App crashes when attempting to swap activities

Question

This is a basic activity swapping.

The app does not crash if i declare a local button inside the configureActivitySwap() method like this:

Button voiceBtn = (findViewById(R.id.goToVoice));

But I have to declare the button in the global scope instead so I can use the button in other methods, mainly activating and deactivating the button when it should/should not be pressed.

I also noticed that if I remove the finish(); method and replace it with something else the app functions normally, but I have to have the finish(); method one way or another.

public class RecogActivity extends AppCompatActivity {

    private Button voiceBtn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        voiceBtn = findViewById(R.id.goToVoice);
        setContentView(R.layout.main_layout);

        // some unrelated code

        configureActivitySwap();
    }

    public void configureActivitySwap(){
        // Button voiceBtn = (findViewById(R.id.goToVoice));

        voiceBtn.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {
                finish();
            }
        });
    }

}

My runtime error logs:

    E/AndroidRuntime: FATAL EXCEPTION: main
        Process: tk.gandriks.gaaudiotransform, PID: 23125
        java.lang.RuntimeException: Unable to start activity ComponentInfo{tk.gandriks.gaaudiotransform/tk.gandriks.gaaudiotransform.RecogActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2957)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032)
            at android.app.ActivityThread.-wrap11(Unknown Source:0)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
            at android.os.Handler.dispatchMessage(Handler.java:105)
            at android.os.Looper.loop(Looper.java:164)
            at android.app.ActivityThread.main(ActivityThread.java:6944)
            at java.lang.reflect.Method.invoke(Native Method)
            at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
         Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
            at tk.gandriks.gaaudiotransform.RecogActivity.configureActivitySwap(RecogActivity.java:140)
            at tk.gandriks.gaaudiotransform.RecogActivity.onCreate(RecogActivity.java:124)
            at android.app.Activity.performCreate(Activity.java:7183)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1220)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2910)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032) 
            at android.app.ActivityThread.-wrap11(Unknown Source:0) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696) 
            at android.os.Handler.dispatchMessage(Handler.java:105) 
            at android.os.Looper.loop(Looper.java:164) 
            at android.app.ActivityThread.main(ActivityThread.java:6944) 
            at java.lang.reflect.Method.invoke(Native Method) 
            at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)

Should you use StartActivity() method in your OnClick Callback method? — Kwnstantinos Nikoloutsos, May 29 '19 at 10:48

score 4 · Accepted Answer · answered May 29 '19 at 10:52

4

You need to call the setContentView() before calling voiceBtn = findViewById(R.id.goToVoice); Since you don't specify the layout the findViewById method will not get the button instance

public class RecogActivity extends AppCompatActivity {

   private Button voiceBtn;

   @Override
   protected void onCreate(Bundle savedInstanceState) {

      super.onCreate(savedInstanceState);
      // set the layout first
      setContentView(R.layout.YOUR_LAYOUT_XML_FILE_NAME)

      voiceBtn = findViewById(R.id.goToVoice);

      // some unrelated code

      configureActivitySwap();
 }

 public void configureActivitySwap(){
    // Button voiceBtn = (findViewById(R.id.goToVoice));

    voiceBtn.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View view) {
        finish();
    }
  });
}

answered May 29 '19 at 10:52

sanoJ

2,935
2
8
18

Pardon me, i made this thread in a rush and i accidentally mistook the `setContentView();` method for unrelated code and didn't mention it. I am already using it, this is not the issue. Here, i edited the code as it currently is, and the issue persists. – Greg May 29 '19 at 11:01
@Greg You must call `setContentView` before `voiceBtn = findViewById(R.id.goToVoice);` – sanoJ May 29 '19 at 11:03
Oh damn... I think i may be an idiot – Greg May 29 '19 at 11:05

score 1 · Answer 2 · answered May 29 '19 at 10:53

Try I guess) In your // some unrelated code is contains setContentView method?

public class RecogActivity extends AppCompatActivity {

    private Button voiceBtn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        voiceBtn = findViewById(R.id.goToVoice);

        setContentView(R.layout.some_layout)
        // some unrelated code

        configureActivitySwap();
    }

    public void configureActivitySwap(){
        // Button voiceBtn = (findViewById(R.id.goToVoice));

        voiceBtn.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {
                finish();
            }
        });
    }

}

You caught NPE because of findViewById is calling on inflated view. You are have been calling findViewById before setContentView in the first case and got the exception. And in the second case - in configureActivitySwap, that going after setContentView. Move setContentView after super.onCreate(savedInstanceState) and all will be working fine.

score 1 · Answer 3 · answered May 29 '19 at 10:54

1

Are you setting layout before trying to find view with findViewById? setContentView(R.layout.main_layout);

answered May 29 '19 at 10:54

Tomislav29

78
1
6

Yes, i do. Pardon for forgetting to mention i am going to edit the code. – Greg May 29 '19 at 10:57

score -2 · Answer 4 · edited May 29 '19 at 11:53

-2

voiceBtn = (Button) findViewById(R.id.goToVoice);

replace the statement in your onCreate() method with the above. It should work.

and use

super.finish() instead of finish()

edited May 29 '19 at 11:53

Sajith

713
6
21

answered May 29 '19 at 10:50

Mian Faizan

23
1
7

1

casting is no longer required in new android studio versions – akshay_shahane May 29 '19 at 10:50
It does it automatically, that's clearly not the problem! – Kwnstantinos Nikoloutsos May 29 '19 at 10:51
This doesn't work. It is the same way to write it, the (Button) part is redundant. – Greg May 29 '19 at 10:52

App crashes when attempting to swap activities

4 Answers4