java - 我的应用程序在启动之前崩溃。这是一个简单的应用程序，其中我利用 Intent 从上一个类中调用另一个类

intentTest.java：

    package intentTest.xyz.com;

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

      public class intentTest extends Activity {
      Button b;
            /** Called when the activity is first created. */
     @Override
      public void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
         b = (Button) findViewById(R.id.b);

        b.setOnClickListener( new View.OnClickListener() {
           public void onClick(View view) {
            Intent intent = new Intent(intentTest.this,seond.class );
            startActivity(intent);

           }

       });

    }
}

main.xml：

 <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:orientation="vertical"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent"
  >
   <TextView
       android:layout_width="fill_parent"
      android:layout_height="wrap_content"
       android:text="First screen"
      />

   <Button
      android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="click"
     android:name="@+id/b"
     />
    </LinearLayout>

seond.java：

package intentTest.xyz.com;

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

public class seond extends Activity{
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.second);

     }
 }

second.xml：

 <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content">


    <TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
   android:text="second screen"
     />

manifest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
  package="intentTest.xyz.com"
  android:versionCode="1"
  android:versionName="1.0">
<application android:icon="@drawable/icon" android:label="@string/app_name">
    <activity android:name=".intentTest"
              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=".seond"></activity>
</application>


</manifest>

我得到以下错误代码：

11-16 01:26:07.156: ERROR/AndroidRuntime(779): Uncaught handler: thread main exiting  due to uncaught exception
11-16 01:26:07.166: ERROR/AndroidRuntime(779): java.lang.RuntimeException: Unable to start activity ComponentInfo{intentTest.xyz.com/intentTest.xyz.com.intentTest}: java.lang.NullPointerException
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2401)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2417)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at android.app.ActivityThread.access$2100(ActivityThread.java:116)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at android.os.Handler.dispatchMessage(Handler.java:99)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at android.os.Looper.loop(Looper.java:123)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at android.app.ActivityThread.main(ActivityThread.java:4203)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at java.lang.reflect.Method.invokeNative(Native Method)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at java.lang.reflect.Method.invoke(Method.java:521)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:549)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at dalvik.system.NativeStart.main(Native Method)
11-16 01:26:07.166: ERROR/AndroidRuntime(779): Caused by: java.lang.NullPointerException
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at intentTest.xyz.com.intentTest.onCreate(intentTest.java:18)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2364)
11-16 01:26:07.166: ERROR/AndroidRuntime(779):     ... 11 more

最佳答案

查看main.xml文件，您犯了一个小错误，有时会引起很多讨论。

android:name="@+id/b"

它应该像这样：

android:id="@+id/b"

这样您就可以在活动中访问此元素，这就是您的应用程序导致

NullPointerException

顺便说一下，您在类名中使用小写字母，在包名中使用大写字母，这对编码人员而言不是一个好习惯。