NullPointException when using GetMethods with one class but not another

Question

I am trying to create a script that will let me change the current microphone focus and I am currently confused by the behavior of my script. I have two sections where I get all methods from two classes (separated into two catch processes). When using "AudioManager" I can getMethods() by using...

Method[] methods1 = audioManager.getClass().getMethods();

-or-

Method[] methods1 = AudioManager.class.getMethods();

Both of which list the entire array of methods once they are broken down into a string.

However, if I run this same identical process against "AudioDeviceInfo" and use the same exact options, one of them will fail.

Method[] methods2 = audioDeviceInfo.getClass().getMethods(); (FAILS)

-or-

Method[] methods2 = AudioDeviceInfo.class.getMethods(); (SUCCEEDS)

package com.example.myfirstapp;

import android.content.Context;
import android.media.AudioDeviceInfo;
import android.media.AudioManager;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.Switch;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;

import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    private Switch sw1;
    TextView slvtxt;
    Button btnGet;
    Context context;
    AudioManager audioManager;
    AudioDeviceInfo audioDeviceInfo;

    private static List<String> getFieldNamesOne(Method[] methods1) {
        List<String> fieldNames1 = new ArrayList<>();
        for (Method method1 : methods1)
            fieldNames1.add(method1.getName());
        return fieldNames1;
    }

    private static List<String> getFieldNamesTwo(Method[] methods2) {
        List<String> fieldNames2 = new ArrayList<>();
        for (Method method2 : methods2)
            fieldNames2.add(method2.getName());
        return fieldNames2;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        context = getApplicationContext();
        audioManager = ((AudioManager)getSystemService(Context.AUDIO_SERVICE));
        setContentView(R.layout.activity_main);
        slvtxt = findViewById(R.id.slaveText);
        sw1 = findViewById(R.id.switch1);
        btnGet = findViewById(R.id.getBtn);
        btnGet.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                try {
                    Method[] methods1 = audioManager.getClass().getMethods();
                    //Method[] methods1 = AudioManager.class.getMethods();
                    List<String> actualFieldNames1 = getFieldNamesOne(methods1);
                    Log.d("tag","FOUND METHOD RETURN: " + Arrays.toString(actualFieldNames1.toArray()));

                    Method methodVar1 = audioManager.getClass().getMethod("setWiredDeviceConnectionState", Integer.TYPE, Integer.TYPE, String.class, String.class);
                    Log.d("tag","Method: " + methodVar1.getName());
                    methodVar1.setAccessible(true);


                } catch (NoSuchMethodException e) {
                    e.printStackTrace();
                    Log.d("tag","No Such Method : WiredDevice");
                } catch (NullPointerException e) {
                    e.printStackTrace();
                    Log.d("tag","Null Point : WiredDevice");
                }

                try {
                    Method[] methods2 = audioDeviceInfo.getClass().getMethods();
                    List<String> actualFieldNames2 = getFieldNamesTwo(methods2);
                    Log.d("tag","FOUND METHOD RETURN: " + Arrays.toString(actualFieldNames2.toArray()));

                } catch (NullPointerException e) {
                     e.printStackTrace();
                    Log.d("tag","Null Point : GetAddress");
                }
            }
        });
    }
}

LogCat Error

W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
        at com.example.myfirstapp.MainActivity$1.onClick(MainActivity.java:91)
        at android.view.View.performClick(View.java:5610)
        at android.view.View$PerformClick.run(View.java:22265)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6077)
        at java.lang.reflect.Method.invoke(Native Method)
W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
D/tag: Null Point : GetAddress
Process 4630 terminated.

I would expect that both versions of each set would have the same results. Additional items of the code I want to use follow the format for using the variable audioManager and audioDeviceInfo rather than their core class type for the getMethod and I want to make sure I am not doing something entirely wrong further up the pipeline before digging deeper.

Where do you call this from? The `getClass()` method should only be used for instance access, while the `.class` way is static. My guese would be that the instance you call the `getClass()` on is `null`. — n247s, Aug 22 '19 at 17:55
And hint: reflection is a **really** advanced topic. Whereas NPEs are daily super basic business. Sounds a bit like you are overburdening yourself. imho: when you need to ask others to help you with a null pointer exception, then you should seriously consider to not use reflection at all. — GhostCat, Aug 22 '19 at 18:03
No doubt there, unfortunately I am trying to solve a problem which (from my research) suggests only reflection can fix (trying to setWiredDeviceConnectionState through AudioManager). Additionally my particular question was directed more at the behavior of the script rather than the errors it could put out (see answere below @AndiCover). — PublicStringName, Aug 23 '19 at 16:26

AndiCover · Accepted Answer · 2019-08-22T18:04:04.213

You need to initialize your variable audioDeviceInfo before you can use it.

Probably you need something like following line:

audioDeviceInfo = audioManager.getDevices(AudioManager.GET_DEVICES_OUTPUTS);

Why does it work with audioManager?

Because you assign a value to it:

audioManager = ((AudioManager)getSystemService(Context.AUDIO_SERVICE));

Why do the other cases work?

In the other two cases you are using the class itself which will never be null:

Method[] methods1 = AudioManager.class.getMethods();
Method[] methods2 = AudioDeviceInfo.class.getMethods();

Please take a look at this answer: https://stackoverflow.com/a/218510/7450414

Thank you, that makes a lot of sense, I was sorta spinning my wheels trying to think of why everything appeared to be the same. — PublicStringName, Aug 23 '19 at 12:41

NullPointException when using GetMethods with one class but not another

1 Answers1