在空对象引用上接收“ android.content.Context android.content.Context.getApplicationContext（）”

Question

I am new to android development and scripting with java. I am trying to create an app that allows me to toggle the internal microphone on and off via a switch. I found two scirpts (a switch script and a microphone control script) and I have peaced them together, correcting any debugging issues as I go and currently the script shows up clean. However when it is run on a phone or simulator it crashes immediately posting the following error,

android.content.Context android.content.Context.getApplicationContext()' on a null object reference"focused around my use of import android.content.Context;.

The error appears to be based on the Context.getApplicationContext request failing to find a result and thus posting NULL.

I have looked at a long list of other people suffering from similar areas, but all solutions seem to focus on changes to different areas of their code which I cannot relate back to my own scripts.

package com.example.myfirstapp;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Context;
import android.media.AudioManager;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Switch;

public class MainActivity extends AppCompatActivity {

    private Switch sw1;
    Button btnGet;
    Context context = getApplicationContext();
    AudioManager audioManager = ((AudioManager)context.getSystemService(Context.AUDIO_SERVICE));
    @Override
    protected void onCreate(Bundle savedInstanceState) {

        audioManager.setStreamMute(AudioManager.STREAM_SYSTEM, true);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        sw1 = findViewById(R.id.switch1);
        btnGet = findViewById(R.id.getBtn);
        btnGet.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (sw1.isChecked()) {
                    audioManager.setMicrophoneMute(false);
                }
                else {
                    audioManager.setMicrophoneMute(true);
                }

            }
        });
    }
}

The expected result, even if the script doesn't work, is that I am able to run the app, currently the app crashes on startup and posts the following error message...

[Logcat]

2019-08-20 13:23:01.710 5559-5559/com.example.myfirstapp E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myfirstapp, PID: 5559
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.myfirstapp/com.example.myfirstapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' 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.content.Context android.content.Context.getApplicationContext()' on a null object reference
        at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:106)
        at com.example.myfirstapp.MainActivity.<init>(MainActivity.java:16)
        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) 

Answer 1

Your issue is that you are attempting to retrieve the context at initialization ie.

Context context = getApplicationContext();

There is no guarantee that getApplicationContext(); will return a valid value until the activity has been created. In this case it is null and when you attempt to access it on the next line you get a null pointer exception. You need to instead assign the variable context within the onCreate() along with the audio manager.

For example, like so:

    Context context;
    AudioManager audioManager;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        context = getApplicationContext();
        audioManager = ((AudioManager)context.getSystemService(Context.AUDIO_SERVICE));
    ...
    }

Another thing to note is that the Activity class is a subclass of Context , so you do not need to retrieve the application context you can instead write

    AudioManager audioManager;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        audioManager = ((AudioManager)getSystemService(Context.AUDIO_SERVICE));
    ...
    }

Answer 2

I guess, the problem here is this line:

Context context = getApplicationContext();

Instead of initializing it as a class field, initialize it inside onCreate() method. For Example:

public class MainActivity extends AppCompatActivity {

private Switch sw1;
Button btnGet;
Context context;
AudioManager audioManager;
@Override
protected void onCreate(Bundle savedInstanceState) {


    super.onCreate(savedInstanceState);
    audioManager = ((AudioManager)getSystemService(Context.AUDIO_SERVICE));
    audioManager.setStreamMute(AudioManager.STREAM_SYSTEM, true);
    context = getApplicationContext();

    setContentView(R.layout.activity_main);
    sw1 = findViewById(R.id.switch1);
    btnGet = findViewById(R.id.getBtn);
    .
    .