23.2 Adding Views to an Activity
The onCreate() method is currently designed to use a resource layout file for the user interface. Begin, therefore, by deleting this line from the method:
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
The next modification is to add a ConstraintLayout object with a single Button view child to the activity. This involves the creation of new instances of the ConstraintLayout and Button classes. The Button view then needs to be added as a child to the ConstraintLayout view which, in turn, is displayed via a call to the setContentView() method of the activity instance:
package com.ebookfrenzy.javalayout;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import androidx.constraintlayout.widget.ConstraintLayout;
...