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java - 应用程序未启动，但可以正确编译

问题描述

我正在使用 YouTube 上的课程在智能手机上制作指南针。

应用程序没有为我打开，我在应用程序控制台中收到错误消息。

我刚开始学习 Android Studio 和 Java，所以我准备发送您的所有评论。

这是代码本身：

package stgamobil.com.compas;

import androidx.appcompat.app.AppCompatActivity;

import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.os.Bundle;
import android.view.animation.Animation;
import android.view.animation.RotateAnimation;
import android.widget.ImageView;
import android.widget.TextView;


public class MainActivity extends AppCompatActivity implements SensorEventListener {
    private ImageView ivDinamic;
    private TextView tvDegree;
    private float current_degree = 0f;
    private SensorManager sensorManager;
    Sensor accelerometer;
    Sensor magnetometer;

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

        sensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);
        accelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        magnetometer = sensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD);
        ivDinamic = findViewById(R.id.iveDinamic);
        tvDegree = findViewById(R.id.tvDegree);

    }

    @Override
    protected void onResume() {
        super.onResume();
        sensorManager.registerListener(this, accelerometer, SensorManager.SENSOR_DELAY_UI);
        sensorManager.registerListener(this, magnetometer, SensorManager.SENSOR_DELAY_UI);
    }

    @Override
    protected void onPause() {
        super.onPause();
        sensorManager.unregisterListener(this);

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

    }
    @Override
    public void onSensorChanged(SensorEvent sensorEvent) {
        float degree = Math.round(0);



        SensorEvent event = null;
        float[] mGravity= null;
        float[] mGeomagnetic = null;
        if (event.sensor.getType() == Sensor.TYPE_ACCELEROMETER)
            mGravity = event.values;


        if (event.sensor.getType() == Sensor.TYPE_MAGNETIC_FIELD)
            mGeomagnetic = event.values;

        if (mGravity != null && mGeomagnetic != null) {
            float R[] = new float[9];
            float I[] = new float[9];

            boolean success = SensorManager.getRotationMatrix(R, I, mGravity, mGeomagnetic);
            if (success) {
                float orientation[] = new float[3];

                SensorManager.getOrientation(R, orientation);
                degree = orientation[0];
                Float.toString(degree);
                tvDegree.setText("Degrees from north: " + (degree));
            }
        }
        RotateAnimation ra = new RotateAnimation(current_degree, -degree, Animation.RELATIVE_TO_SELF,
                0.5f, Animation.RELATIVE_TO_SELF, 0.5f);

        ra.setDuration(210);

        ra.setFillAfter(true);

        ivDinamic.startAnimation(ra);
        current_degree = -degree;

    }


}

最好点击链接，你可以在那里看到更好 这是我的应用程序控制台

在此处输入图像描述

09/30 22:02:42: Launching 'app' on Pixel 3 API 28.
Install successfully finished in 2 s 564 ms.
$ adb shell am start -n "stgamobil.com.compas/stgamobil.com.compas.MainActivity" -a android.intent.action.MAIN -c android.intent.category.LAUNCHER
Connected to process 5209 on device 'emulator-5554'.
Capturing and displaying logcat messages from application. This behavior can be disabled in the "Logcat output" section of the "Debugger" settings page.
W/obil.com.compa: Accessing hidden method Landroid/graphics/drawable/Drawable;->getOpticalInsets()Landroid/graphics/Insets; (light greylist, linking)
    Accessing hidden field Landroid/graphics/Insets;->left:I (light greylist, linking)
    Accessing hidden field Landroid/graphics/Insets;->right:I (light greylist, linking)
    Accessing hidden field Landroid/graphics/Insets;->top:I (light greylist, linking)
    Accessing hidden field Landroid/graphics/Insets;->bottom:I (light greylist, linking)
W/obil.com.compa: Accessing hidden field Landroid/view/WindowInsets;->CONSUMED:Landroid/view/WindowInsets; (light greylist, reflection)
W/obil.com.compa: Accessing hidden method Landroid/view/View;->getAccessibilityDelegate()Landroid/view/View$AccessibilityDelegate; (light greylist, linking)
W/obil.com.compa: Accessing hidden method Landroid/view/View;->computeFitSystemWindows(Landroid/graphics/Rect;Landroid/graphics/Rect;)Z (light greylist, reflection)
W/obil.com.compa: Accessing hidden method Landroid/view/ViewGroup;->makeOptionalFitsSystemWindows()V (light greylist, reflection)
D/OpenGLRenderer: HWUI GL Pipeline
E/SensorManager: Exception dispatching input event.

D/AndroidRuntime: Shutting down VM
    
    
    --------- beginning of crash
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: stgamobil.com.compas, PID: 5209
    java.lang.NullPointerException: Attempt to read from field 'android.hardware.Sensor android.hardware.SensorEvent.sensor' on a null object reference

at stgamobil.com.compas.MainActivity.onSensorChanged(MainActivity.java:62)
at
android.hardware.SystemSensorManager$SensorEventQueue.dispatchSensorEvent(SystemSensorManager.java:833)
at android.os.MessageQueue.nativePollOnce(Native Method)

at android.os.MessageQueue.next(MessageQueue.java:326)

at android.os.Looper.loop(Looper.java:160)

at android.app.ActivityThread.main(ActivityThread.java:6669)

at java.lang.reflect.Method.invoke(Native Method)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

I/Process: Sending signal. PID: 5209 SIG: 9

标签： javaandroidandroid-studio

解决方案

如果您专注于您的代码，您会发现您正在onSensorChanged方法中创建具有空值的事件变量

    SensorEvent event = null;

在接下来的几行中，您将在 if 条件下使用它

    if (event.sensor.getType() == Sensor.TYPE_ACCELEROMETER)
        mGravity = event.values;

但你实际上需要使用sensorEvent哪个是函数参数，你正在接收

 @Override
public void onSensorChanged(SensorEvent sensorEvent){..}

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