java.lang.IllegalStateException（找不到方法-初学者的Android编程书ERROR [duplicate]） _Android编程

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此问题已经在这里有了答案：java.lang.IllegalStateException: Could not find method in a parent or ancestor Context for android:onClick attribute（3个答案）java.lang.IllegalStateExeption: Could not find a method finishA(View) in the activity class（2个答案）Why can't android:onClick recognize the specfied function?（4个答案）关闭3分钟前。我最近开始学习android，在《初学者的Android编程》一书中工作。
在第四章中，我编写了以下代码：
mainActivity.java：

package com.gamecodeschool.exploringlayouts; import androidx.appcompat.app.AppCompatActivity; import android.os.Bundle; import android.view.View; public class MainActivity extends AppCompatActivity {@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main_menu); }void loadConstraintLayout(View v){ setContentView(R.layout.activity_main); }void loadTableLayout(View v){ //setContentView(R.layout.my_table_layout); }void loadMenuLayout(View v){ setContentView(R.layout.main_menu); } }

main_menu.xml

< ?xml version="1.0" encoding="utf-8"?> < LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="match_parent" android:layout_height="match_parent"> < TextView android:id="@+id/textView4" android:layout_width="match_parent" android:layout_height="wrap_content" android:gravity="center_horizontal" android:text="Menu" android:textSize="50sp" /> < EditText android:id="@+id/editText" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_marginBottom="100sp" android:ems="10" android:inputType="textMultiLine" android:padding="15sp" android:text="Select a layout type to view an example. The onClick attribute of each button will call a method which executes setContentView to load the new layout" /> < LinearLayout android:layout_width="match_parent" android:layout_height="wrap_content" android:orientation="horizontal"> < TextView android:id="@+id/textView5" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_gravity="center_vertical" android:layout_weight=".7" android:text="Load ConstraintLayout" android:textSize="20sp" /> < Button android:id="@+id/button3" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_weight=".1" android:onClick="loadConstraintLayout" android:text="Load" /> < /LinearLayout> < LinearLayout android:layout_width="match_parent" android:layout_height="wrap_content" android:orientation="horizontal"> < TextView android:id="@+id/textView6" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_gravity="center_vertical" android:layout_weight=".7" android:text="Load TableLayout" android:textSize="20sp" /> < Button android:id="@+id/button4" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_weight=".1" android:onClick="loadTableLayout" android:text="Load" /> < /LinearLayout> < RatingBar android:id="@+id/ratingBar2" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_marginLeft="15sp" android:layout_marginTop="75sp" android:layout_marginRight="15sp" /> < /LinearLayout>

[运行程序并单击任意一个按钮时，程序崩溃并返回以下错误：

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.gamecodeschool.exploringlayouts, PID: 10078 java.lang.IllegalStateException: Could not find method loadConstraintLayout(View) in a parent or ancestor Context for android:onClick attribute defined on view class androidx.appcompat.widget.AppCompatButton with id 'button3' at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.resolveMethod(AppCompatViewInflater.java:436) at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:393) at android.view.View.performClick(View.java:7125) at android.view.View.performClickInternal(View.java:7102) at android.view.View.access$3500(View.java:801) at android.view.View$PerformClick.run(View.java:27336) at android.os.Handler.handleCallback(Handler.java:883) at android.os.Handler.dispatchMessage(Handler.java:100) 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)

【java.lang.IllegalStateException（找不到方法-初学者的Android编程书ERROR [duplicate]）】我以为可能是我做过的事情，所以我直接从发布者那里下载了代码，它返回了相同的错误。
答案方法loadConstraintLayout应为public：