAL,false)

        // ABOVE LINE MAKE IT'S SHOWING "mainRvRef can't be null"

        binding.mainRvRef.adapter = MainAdapters(questions,this)
        binding.fabR.setOnClickListener{
            val intent = Intent(this, AskActivity::class.java)
            startActivity(intent)
        }
    }
}

根据id名称更改你的id

更新

您需要在 build.gradle

中添加这些行

buildFeatures {
        viewBinding true
    }

在您需要同步和重建您的项目之后。这是您的mainactivity.kt 示例

package com.example.loginkt

import android.content.Intent
import android.os.Bundle
import android.widget.Button
import android.widget.TextView
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import com.example.loginkt.databinding.ActivityMainBinding
import com.google.firebase.auth.FirebaseAuth


lateinit var emailG: String
lateinit var passG: String
lateinit var auth: FirebaseAuth

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    private fun firebaseLogin(email: String, pass: String) {
        if (email != "" || pass != "") {
            auth.signInWithEmailAndPassword(email, pass).addOnCompleteListener { task ->
                if (task.isSuccessful) {
                    getUser()
                    val intent = Intent(this, HomeActivity::class.java)
                    startActivity(intent)
                    finish()
                }
            }.addOnFailureListener { exception ->
                Toast.makeText(applicationContext, exception.localizedMessage, Toast.LENGTH_LONG)
                    .show()
            }
        } else {
            Toast.makeText(applicationContext, "Enter all the fields", Toast.LENGTH_LONG).show()
        }
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        auth = FirebaseAuth.getInstance()
        title = "Login"
        binding = ActivityMainBinding.inflate(layoutInflater)
        super.onCreate(savedInstanceState)
        setContentView(binding.root)
        binding.SupTB.setOnClickListener {
            val intent = Intent(this, RegisterActivity::class.java)
            startActivity(intent)
        }
        binding.loginBtn.setOnClickListener {
            emailG = binding.emailTxt.text.toString()
            passG = binding.passwordTxt.text.toString()
            firebaseLogin(emailG, passG)
        }

    }
}

Answer 2

这其实是我粗心的错，我的笨蛋，我忘记在 v24 中更新我的 activity_home.xml：|

mainRvref 没有引用 layout_v24 xml 文件中的任何内容，因此请检查您的所有布局文件