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

java.lang.NullPointerException: mainRvRef must not be null ,where mainRvRef is referencing a reyclerView

提问人：krish cyber 提问时间：1/14/2022 更新时间：1/14/2022 访问量：548

问：

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

这是我的 HomeActivity 类

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

我不知道为什么这个错误在我像往常一样初始化时出现， 请忽略我糟糕的英语！mainRvRef

java android kotlin android-recyclerview nullpointerexception

评论

答：

1赞 Kotlin Learner 1/14/2022 #1

我建议你在 kotlin 中使用 viewbinding。这将简化您的ID参考。如果使用 viewbinding，则代码将如下所示

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名称更改您的ID

更新

您需要在 build.gradle 中添加这些行

buildFeatures {
        viewBinding true
    }

enter image description here

在需要同步并重新生成项目之后。这是你的例子mainactivity.kt

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)
        }

    }
}

评论

0赞 krish cyber 1/14/2022
未解析的引用 ActivityHomeBinder，我发现没有要导入🙂的内容
0赞 Kotlin Learner 1/14/2022
你能分享你的项目吗？我可以帮你
0赞 krish cyber 1/14/2022
github.com/krish-dev-7/United-versity repo 链接
0赞 Kotlin Learner 1/14/2022
您可以在此处添加链接。
0赞 Kotlin Learner 1/14/2022
@krishcyber查看我更新的答案
-1赞 krish cyber 1/14/2022 #2

这实际上是我粗心大意的错，我的愚蠢，我忘记在 v24 中更新我的activity_home.xml：|

mainRvref未引用 XML 文件中的任何内容layout_v24因此请检查所有布局文件

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