| No enclosing instance of type Ventana is accessible. Must qualify the allocation with an enclosing instance of type Ventana (e.g. x.new A() where x is an instance of Ventana). | 
| import java.awt.Color; import java.awt.Toolkit; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import javax.swing.JButton; import javax.swing.JFrame; import javax.swing.JPanel; public class Ventana { class ventanita extends JFrame{ ventanita(){ JFrame ventana = new JFrame("TITULO"); this.setSize(Toolkit.getDefaultToolkit().getScreenSize()); this.setResizable(false); panel panel = new panel(); this.add(panel); this.setVisible(true); } } class panel extends JPanel{ panel(){ JPanel panel = new JPanel(); this.setBackground(Color.black); botoncierre b1 = new botoncierre(); this.add(b1); } } class botoncierre extends JButton{ JButton close = new JButton(); botoncierre(){ cierre cerrar = new cierre(); this.addActionListener(cerrar); } } class cierre implements ActionListener{ public void actionPerformed(ActionEvent e){ System.exit(0); } } public static void main(String[]args){ ventanita demo = new ventanita(); } }  | 
import java.awt.Color;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Ventana {
   public static void main(String[]args){
      ventanita demo = new ventanita();
   }
   
}
import java.awt.Color;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
class ventanita extends JFrame{
      
      ventanita(){
         
         this.setSize(Toolkit.getDefaultToolkit().getScreenSize());
         this.setResizable(false);
         panel panel = new panel();
         this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         this.add(panel);
         this.setVisible(true);
      }
   }
import java.awt.Color;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
class panel extends JPanel{
      
      panel(){
         JPanel panel = new JPanel();
         this.setBackground(Color.black);
         botoncierre b1 = new botoncierre();
         this.add(b1);
      }
   }
import java.awt.Color;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
class cierre implements ActionListener{
      public void actionPerformed(ActionEvent e){
         System.exit(0);
      }
   }
javac *.java
java Ventana