Calling the methods in java

I know what it is the Calling the methods in java and before the question, I read about Java grammar very much.I know what it is the this in java, but I do not know, How is invoked the following code in MainActivity. java

    public void onDraw(Canvas canvas) {
      paint.setColor(Color.BLUE);
        canvas.drawCircle(120,120,40,paint);
        }

Now. I put the full code:

MainActivity.java

public class MainActivity extends Activity {
    Draw draw;
    Cal cal;
    TextView textView;
    RelativeLayout linearLayout;

    public void onCreate(Bundle s) {
        super.onCreate(s);
        setContentView(R.layout.activity_main);

        linearLayout = (RelativeLayout) findViewById(R.id.t);
        cal = new Cal(this);
        cal.cal();

        textView = new TextView(getApplicationContext());
        textView.setText("" + cal.result);
        textView.setTextColor(Color.RED);

        draw = new Draw(this);
        linearLayout.addView(textView);
        linearLayout.addView(draw);
    }}

Cal.java

public class Cal extends View {
    Cal(Context context){
        super(context);
    }
    public double result;
    double parameter = (Math.pow(40,2)) * 3.14;
    public void cal(){
        result = Math.sqrt(parameter);

    }
}

Draw.java

public class Draw extends View {
Paint paint = new Paint();
Draw(Context context) {
    super(context);
}
public void onDraw(Canvas canvas) {
  paint.setColor(Color.BLUE);
    canvas.drawCircle(120,120,40,paint);
    }
}

The public void cal(){ result = Math.sqrt(parameter);} is invoked by cal.cal(); in MainActivity But I do not know, How is invoked the

public void onDraw(Canvas canvas) {
  paint.setColor(Color.BLUE);
    canvas.drawCircle(120,120,40,paint);
    }

in MainActivity.java ??
in MainActivity.java, we have only the draw = new Draw(this); for calling the Draw(Context context) {super(context);} in Draw.java


Source: java

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