如何从匿名内部类方法访问资源 ID？

Question

• I created a new project with empty activity selected.
• Then I added a new layout file called layout_extra.xml with id of layout_extra and it just contains one switch with id of switch1 .
• The activity_main.xml contains a button with id of button to navigate to the layout_extra .
• The content of MainActivity.java is as follows:

public class MainActivity extends AppCompatActivity {

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

        findViewById(R.id.button).setOnClickListener(v->{
            setContentView(R.layout.layout_extra);
        });

        findViewById(R.id.switch1).setOnClickListener(v->{
            findViewById(R.layout.layout_extra).setBackgroundColor(Color.RED);
        });
    }
}

Question

I got the following error on the Android Studio.

The tooltip gives the following info.

I did all the given suggestions but none works (the application crashes).

How to properly call resource id from within an anonymous class method?

Answer 1

Hum, the method "findViewById()" is asking the ID of the Object not the Resource.

Basically what i believe you should do is set the "ID" in the "XML" of your main layout, than you can use the method "findViewById()", any object has an id in "XML" even the layout can be set an ID.

Ex: Set in the XML the id of you main object:

android:id="@+id/layout_extra"

in the code:

findViewById(R.id.layout_extra).setBackgroundColor(Color.RED);

Answer 2

I think I have got the error. You are calling setOnClickListener() on a null object. Hence, the updated code should be:

public class MainActivity extends AppCompatActivity {

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

        // The following does not set the layout 'layout_extra' as root view until the button press
        findViewById(R.id.button).setOnClickListener(v->{
            // This block executes on button press
            setContentView(R.layout.layout_extra);
            final View layout_extra = findViewById(R.id.layout_extra);
            
            findViewById(R.id.switch1).setOnClickListener(v2->{
                layout_extra.setBackgroundColor(Color.RED);
            });
        });
    }
}

where R.id.layout_extra refers to the id of the root element of layout_extra.xml .

Answer 3

The setOnClickListener() use a new view so findViewById(R.layout.layout_extra).setBackgroundColor(Color.RED); didn't work, you should try :

public class MainActivity extends AppCompatActivity {

    private Layout layout_extra; // TextView, Button ...

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

        findViewById(R.id.button).setOnClickListener(v->{
            setContentView(R.layout.layout_extra);
        });

        layout_extra = findViewById(R.layout.layout_extra);

        findViewById(R.id.switch1).setOnClickListener(v->{
            layout_extra.setBackgroundColor(Color.RED);
        });
    }
}