我不知道为什么我的应用程序不会停止崩溃

Question

我正在编写一个需要该位置的应用程序。 Logcat 告诉我我的代码很清晰，没有错误。 但是当我启动应用程序（模拟或在我的手机上）时，它会崩溃。

这是我的代码：

package com.example.a291019;


import androidx.appcompat.app.AppCompatActivity;
import androidx.core.content.ContextCompat;
import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;


public class MainActivity extends AppCompatActivity {

    TextView txtLocation = findViewById(R.id.txtLocation);

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

        LocationManager locationManager = (LocationManager)getSystemService(Context.LOCATION_SERVICE);
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_DENIED || ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_DENIED) {

            assert locationManager != null;
            locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 500, 10, new LocationListener() {
                @Override
                public void onLocationChanged(Location location) {
                    Log.d("GPS", "Latitude" + location.getLatitude() + " et longitude " + location.getLongitude());
                    txtLocation.setText(Log.d("GPS", String.format("Latitude%s et longitude %s", location.getLatitude(), location.getLongitude())));
                }

                @Override
                public void onStatusChanged(String provider, int status, Bundle extras) {

                }

                @Override
                public void onProviderEnabled(String provider) {

                }

                @Override
                public void onProviderDisabled(String provider) {

                }

            });
        }


    }
}

Answer 1

从我的角度来看，那条线的初始化 position 是错误的：

public class MainActivity extends AppCompatActivity {
    TextView txtLocation = findViewById(R.id.txtLocation);

它必须放在这一行之后：

setContentView(R.layout.activity_main);

它是 position，您可以在其中定义该活动的负责布局。

因此，为 class 成员调用findViewById无法正常工作。

Answer 2

1) 将此行移至onCreate并使其成为final ：

final TextView txtLocation = findViewById(R.id.txtLocation);

2) Log.d()返回 Integer 并且不应该是setText方法的参数，这样做而不是那样：

txtLocation.setText(String.format("Latitude%s et longitude %s", 
    location.getLatitude(), location.getLongitude()));

固定代码：

package com.example.a291019;

import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.content.ContextCompat;


public class MainActivity extends AppCompatActivity {

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

    final TextView txtLocation = findViewById(R.id.txtLocation);

    LocationManager locationManager = (LocationManager) 
 getSystemService(Context.LOCATION_SERVICE);
    if (ContextCompat.checkSelfPermission(this, 
 Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_DENIED || 
 ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) 
 != PackageManager.PERMISSION_DENIED) {

        assert locationManager != null;
        locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 500, 10, 
          new LocationListener() {
            @Override
            public void onLocationChanged(Location location) {
                Log.d("GPS", "Latitude" + location.getLatitude() + " et longitude " + 
                    location.getLongitude());
                txtLocation.setText(String.format("Latitude%s et longitude %s", 
                    location.getLatitude(), location.getLongitude()));
            }

            @Override
            public void onStatusChanged(String provider, int status, Bundle extras) {

            }

            @Override
            public void onProviderEnabled(String provider) {

            }

            @Override
            public void onProviderDisabled(String provider) {

            }

        });
    }

  }
}

Answer 3

下面是红色部分：

 E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.a291019, PID: 11205 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.a291019/com.example.a291019.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6077) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference at androidx.appcompat.app.AppCompatDelegateImpl.<init>(AppCompatDelegateImpl.java:249) at androidx.appcompat.app.AppCompatDelegate.create(AppCompatDelegate.java:182) at androidx.appcompat.app.AppCompatActivity.getDelegate(AppCompatActivity.java:520) at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:191) at com.example.a291019.MainActivity.<init>(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:2538) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)  at android.app.ActivityThread.-wrap12(ActivityThread.java)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:154)  at android.app.ActivityThread.main(ActivityThread.java:6077)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)