例外：尝试在空对象引用上调用虚拟方法“android.content.res.Resources android.content.Context.getResources()”

Question

When i am trying to add two string variable msg and phoneNo in the TextView through setter method present in MainActivity.java (which is called by dow() method which is present in MyReceiver.java) a Runtime Exception (shown below) occurs When onReceive method in MyReceiver.java file is called by broadcastReceiver . I am unable to understand the Exceptio

MainActivity.java

package com.example.smsreceiver;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends AppCompatActivity {

    private static final int MY_PERMISSION_REQUEST_RECEIVE_SMS = 0;
    TextView textView;

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

        textView = findViewById(R.id.textView2);

        //check if the permission is not granted
        if(ContextCompat.checkSelfPermission(this, Manifest.permission.RECEIVE_SMS) != PackageManager.PERMISSION_GRANTED) {
            if(!ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.RECEIVE_SMS)) {
                //a pop up will appear asking for required permission i.e. Allow or Deny
                ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.RECEIVE_SMS}, MY_PERMISSION_REQUEST_RECEIVE_SMS);
            }
        }

        //this.setter();
    }//onCreate
    //after getting the result of permission request the result will be passed through this method

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults)
    {

        //will check the requestCode
        if (requestCode == MY_PERMISSION_REQUEST_RECEIVE_SMS) {//check whether the length of grantResults is greater than 0 and is equal to PERMISSION_GRANTED
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                //Now broadcast receiver work in background
                Toast.makeText(this, "Thank you for permitting!", Toast.LENGTH_LONG).show();

            } else {
                Toast.makeText(this, "Well I can't do anything until you permit me", Toast.LENGTH_LONG).show();
            }
        }
    }

    public void setter(String msg, String phoneNo){
        setContentView(R.layout.activity_main);
        textView = findViewById(R.id.textView2);
        String hi = msg + phoneNo;
        textView.setText(hi);
        Toast.makeText(this, "message: " + msg + "\n Number: " + phoneNo, Toast.LENGTH_LONG).show();

    }

}

MyReceiver.java

package com.example.smsreceiver;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.telephony.SmsMessage;
import android.util.Log;
import android.widget.Toast;

import java.util.Objects;

public class MyReceiver extends BroadcastReceiver {

    private static final  String SMS_RECEIVED = "android.provider.Telephony.SMS_RECEIVED";
    private static final String TAG = "SmsBroadcastReceiver";
    public static String msg, phoneNo = "";

    @Override
    public void onReceive(Context context, Intent intent) {

        Log.i(TAG, "Intent Received: " + intent.getAction());
        if(Objects.equals(intent.getAction(), SMS_RECEIVED)) {

            //retrieves a map of extended data from the intent
            Bundle dataBundle = intent.getExtras();

            if (dataBundle != null) {

                //create an PDU(Protocol Data Unit) object which is a protocol for transferring message
                Object[] mypdu = (Object[])dataBundle.get("pdus");
                final SmsMessage[] message = new SmsMessage[mypdu.length];

                for (int i = 0; i < mypdu.length; i++) {

                    //for build version >= API Level 23 (Build.VERSION_CODES.M)

                    String format = dataBundle.getString("format");
                    //from PDU we get all abject and SmsMessage Object using following line of code
                    message[i] = SmsMessage.createFromPdu((byte[]) mypdu[i], format);
                    msg = message[i].getMessageBody();
                    phoneNo = message[i].getOriginatingAddress();
                }
                Toast.makeText( context, "message: " + msg + "\n NUmber: " + phoneNo, Toast.LENGTH_LONG).show();
                dow(msg, phoneNo);
            }
        }
    }
    public void  dow(String msg, String phoneNo){
        MainActivity mainActivity = new MainActivity();
        mainActivity.setter(msg, phoneNo);
    }

}

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=".MainActivity">


    <TextView
        android:id="@+id/textView2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:textSize="24sp"
        />

</LinearLayout>

Exception shown by Logcat

Process: com.example.smsreceiver, PID: 18414
    java.lang.RuntimeException: Unable to start receiver com.example.smsreceiver.MyReceiver: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
        at android.app.ActivityThread.handleReceiver(ActivityThread.java:3665)
        at android.app.ActivityThread.access$1500(ActivityThread.java:219)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1850)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:227)
        at android.app.ActivityThread.main(ActivityThread.java:7200)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:575)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:903)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
        at android.content.ContextWrapper.getResources(ContextWrapper.java:91)
        at android.view.ContextThemeWrapper.getResourcesInternal(ContextThemeWrapper.java:127)
        at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:121)
        at androidx.appcompat.app.AppCompatActivity.getResources(AppCompatActivity.java:543)
        at android.widget.Toast.<init>(Toast.java:139)
        at android.widget.Toast.makeText(Toast.java:306)
        at android.widget.Toast.makeText(Toast.java:296)
        at com.example.smsreceiver.MainActivity.setter(MainActivity.java:60)
        at com.example.smsreceiver.MyReceiver.dow(MyReceiver.java:51)
        at com.example.smsreceiver.MyReceiver.onReceive(MyReceiver.java:45)
        at android.app.ActivityThread.handleReceiver(ActivityThread.java:3649)
        at android.app.ActivityThread.access$1500(ActivityThread.java:219) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1850) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:227) 
        at android.app.ActivityThread.main(ActivityThread.java:7200) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:575) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:903) 

Answer 1

You are trying to instantiate MainActivity on your own. Instead, do the following : 1)Create MyReceiver as an inner class inside MainActivity and register it as usual. 2)Receive the broadcast and update the UI from within onReceive().