java - 应用程序意外停止。由 : Java. lang.NullPointerException 引起的错误

Question

主.java

package com.learnactivities;

import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class Main extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity);

    Button b = (Button) findViewById(R.id.button1);
    b.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            startActivity(new Intent(Main.this, Second.class));

        }
    });
}

Activity .xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".Main" >

<TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button1" />

</RelativeLayout>

第二个.java

package com.learnactivities;

import android.app.Activity;
import android.os.Bundle;

public class Second extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.second);
}

第二个.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".Second" >

<TextView
    android:id="@+id/button1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true"
    android:layout_marginLeft="16dp"
    android:layout_marginTop="15dp"
    android:text="@string/nd" />

</RelativeLayout>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.learnactivities"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="17" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name="com.learnactivities.Main"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name=".second" />
</application>

</manifest>

我收到以下错误。我可以看到由 java.lang.NullPointerException 引起的错误。我在这里和那里尽了最大的努力，但找不到解决方案。Plz，有人帮我解决这个问题吗？

08-25 19:08:44.442: D/AndroidRuntime(276): Shutting down VM
08-25 19:08:44.442: W/dalvikvm(276): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
08-25 19:08:44.472: E/AndroidRuntime(276): FATAL EXCEPTION: main
08-25 19:08:44.472: E/AndroidRuntime(276): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.learnactivities/com.learnactivities.Main}: java.lang.NullPointerException
08-25 19:08:44.472: E/AndroidRuntime(276):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
08-25 19:08:44.472: E/AndroidRuntime(276):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
08-25 19:08:44.472: E/AndroidRuntime(276):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
08-25 19:08:44.472: E/AndroidRuntime(276):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
08-25 19:08:44.472: E/AndroidRuntime(276):  at android.os.Handler.dispatchMessage(Handler.java:99)
08-25 19:08:44.472: E/AndroidRuntime(276):  at android.os.Looper.loop(Looper.java:123)
08-25 19:08:44.472: E/AndroidRuntime(276):  at android.app.ActivityThread.main(ActivityThread.java:4627)
08-25 19:08:44.472: E/AndroidRuntime(276):  at java.lang.reflect.Method.invokeNative(Native Method)
08-25 19:08:44.472: E/AndroidRuntime(276):  at java.lang.reflect.Method.invoke(Method.java:521)
08-25 19:08:44.472: E/AndroidRuntime(276):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
08-25 19:08:44.472: E/AndroidRuntime(276):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
08-25 19:08:44.472: E/AndroidRuntime(276):  at dalvik.system.NativeStart.main(Native Method)
08-25 19:08:44.472: E/AndroidRuntime(276): Caused by: java.lang.NullPointerException
08-25 19:08:44.472: E/AndroidRuntime(276):  at com.learnactivities.Main.onCreate(Main.java:19)
08-25 19:08:44.472: E/AndroidRuntime(276):  at `android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)`
`08-25 19:08:44.472: E/AndroidRuntime(276):     at` `android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)`
08-25 19:08:44.472: E/AndroidRuntime(276):  ... 11 more

Answer 1

R.id.button1 在 second.xml 中声明，但您在 activity.xml 中查找它。所以在 Main 中，当你这样做的时候

   Button b = (Button) findViewById(R.id.button1);
    b.setOnClickListener(new OnClickListener()

findViewById 返回 null