java - 使用加速度传感器时 Android 应用程序崩溃

Question

我正在尝试在 avd 中运行该应用程序API 级别为 25。

首先，我创建了一个简单的空白 Activity 并启动了应用程序，它可以工作。然后我添加了传感器数据的代码，它崩溃了。

这是我的MainActivity.java :

public class MainActivity extends AppCompatActivity implements SensorEventListener{

    private SensorManager sensorManager;
    private Sensor accelerometer;
    private long lastUpdate = 0;

    private TextView x_val = (TextView)findViewById(R.id.x);
    private TextView y_val = (TextView)findViewById(R.id.y);
    private TextView z_val = (TextView)findViewById(R.id.z);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        sensorManager = (SensorManager)getSystemService(Context.SENSOR_SERVICE);
        accelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        sensorManager.registerListener(this,accelerometer,SensorManager.SENSOR_DELAY_NORMAL);
    }

    @Override
    public void onSensorChanged(SensorEvent event) {
        Sensor sensor = event.sensor;

        if(sensor.getType() == Sensor.TYPE_ACCELEROMETER)
        {
            float x = event.values[0];
            float y = event.values[1];
            float z = event.values[2];

            long curTime = System.currentTimeMillis();
            if(curTime-lastUpdate > 100) //keep a difference of 100ms to avoid too much data
            {
                x_val.setText(x+"");
                y_val.setText(y+"");
                z_val.setText(z+"");
            }
        }
    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int accuracy) {

    }

    public void onPause()
    {
        super.onPause();
        sensorManager.unregisterListener(this);
    }

    public void onResume()
    {
        super.onResume();
        sensorManager.registerListener(this,accelerometer,SensorManager.SENSOR_DELAY_NORMAL);
    }
}

这是activity_main.xml :

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
    tools:context="com.example.naveen.accelerometer.MainActivity">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/x"
        android:text="X : 0"
         />
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/y"
        android:text="Y : 0"
         />
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/z"
        android:text="Z : 0"
         />

</LinearLayout>

在 logcat 中，我可以看到以下行 at com.example.naveen.accelerometer.MainActivity.<init>(MainActivity.java:18)

完整日志:

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.naveen.accelerometer/com.example.naveen.accelerometer.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2567) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6119) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference at android.support.v7.app.AppCompatDelegateImplBase.(AppCompatDelegateImplBase.java:120) at android.support.v7.app.AppCompatDelegateImplV9.(AppCompatDelegateImplV9.java:155) at android.support.v7.app.AppCompatDelegateImplV11.(AppCompatDelegateImplV11.java:31) at android.support.v7.app.AppCompatDelegateImplV14.(AppCompatDelegateImplV14.java:55) at android.support.v7.app.AppCompatDelegateImplV23.(AppCompatDelegateImplV23.java:33) at android.support.v7.app.AppCompatDelegateImplN.(AppCompatDelegateImplN.java:33) at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:201) at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:185) at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519) at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190) at com.example.naveen.accelerometer.MainActivity.(MainActivity.java:18) at java.lang.Class.newInstance(Native Method) at android.app.Instrumentation.newActivity(Instrumentation.java:1078) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2557) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)  at android.app.ActivityThread.-wrap12(ActivityThread.java)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:154)  at android.app.ActivityThread.main(ActivityThread.java:6119)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Answer 1

永远不要调用findViewById()，直到之后您创建了小部件(在您的情况下，通过调用setContentView())。