英文:
How to handle with "java.lang.NullPointerException: null cannot be cast to non-null type android.widget.TextView"?
问题
我是你的中文翻译，以下是你提供的代码部分的翻译：
在Android开发中，我是新手。所以我决定开始制作一个待办事项应用程序。我想要创建一个窗口，用户可以在其中输入任务的标题和描述。在使用Fragment和RecyclerView之后，我开始遇到以下错误：
致命错误：主线程
进程：com.bignerdranch.android.taskmaster，PID：8887
java.lang.RuntimeException：无法启动Activity ComponentInfo {com.bignerdranch.android.taskmaster / com.bignerdranch.android.taskmaster.MainActivity}：java.lang.NullPointerException：无法将null强制转换为非null类型android.widget.TextView
但我无法理解出错的原因。我已经检查了我的MainActivity、Fragment和RecyclerView的适配器。这个错误一直导致我的应用程序停止运行。我将非常感谢您的帮助！
MainActivity.kt
package com.bignerdranch.android.taskmaster
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView
import androidx.lifecycle.ViewModelProvider
import androidx.recyclerview.widget.LinearLayoutManager
import com.bignerdranch.android.taskmaster.databinding.ActivityMainBinding
class MainActivity : AppCompatActivity() {
private lateinit var todoAdapter: TodoAdapter
private lateinit var binding: ActivityMainBinding
private lateinit var taskViewModel: TaskViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
val tvTodoTitle = findViewById<TextView>(R.id.tvTodoTitle)
taskViewModel = ViewModelProvider(this).get(TaskViewModel::class.java)
todoAdapter = TodoAdapter(mutableListOf())
binding.rvTodoItems.adapter = todoAdapter
binding.rvTodoItems.layoutManager = LinearLayoutManager(this)
binding.btnAddTodo.setOnClickListener{
NewTaskSheet().show(supportFragmentManager,"NewTaskTag")
}
taskViewModel.title.observe(this){
tvTodoTitle.text=String.format("Task:%s",it)
}
}
}
NewTaskSheet
package com.bignerdranch.android.taskmaster
import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import
<details>
<summary>英文:</summary>
I'm new in android dev. So I decided to start making todo app. I wanted to make a window where a user may enter title and description of his tasks. After using fragment+recycler view I started to get this error:
FATAL EXCEPTION: main
Process: com.bignerdranch.android.taskmaster, PID: 8887
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bignerdranch.android.taskmaster/com.bignerdranch.android.taskmaster.MainActivity}: java.lang.NullPointerException: null cannot be cast to non-null type android.widget.TextView
But I cannot understand what is wrong. I've checked my MainActivity, Fragment and Adapter for recycler view. This error makes my app stop all the time. I would appreciate your help!
MainActivity.kt
package com.bignerdranch.android.taskmaster
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView
import androidx.lifecycle.ViewModelProvider
import androidx.recyclerview.widget.LinearLayoutManager
import com.bignerdranch.android.taskmaster.databinding.ActivityMainBinding
class MainActivity : AppCompatActivity() {
private lateinit var todoAdapter:TodoAdapter
private lateinit var binding: ActivityMainBinding
private lateinit var taskViewModel: TaskViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
val tvTodoTitle = findViewById<TextView>(R.id.tvTodoTitle)
taskViewModel = ViewModelProvider(this).get(TaskViewModel::class.java)
todoAdapter = TodoAdapter(mutableListOf())
binding.rvTodoItems.adapter = todoAdapter
binding.rvTodoItems.layoutManager = LinearLayoutManager(this)
binding.btnAddTodo.setOnClickListener{
NewTaskSheet().show(supportFragmentManager,"NewTaskTag")
}
taskViewModel.title.observe(this){
tvTodoTitle.text=String.format("Task:%s",it)
}
}
}
NewTaskSheet
package com.bignerdranch.android.taskmaster
import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.lifecycle.ViewModelProvider
import com.bignerdranch.android.taskmaster.databinding.FragmentNewTaskSheetBinding
import com.google.android.material.bottomsheet.BottomSheetDialogFragment
class NewTaskSheet : BottomSheetDialogFragment(){
private lateinit var binding: FragmentNewTaskSheetBinding
private lateinit var taskViewModel: TaskViewModel
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
binding = FragmentNewTaskSheetBinding.inflate(inflater,container,false)
// Inflate the layout for this fragment
return binding.root
}
override fun onViewCreated(view:View,savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val activity = requireActivity()
taskViewModel= ViewModelProvider(activity).get(TaskViewModel::class.java)
binding.btnSave.setOnClickListener{
saveAction()
}
}
private fun saveAction(){
taskViewModel.title.value=binding.title.text.toString()
taskViewModel.description.value=binding.description.text.toString()
binding.title.setText("")
binding.description.setText("")
dismiss()
}
}
TodoAdapter
package com.bignerdranch.android.taskmaster
import android.graphics.Paint.STRIKE_THRU_TEXT_FLAG
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.CheckBox
import android.widget.TextView
import androidx.recyclerview.widget.RecyclerView
import com.bignerdranch.android.taskmaster.databinding.ItemTodoBinding
class TodoAdapter(
private val todos: MutableList<Todo>
):RecyclerView.Adapter<TodoAdapter.TodoViewHolder>() {
class TodoViewHolder( ItemTodoBinding: ItemTodoBinding): RecyclerView.ViewHolder(ItemTodoBinding.root)
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): TodoViewHolder {
val layoutInflater = LayoutInflater.from(parent.context)
val ItemTodoBinding = ItemTodoBinding.inflate(layoutInflater, parent, false)
return TodoViewHolder(
return TodoViewHolder(ItemTodoBinding)
)
}
private fun toggleStrikeThrough(tvTodoTitle: TextView, isChecked:Boolean){
if(isChecked){
tvTodoTitle.paintFlags = tvTodoTitle.paintFlags or STRIKE_THRU_TEXT_FLAG
}else{
tvTodoTitle.paintFlags = tvTodoTitle.paintFlags and STRIKE_THRU_TEXT_FLAG.inv()
}
}
override fun onBindViewHolder(holder: TodoViewHolder, position: Int) {
val curTodo = todos[position]
holder.itemView.apply{
val tvTodoTitle = findViewById(R.id.tvTodoTitle) as TextView
tvTodoTitle.text = curTodo.title
val cbDone = findViewById(R.id.cbDone) as CheckBox
cbDone.isChecked = curTodo.isChecked
toggleStrikeThrough(tvTodoTitle, curTodo.isChecked)
cbDone.setOnCheckedChangeListener { _, isChecked ->
toggleStrikeThrough(tvTodoTitle, isChecked)
curTodo.isChecked = !curTodo.isChecked
}
cbDone.setOnClickListener(View.OnClickListener{
todos.removeAll{todo->
todo.isChecked
}
notifyDataSetChanged()
})
}
}
override fun getItemCount(): Int {
return todos.size
}
}
activity_main
<?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"
tools:context=".MainActivity">
<androidx.recyclerview.widget.RecyclerView
android:id="@+id/rvTodoItems"
android:layout_width="match_parent"
android:layout_height="0dp"
app:layout_constraintBottom_toTopOf="@+id/btnAddTodo"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<Button
android:id="@+id/btnAddTodo"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="20dp"
android:text="Add"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toStartOf="@+id/btnTodoDone"
/>
<Button
android:id="@+id/btnTodoDone"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginEnd="8dp"
android:text="Done"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
item_todo
<?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="80dp"
android:paddingStart="8dp"
android:paddingEnd="8dp">
<TextView
android:id="@+id/tvTodoTitle"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:text="Example"
android:textSize="24sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toStartOf="@+id/cbDone"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<CheckBox
android:id="@+id/cbDone"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
P.S The description of this error tells me that it is all in 25 line in MainActivity but I don't know what is the problem there. Thanks in advance
 
</details>
# 答案1
**得分**: 1
由于您正在使用View Binding功能，无需使用此行来访问您的TextView：
```kotlin
val tvTodoTitle = findViewById<TextView>(R.id.tvTodoTitle)
当您想要设置标题时，只需执行以下操作：
binding.tvTodoTitle.text = "....."
这假设activity_main 中存在一个名为tvTodoTitle 的TextView。
英文:
As you're using the View Binding feature there's no need to use this line to access your TextView
val tvTodoTitle = findViewById<TextView>(R.id.tvTodoTitle)
When you want to set your Title you can just do
binding.tvTodoTitle.text = "....."
That's assuming that a TextView with the name tvTodoTitle exists in activity_main
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