Android Studio KT NullPointerException 错误答案

【问题标题】：Android Studio KT NullPointerException errorAndroid Studio KT NullPointerException 错误
【发布时间】：2021-02-23 21:11:30
【问题描述】：

我在我的android studio项目的主activivty.kt中添加了几个按钮功能，据我所知，我初始化并调用了变量。


package com.dev.afp

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.animation.AnimationUtils
import android.widget.Toast
import android.widget.Toast.makeText
import com.google.android.material.tabs.TabLayout
import com.google.android.material.tabs.TabLayoutMediator
import kotlinx.android.synthetic.main.activity_main.*
import kotlinx.android.synthetic.main.splashscreen.*
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)


        val images = listOf(
            R.drawable.thumbnail_image001,
            R.drawable.image00,
            R.drawable.image01,
            R.drawable.image03,
            R.drawable.image04,
            R.drawable.image10,
            R.drawable.image13
        )

        val adapter = ViewPagerAdapter(images)
        viewPager.adapter = adapter

        TabLayoutMediator(tabLayout, viewPager) { tab, position ->
            tab.text = "${position + 1}"
        }.attach()

        tabLayout.addOnTabSelectedListener(object : TabLayout.OnTabSelectedListener {
            override fun onTabSelected(tab: TabLayout.Tab?) {
                makeText(
                    this@MainActivity,
                    "Reselected Tool Segment ${tab?.text}",
                    Toast.LENGTH_SHORT
                ).show()
            }

            override fun onTabUnselected(tab: TabLayout.Tab?) {
                makeText(
                    this@MainActivity,
                    "Unselected Tool Segment ${tab?.text}",
                    Toast.LENGTH_SHORT
                ).show()
            }

            override fun onTabReselected(tab: TabLayout.Tab?) {
                makeText(
                    this@MainActivity,
                    "Selected Tool Segment ${tab?.text}",
                    Toast.LENGTH_SHORT
                ).show()
            }
        })
        btnZoomIn.setOnClickListener {
            val animation = AnimationUtils.loadAnimation(this,R.anim.zoom_in)
            textView.startAnimation(animation)
            makeText(this@MainActivity, "Zoomed In", Toast.LENGTH_SHORT).show()
        }
        btnZoomOut.setOnClickListener {
            val animation = AnimationUtils.loadAnimation(this,R.anim.zoom_out)
            textView.startAnimation(animation)
            makeText(this@MainActivity, "Zoomed Out", Toast.LENGTH_SHORT).show()
        }
        slide_down.setOnClickListener {
            val animation = AnimationUtils.loadAnimation(this,R.anim.slide_down)
            textView.startAnimation(animation)
            makeText(this@MainActivity, "Moving Down", Toast.LENGTH_SHORT).show()
        }
        slide_up.setOnClickListener {
            val animation = AnimationUtils.loadAnimation(this,R.anim.slide_up)
            textView.startAnimation(animation)
            makeText(this@MainActivity, "Moving Up", Toast.LENGTH_SHORT).show()
        }

这是我在模拟器中单击按钮时出现的错误。

    Process: com.dev.afp, PID: 13840
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.startAnimation(android.view.animation.Animation)' on a null object reference
        at com.dev.afp.MainActivity$onCreate$1.onClick(MainActivity.kt:21)
        at android.view.View.performClick(View.java:7448)
        at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:992)
        at android.view.View.performClickInternal(View.java:7425)
        at android.view.View.access$3600(View.java:810)
        at android.view.View$PerformClick.run(View.java:28305)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

一切都是进口的，那里没有问题。我知道 NullPointerException 告诉我我正在调用的对象为空，但我不确定我哪里出错了。感谢您的帮助。

好的：我正在添加 xml 文件。

activitymain 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"
    android:orientation="vertical"
    android:background="@color/red"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/slide_up"
        android:layout_width="100dp"
        android:layout_height="60dp"
        android:layout_marginTop="4dp"
        android:layout_weight="1"
        android:baselineAligned="false"
        android:background="#000000"
        android:text="Slide Up"
        app:layout_constraintStart_toEndOf="@+id/btnZoomOut"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/btnZoomIn"
        android:layout_width="100dp"
        android:layout_height="60dp"
        android:layout_marginStart="4dp"
        android:layout_marginLeft="4dp"
        android:layout_marginTop="4dp"
        android:text="Zoom In"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <com.google.android.material.tabs.TabLayout
        android:id="@+id/tabLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintStart_toStartOf="parent"
        android:background="@color/red"
        app:layout_constraintTop_toBottomOf="@+id/btnZoomIn" />

    <Button
        android:id="@+id/btnZoomOut"
        android:layout_width="100dp"
        android:layout_height="60dp"
        android:layout_marginTop="4dp"
        android:text="@string/zoom_out"
        app:layout_constraintStart_toEndOf="@+id/btnZoomIn"
        android:background="#000000"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/slide_down"
        android:layout_width="100dp"
        android:layout_height="60dp"
        android:layout_marginStart="100dp"
        android:layout_marginLeft="100dp"
        android:layout_marginTop="4dp"
        android:text="@string/slide_down"
        android:background="#000000"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.971"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <androidx.viewpager2.widget.ViewPager2
        android:id="@+id/viewPager"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:layout_editor_absoluteX="25dp" />


</androidx.constraintlayout.widget.ConstraintLayout>

浏览器


<?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">

    <ImageView
        android:id="@+id/ivImage"
        android:layout_width="384dp"
        android:layout_height="395dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />


</androidx.constraintlayout.widget.ConstraintLayout>

【问题讨论】：

如果您提供失败的行的内容会很有帮助：at com.dev.afp.MainActivity$onCreate$1.onClick(MainActivity.kt:21)。此外，如果您可以共享整个 onCreate 方法，它也会提供一些上下文。
我们还需要查看xml文件
这就是不鼓励合成视图引用的原因。我推荐视图绑定。 developer.android.com/topic/libraries/view-binding

标签： java android android-studio user-interface kotlin

【解决方案1】：

NPE 发生在您在 OnClickListener 中引用的 textView 上，但它似乎未包含在您所引用的布局文件中。请验证您要制作动画的视图或布局文件。

【讨论】：

【解决方案2】：

我相信您希望在单击按钮时开始动画。您将首先必须初始化这些按钮，然后继续并在它们上启动动画。在您的一个按钮上启动动画的示例

val btnZoomIn = findViewById<Button>(R.id.btnZoomIn)
btnZoomIn.setOnClickListener {
            val animation = AnimationUtils.loadAnimation(this,R.anim.zoom_in)
            btnZoomIn.startAnimation(animation)
            makeText(this@MainActivity, "Zoomed In", Toast.LENGTH_SHORT).show()
        }

【讨论】：