PROMEDIO 2 DE CALIFICACIONES POR CAMPO COMPROBADO en 9:59

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import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;
public class promedio2 extends JApplet implements ActionListener
{
    JTextField t1,t2,t3,t4;
    JButton b1,b2;
    public void init()
    {
    Container c=getContentPane();
    c.setLayout(new FlowLayout());
    t1=new JTextField(10);
    t2=new JTextField(10);
    t3=new JTextField(15);
    t4=new JTextField(15);
    b1=new JButton("PROMEDIAR");
    b2=new JButton("LIMPIAR");
    c.add(t1);
    c.add(t2);
    c.add(t3);
    c.add(t4);
    c.add(b1);
    c.add(b2);
    b1.addActionListener(this);
    b2.addActionListener(this);
    }
    public void actionPerformed(ActionEvent e)
    {
        double a,b,r;
    if(e.getSource()==b1)
    {
        a=Double.parseDouble(t1.getText());
        b=Double.parseDouble(t2.getText());
       if(a<0 || a>10 )
       {
           JOptionPane.showMessageDialog(null,"CALIFICACIÓN UNO ( NO VALIDA )");
           t1.setText(" ");
           t2.setText(" ");
       }
       
        if(b<0 || b>10 )
       {
           JOptionPane.showMessageDialog(null,"CALIFICACIÓN DOS ( NO VALIDA )");
           t1.setText(" ");
           t2.setText(" ");
       }
      
       if(a>=0 && a<=10 && b>=0 && b<=10)
       {
       r=(a+b)/2;
       if(r>=0 && r<=5.9)
       {
        t3.setText(String.valueOf(r));  
        t4.setText("REPROBADO");
       }//r
        if(r>=6 && r<=10)
        {
            t3.setText(String.valueOf(r));  
        t4.setText("APROBADO");
        }
       }

      
    }//boton

    if(e.getSource()==b2)
    {
        t1.setText(" ");
        t2.setText(" ");
        t3.setText(" ");
        t4.setText(" ");
    }
    }//ACTION
}//CLASE