Android Studio - 错误：应用程序在虚拟设备上崩溃

Question

So, I am a new programmer and also new here on this platform. I have got a Problem. I tried everything like to watch Videos, read older Posts on StackOverflow, but nothing helped me really. That's why I wanted to ask you for help.

This is my Logcat after the crash report:

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 de.S8S7.Spiel.onCreate(Spiel.java:32)

And at this point it Shows the error:

btnAntwortEins.setOnClickListener(this);
btnAntwortZwei.setOnClickListener(this);
btnAntwortDrei.setOnClickListener(this);
btnAntwortVier.setOnClickListener(this);

And yeah, I am not native English speaker.

Answer 1

Beforde you can set the OnClickListeners, you have to load the instances of the Buttons like this:

btnAntwortEins = findViewById(R.id.btnIdAntwortEins);
btnAntwortZwei = findViewById(R.id.btnIdAntwortZwei);
btnAntwortDrei = findViewById(R.id.btnIdAntwortDrei);
btnAntwortVier = findViewById(R.id.btnIdAntwortVier);

btnAntwortEins.setOnClickListener(this);
btnAntwortZwei.setOnClickListener(this);
btnAntwortDrei.setOnClickListener(this);
btnAntwortVier.setOnClickListener(this);

The Id (eg btnIdAntwortEins) is what you have written into the Id field of each Button in the layout editor.

Answer 2

if its only crashing in virtual device then may be u have used google play service in your app and you have not istalled a emulator with play servivce. try to instyall emulator with play services.

Answer 3

There might be a scenario of having such crash when you dont intialize the button correctly in android.So as you are new to Android development consider these things when you initialize the buttons in the Activity

You can mention the Button Globally like:

Button button;

you have to give the reference id which you have defined in the xml like;

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

    githubRepoUrlRecyclerView = findViewById(R.id.button);

//mention on click listener after you have given the reference id.

button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //TODO task.
            }
        });