Minimal CustomView

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
    tools:context="com.example.myapplication.MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!" />

    <com.example.myapplication.CustomRectangle
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

CustomRectangle.java

public class CustomRectangle extends View {
    private Rect rectangle;
    private Paint paint;

    public CustomRectangle(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);

        rectangle = new Rect(100, 100, 300, 250);

        paint = new Paint();
        paint.setColor(Color.YELLOW);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        canvas.drawColor(Color.BLUE);
        canvas.drawRect(rectangle, paint);
    }
}