Android Click Event Error

Question

I get this strange error, when I activate my clickListener

Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (Build.VERSION.SDK_INT < 16) {
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN);
    }
    getWindow().addFlags(WindowManager.LayoutParams.FLAG_DISMISS_KEYGUARD);

    setContentView(R.layout.activity_main);
    WebView webView = (WebView) findViewById(R.id.webView);
    webView.loadData("test ", "text/html", "utf-8");
    webView.loadUrl("https://www.google.de/");
    webView.getSettings().setDomStorageEnabled(true);
    PrefUtils.setKioskModeActive(true, getApplicationContext());

}
   @Override
    public void onBackPressed() {
        Context context = getApplicationContext();
        CharSequence text = "password to deactivate mode!";
        int duration = Toast.LENGTH_SHORT;
        Toast.makeText(context, text, duration).show();
        myDialog = new Dialog(this);
        myDialog.setContentView(R.layout.dialog_signin);
        myDialog.setCancelable(false);
        password = (EditText) myDialog.findViewById(R.id.password);
        myDialog.show();
        // Error probably because of this
        Button lbtn = (Button)findViewById(R.id.loginButton);
        lbtn.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                if (password.getText().toString().equals("123")) {
                    Log.d("myapp", "test1");
                } else {
                    Log.d("myapp", "test2");
                }
            }
        });
    }

so basically a dialog textfield windows appears, when I click on the back button. Inside this I check if the password of 123 is correct or not.

Here is my dialog_signin.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content">
    <EditText
        android:id="@+id/password"
        android:inputType="textPassword"
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="4dp"
        android:layout_marginLeft="4dp"
        android:layout_marginRight="4dp"
        android:layout_marginBottom="16dp"
        android:fontFamily="sans-serif" />
        <!--android:hint="@string/password"-->
    <Button
        android:id="@+id/loginButton"
        android:layout_width="200dp"
        android:layout_height="30dp"
        android:background="@color/red"/>
</LinearLayout>

and this is my activity_main.xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:keepScreenOn="true"
    tools:context=".MainActivity">

    <WebView
        android:id="@+id/webView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true">

    </WebView>
</RelativeLayout>

Answer 1

Replace your code with this :

@Override
public void onBackPressed() {
    Context context = getApplicationContext();
    CharSequence text = "password to deactivate mode!";
    int duration = Toast.LENGTH_SHORT;
    Toast.makeText(context, text, duration).show();
    myDialog = new Dialog(this);
    myDialog.setContentView(R.layout.dialog_signin);
    myDialog.setCancelable(false);
    password = (EditText) myDialog.findViewById(R.id.password);
    myDialog.show();
    // Error probably because of this
    Button lbtn = (Button)myDialog.findViewById(R.id.loginButton);
    lbtn.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            if (password.getText().toString().equals("123")) {
                Log.d("myapp", "test1");
            } else {
                Log.d("myapp", "test2");
            }
        }
    });

Answer 2

You need to get the Button from the view it is in. If you use findViewById without any view reference then it would try to find the view in you activity xml in this case activity_main.xml. loginButton is not in this xml but in the dialog you have created that's why you are getting NPE. So, change

Button lbtn = (Button)findViewById(R.id.loginButton);

to

Button lbtn = (Button)myDialog.findViewById(R.id.loginButton);

Answer 3

You are attempting for find the Button in your Activity or Fragment layout, not in the Dialog:

    Button lbtn = (Button)findViewById(R.id.loginButton);

should be

    Button lbtn = (Button)myDialog.findViewById(R.id.loginButton);