Unable to get radio button id

I am trying to get radio button value on click of a button but it is showing red line error below this line:

I want to know why this error is showing as I am unable to understand.

radioSexButton = (RadioButton)findViewById(selectedId);

inconvertible types,cannot cast android.view.view to RadioButton

Below is my code:

XML code:

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".RadioButton"
    android:padding="20dp">

    <RadioGroup
        android:id="@+id/radioSex"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" >

        <RadioButton
            android:id="@+id/radioMale"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Male"
            android:checked="true" />

        <RadioButton
            android:id="@+id/radioFemale"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Female" />

    </RadioGroup>

    <Button
        android:id="@+id/btnDisplay"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Display" />

</LinearLayout>

Java code:

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RadioGroup;
import android.widget.Toast;

public class RadioButton extends AppCompatActivity {

private RadioGroup radioSexGroup;
private RadioButton radioSexButton;
private Button btnDisplay;

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

    radioSexGroup = findViewById(R.id.radioSex);
    btnDisplay = findViewById(R.id.btnDisplay);

    btnDisplay.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View v) {

             int selectedId = radioSexGroup.getCheckedRadioButtonId();
             radioSexButton = (RadioButton)findViewById(selectedId);

             // find the radiobutton by returned id
             Toast.makeText(getApplicationContext(),
                     radioSexButton.getText(), Toast.LENGTH_SHORT).show();
         }
      });
   }
}

How can I resolve this issue?

According to the exception message you're getting it appears to be an issue with casting the view returned by findViewById(int). Try the following steps below;

1. Rename your activity so that it's name doesn't clash with the view's type
2. Don't cast the return value of findViewById(int). It will automatically do that for you based on the type of your variable.
3. Make sure you have the correct imports.

After taking the above steps the resulting activity should be like below

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RadioButton; // this is the important part
import android.widget.RadioGroup;
import android.widget.Toast;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

// activity name changed!
public class RadioButtonActivity extends AppCompatActivity {

    private RadioGroup radioSexGroup;
    private RadioButton radioSexButton;
    private Button btnDisplay;

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

        radioSexGroup = findViewById(R.id.radioSex);
        btnDisplay = findViewById(R.id.btnDisplay);

        btnDisplay.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                int selectedId = radioSexGroup.getCheckedRadioButtonId();
                radioSexButton = findViewById(selectedId);

                // find the radiobutton by returned id
                Toast.makeText(getApplicationContext(),
                        radioSexButton.getText(), Toast.LENGTH_SHORT).show();
            }
        });
    }
}

You're encountering the issue because you're attempting to cast the return value of findViewById(int) into "RadioButton" which in your code is referring to the activity instead of android.widget.RadioButton