无法修复 NullPointer 异常

Question

我是一名业余程序员，刚刚完成他的应用程序，现在正在调试；直到应用程序在没有询问的情况下崩溃:(.

我从 lolcat 做了一个堆栈跟踪，下面在 ContextWrapper 中实现了 NullPointerError。

@Override
public PackageManager getPackageManager() {
    return mBase.getPackageManager();
}

但我看不出有什么问题。应用代码如下：

package com.example.denny.myapplication;

import android.Manifest;
import android.Manifest.permission;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.location.Location;
import android.location.LocationManager;
import android.os.Build;
import android.os.Bundle;
import android.os.Handler;
import android.provider.Settings;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v4.content.ContextCompat;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    PackageManager pm = getPackageManager();
    if (pm.checkPermission(permission.ACCESS_FINE_LOCATION, getPackageName()) == PackageManager.PERMISSION_DENIED) {
        alertboxGPS();
    }
    mHandler = new Handler();
    startRepeatingTask();
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
                               @Override
                               public void onClick(View view) {
                                   Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                                           .setAction("Action", null).show();

                               }
                           }

    );
    getSystemService(Context.LOCATION_SERVICE);

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public PackageManager getPackageManager() {
    return super.getPackageManager();
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.

    //Also, chuck the subroutine settings here. :3
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

PackageManager pm = getPackageManager();

protected void alertboxGPS() {
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setMessage("Your Device's GPS is Disabled")
            .setCancelable(true)
            .setTitle("** Gps Status **")
            .setPositiveButton("Gps On",
                    new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int id) {
                            // finish the current activity
                            // AlertBoxAdvance.this.finish();
                            Intent myIntent = new Intent(
                                    Settings.ACTION_SECURITY_SETTINGS);
                            startActivity(myIntent);
                            dialog.cancel();
                        }
                    })
            .setNegativeButton("Cancel",
                    new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int id) {
                            // cancel the dialog box
                            dialog.cancel();
                        }
                    });
    AlertDialog alert = builder.create();
    alert.show();
}

protected void alertboxViolation() {
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setMessage("Please stop walking. You may get in an accident.")
            .setCancelable(false)
            .setTitle("Walking Warning")
            .setPositiveButton("Stop Walking",
                    new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int id) {
                            // finish the current activity
                            // AlertBoxAdvance.this.finish();
                            Intent myIntent = new Intent(
                                    Settings.ACTION_SECURITY_SETTINGS);
                            startActivity(myIntent);
                            dialog.cancel();
                        }
                    })
            .setNegativeButton("Cancel",
                    new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int id) {
                            // cancel the dialog box
                            dialog.cancel();
                        }
                    });
    AlertDialog alert = builder.create();
    alert.show();
}

protected void alertboxGranted() {
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setMessage("Access Granted")
            .setCancelable(true)
            .setTitle("Access to GPS")
            .setPositiveButton("Ok",
                    new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int id) {
                            // finish the current activity
                            // AlertBoxAdvance.this.finish();
                            Intent myIntent = new Intent(
                                    Settings.ACTION_SECURITY_SETTINGS);
                            startActivity(myIntent);
                            dialog.cancel();
                        }
                    })
            .setNegativeButton("Cancel",
                    new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int id) {
                            // cancel the dialog box
                            dialog.cancel();
                        }
                    });
    AlertDialog alert = builder.create();
    alert.show();
}

//Some Default Integers
int ViolationLimit = 10;
int SpeedLimit = 100;
int ExceedInstance = 0;

//Default Value
LocationManager lm = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
Location location1;
Location location2;

private boolean canGetLocation() {
    return(ContextCompat.checkSelfPermission(this,
            Manifest.permission.ACCESS_FINE_LOCATION)==
            PackageManager.PERMISSION_GRANTED);
}

private double GetSpeed(Location location1, Location location2) {

    if (location1 == null) {

        location1 = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);
        location2 = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);
    } else {
        location1 = location2;

        }
            if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION)== PackageManager.PERMISSION_GRANTED) {
                alertboxGranted();
        }
        location2 = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);

    double Distance;
    Distance = location1.distanceTo(location2);
    double Speed = Distance / mInterval;
    return Speed;
}


private int Judge(double Speed, int ViolationLimit, int SpeedLimit, int ExceedInstance) {
    if (Speed >= 1 && Speed <= SpeedLimit) {
        ExceedInstance += 1;
    }

    if (ExceedInstance >= ViolationLimit) {
        alertboxViolation();
    }
    return ExceedInstance;
}

void stopRepeatingTask() {
    mHandler.removeCallbacks(mStatusChecker);
}

private int mInterval = 3000; // 3 seconds by default, can be changed later
private Handler mHandler;
Runnable mStatusChecker = new Runnable() {
    @Override
    public void run() {
        updateStatus();
        mHandler.postDelayed(mStatusChecker, mInterval);
    }
};

private void updateStatus() {
    //Is used for updating mInterval
}

void startRepeatingTask() {
    double Speed = GetSpeed(location1, location2);
    Judge(Speed, ViolationLimit, SpeedLimit, ExceedInstance);
    mStatusChecker.run();
}

}

笑笑猫：

10-17 10:41:39.663 7874-7874/com.example.denny.myapplication D/AndroidRuntime: Shutting down VM
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0x41967898)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime: java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.denny.myapplication/com.example.denny.myapplication.MainActivity}: java.lang.NullPointerException
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2219)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2349)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.ActivityThread.access0(ActivityThread.java:159)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1316)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:99)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:137)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5419)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at java.lang.reflect.Method.invokeNative(Native Method)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Method.java:525)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at dalvik.system.NativeStart.main(Native Method)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:  Caused by: java.lang.NullPointerException
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.content.ContextWrapper.getPackageManager(ContextWrapper.java:94)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at com.example.denny.myapplication.MainActivity.getPackageManager(MainActivity.java:63)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at com.example.denny.myapplication.MainActivity.<init>(MainActivity.java:83)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at java.lang.Class.newInstanceImpl(Native Method)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at java.lang.Class.newInstance(Class.java:1130)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2210)
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2349) 
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.ActivityThread.access0(ActivityThread.java:159) 
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1316) 
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:99) 
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:137) 
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5419) 
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at java.lang.reflect.Method.invokeNative(Native Method) 
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Method.java:525) 
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187) 
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003) 
10-17 10:41:39.663 7874-7874/com.example.denny.myapplication E/AndroidRuntime:     at dalvik.system.NativeStart.main(Native Method) 
10-17 10:41:42.556 7874-7874/com.example.denny.myapplication I/Process: Sending signal. PID: 7874 SIG: 9

我的代码有什么问题？

Answer 1

据我所知，当 location1 和 location2 未在您的方法中初始化时，您可以调用它们 startRepeatingTask()。它们需要像在 getspeed.

中那样初始化和处理空异常

Location location1 = null;
Location location2 = null;

/.../

private double GetSpeed(Location location1, Location location2) {

    if (location1 == null) {

        location1 = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);
        location2 = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);
    } else {
        location1 = location2;
    }

/.../


void startRepeatingTask() {
    if (location1 != null && location2 != null) {
        /.../
        double Speed = GetSpeed(location1, location2);
        Judge(Speed, ViolationLimit, SpeedLimit, ExceedInstance);
        mStatusChecker.run();
}

看到你的堆栈跟踪后：
空指针异常出现在您使用的两行中：

PackageManager pm = getPackageManager();

首先：在你的 class 范围内声明 PackageManager pm; Once 然后在你的范围内初始化它 onCreate.

pm= getPackageManager();

通过在 Activity class 和 onCreate 中声明 PackckageManager pm = getPackageManager() ，您是否意识到您正在处理两个具有相同名称和不同范围的独立实例你的申请。

老实说，我对你的代码感到困惑。

所以我建议除了我上面说的以外，你需要研究并尝试理解以下内容：

范围
初始化对象 -> 调用未初始化的对象将导致空指针异常。
包管理器

这将真正帮助您进步。很难在本网站内解释所有这些。所以我提供了一些链接以配合每一点。

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