import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class fangcheng extends Applet  implements  ActionListener
{
      Label label1,label2,label3,result;
      TextField input1,input2,input3,output;
      Button btn;
      double A,B,C;

      public void init()
      {
         label1 = new Label("A:");
         label2 = new Label("B:");
         label3 = new Label("C:");
         result = new Label("   ");
         input1 = new TextField(10);
         input2 = new TextField(10);
         input3 = new TextField(10);
         btn = new Button("求解");
         add(label1); add(input1);
         add(label2); add(input2);
         add(label3); add(input3);
         add(btn); add(result);
         input1.addActionListener(this);
         input2.addActionListener(this);
         input3.addActionListener(this);
         btn.addActionListener(this);
       }

       public void actionPerformed(ActionEvent e)
       {
         if(e.getSource()==btn)
         {
           A = Double.valueOf(input1.getText()).doubleValue();
           B = Double.valueOf(input2.getText()).doubleValue();
           C = Double.valueOf(input3.getText()).doubleValue();
           result.setText("方程的解是："+root(A,B,C));
         }
       }

       public double root(double A, double B, double C) throws IllegalArgumentException
       {
         if (A == 0)
         {
           throw new IllegalArgumentException("A不能為零!");
         }
         else
         {
           double disc = B*B - 4*A*C;
           if (disc < 0)
           throw new IllegalArgumentException("b*b-4ac值不能小于零!");
           return  (-B + Math.sqrt(disc)) / (2*A);
         }
       }

}