AndroidStudio Projects: Circle how to

package ... //this is the first line in your MainActivity.java 

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initButton();
    }
    private void initButton()
    {
        Button button = (Button)findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                EditText rad = (EditText) findViewById(R.id.editTextRadius);
                double r = Double.parseDouble(rad.getText().toString());
                if (r <= 0)
                    r = r * -1;

                Circle circ= new Circle(r);

                TextView textViewA = (TextView) findViewById(R.id.textViewArea);
                TextView textViewC = (TextView) findViewById(R.id.textViewCircumference);
                textViewA.setText(String.format("Area: %.2f", circ.getArea()));
                textViewC.setText(String.format("Circumference: %.2f", circ.getCircumference()));

            }
        });
    }
}

--snip
package ... //this is the first line in your Circle.java file

public class Circle {
    double radius;

    public double getRadius() {
        return radius;
    }

    public void setRadius(double radius) {
        this.radius = radius;
    }

    public Circle(double radius){
        this.radius = radius;
    }

    public double getArea(){
        return Math.PI * radius * radius;
    }

    public double getCircumference(){
        return 2 * Math.PI * radius;
    }

}