Import of libraries

I have a problem with this part and it is that I can not import libraries that I used previously as import android.Widget.Button

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;


public class MainActivity extends AppCompatActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState)
  {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

  }

When it happens that I add a variable of type Button and it is not recognized, I tried to update the gradle but it still does not work.