在空对象引用上调用虚拟方法'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)'的诱惑

Question

当我在模拟器上启动程序时，错误：项目"不断停止"。在logcat-java.lang.RuntimeException中：无法启动活动ComponentInfo｛com.example.prroject.com/example.product.MainActivity｝：java.lang.NullPointerException：尝试在null对象引用上调用虚拟方法"void android.widget.Button.setOnClickListener(android.view.view$OnClickListen(">

logcat错误

2022-03-06 00:26:59.113 15272-15272/com.example.project E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.project, PID: 15272
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.project/com.example.project.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
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$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.project.MainActivity.onCreate(MainActivity.java:20)
at android.app.Activity.performCreate(Activity.java:8000)
at android.app.Activity.performCreate(Activity.java:7984)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
at android.os.Handler.dispatchMessage(Handler.java:106) 
at android.os.Looper.loop(Looper.java:223) 
at android.app.ActivityThread.main(ActivityThread.java:7656) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
2022-03-06 00:27:00.347 15272-15289/com.example.project W/System: A resource failed to call close. 

MainActivity.java

package com.example.project;
import static com.example.project.R.layout.activity_2;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import androidx.appcompat.app.AppCompatActivity;
public class MainActivity extends AppCompatActivity {
private Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(activity_2);
button = findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
openactivity2();
}
});
}
public void openactivity2(){
Intent intent = new Intent(this, activity2.class);
startActivity(intent);
}
}

activity2.java

package com.example.project;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
public class activity2 extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_2);
}
}

activity_main.xml

<RelativeLayout 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">
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/text_name"
android:textColor="@color/black"
android:layout_centerHorizontal="true"
android:layout_marginTop="250dp"
/>

<Button
android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_name"
android:layout_below="@+id/textView"
android:layout_centerHorizontal="true"
android:layout_marginTop="10dp"

/>

</RelativeLayout>

activity_2.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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">
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/text_name2"
android:textColor="@color/black"
android:layout_centerHorizontal="true"
android:layout_marginTop="250dp"
/>

</RelativeLayout> 

Answer 1

您的MainActivity中有setContentView(activity_2)，而activity_2.xml没有id为button的视图。

您可能应该在MainActivity中使用setContentView(R.layout.activity_main)。