为什么这两个对象为null？

Question

I'm writing an Android app and two buttons ("btnGPSShowLocation" and "button") that I am using are null. I've instantiated them both so I don't know why they are null. Here is my code:

public class AndroidLocationActivity extends Activity {
Button btnGPSShowLocation;
Button button;


AppLocationService appLocationService;
private final static int INTERVAL = 1000 * 60 * 2; //2 minutes
Handler mHandler = new Handler();


@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    appLocationService = new AppLocationService(
            AndroidLocationActivity.this);

    btnGPSShowLocation = (Button) findViewById(R.id.btnGPSShowLocation);

    btnGPSShowLocation.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View arg0) {

            Location gpsLocation = appLocationService
                    .getLocation(LocationManager.GPS_PROVIDER);

            if (gpsLocation != null) {
                double latitude = gpsLocation.getLatitude();
                double longitude = gpsLocation.getLongitude();
                Toast.makeText(
                        getApplicationContext(),
                        "Mobile Location (GPS): \nLatitude: " + latitude
                                + "\nLongitude: " + longitude,
                        Toast.LENGTH_LONG).show();
            } else {
                showSettingsAlert("GPS");
            }

        }
    });

    button = (Button) findViewById(R.id.button12);

    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent("locationActivity"));
        }
    });
} 

And here is the XML that may be relevant:

<?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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="it226.myapplicationit226androidapp.AndroidLocationActivity"
tools:showIn="@layout/activity_gps">

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Show Location\n(GPS)"
    android:id="@+id/btnGPSShowLocation"
    android:layout_alignParentTop="true"
    android:layout_marginTop="83dp"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/nextgps"
    android:id="@+id/button12"
    android:layout_alignParentBottom="true"
    android:layout_centerHorizontal="true"
    android:layout_marginBottom="106dp" />
</RelativeLayout>

Edit: here is the logcat

> 04-27 23:34:30.525 18310-18310/it226.myapplicationit226androidapp
> E/AndroidRuntime: FATAL EXCEPTION: main
>                                                                                     Process: it226.myapplicationit226androidapp, PID: 18310
>                                                                                     java.lang.RuntimeException: Unable to start activity
> ComponentInfo{it226.myapplicationit226androidapp/it226.myapplicationit226androidapp.AndroidLocationActivity}:
> java.lang.NullPointerException: Attempt to invoke virtual method 'void
> android.widget.Button.setOnClickListener(android.view.View$OnClickListener)'
> on a null object reference
>                                                                                         at
> android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
>                                                                                         at
> android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
>                                                                                         at android.app.ActivityThread.-wrap11(ActivityThread.java)
>                                                                                         at
> android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
>                                                                                         at android.os.Handler.dispatchMessage(Handler.java:102)
>                                                                                         at android.os.Looper.loop(Looper.java:148)
>                                                                                         at android.app.ActivityThread.main(ActivityThread.java:5417)
>                                                                                         at java.lang.reflect.Method.invoke(Native Method)
>                                                                                         at
> com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
>                                                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
>                                                                                      Caused by: java.lang.NullPointerException: Attempt to invoke virtual
> method 'void
> android.widget.Button.setOnClickListener(android.view.View$OnClickListener)'
> on a null object reference
>                                                                                         at
> it226.myapplicationit226androidapp.AndroidLocationActivity.onCreate(AndroidLocationActivity.java:39)
>                                                                                         at android.app.Activity.performCreate(Activity.java:6237)
>                                                                                         at
> android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
>                                                                                         at
> android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
>                                                                                         at
> android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
>                                                                                         at android.app.ActivityThread.-wrap11(ActivityThread.java) 
>                                                                                         at
> android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
>                                                                                         at android.os.Handler.dispatchMessage(Handler.java:102) 
>                                                                                         at android.os.Looper.loop(Looper.java:148) 
>                                                                                         at android.app.ActivityThread.main(ActivityThread.java:5417) 
>  
                                                                                   at java.lang.reflect.Method.invoke(Native Method)

Answer 1

Remove tools:showIn="@layout/activity_gps"

This attribute set on the root element of a layout that 'ed by another layout. This allows you to point to one of the layouts which includes this layout, and at designtime this included layout will be rendered with the outer layout surrounding it.

See here for reference