Simple program to demonstrate how to register an event with a button

SimpleEvent.java

//
// Simple program to demonstrate how to register an event with a button.
//
// There are two inputs, a result field, and a button. When the button is pushed,
//    an event is fired which finds the root of the sum of the squares.  
//
 
import java.awt.event.*;
import javax.swing.*;

public class SimpleEvent extends JFrame implements ActionListener {
    JTextField field_1 = new JTextField("3",20);
    JTextField field_2 = new JTextField("4",20);
    JTextField field_result = new JTextField(20);
    JButton button_add = new JButton("FIND HYPOTENUSE");
 
    public static void main(String[] args) {
        SimpleEvent myWindow = new SimpleEvent("Pythagorean");
        myWindow.setSize(350,200);
        myWindow.setVisible(true);
    }
 
    public SimpleEvent(String title) {
        super(title);
        setLayout(new java.awt.FlowLayout());
 
        field_result.setEditable(false);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        this.add(field_1);
        this.add(field_2);
        this.add(field_result);
        this.add(button_add);
        
        // !! This is the most important line of code for this little program. 
        //  Notice the method signature looks like: void addActionListener( ActionListener ). So,
        //    as a parameter it takes an ActionListener, which SimpleEvent implements and actionPerformed
        //    is implemented. Thus when the butten triggers an event, the actionPerformed method is called
        //    and the fields are updated. 
        //
        // See http://docs.oracle.com/javase/7/docs/api/java/awt/TextField.html#addActionListener(java.awt.event.ActionListener) 
        //  
        button_add.addActionListener(this);
    }
 
    @Override
    public void actionPerformed(ActionEvent e) {
        // if the input fields are empty, do nothing
        if ( field_1.getText().length() > 0 && field_1.getText().length() > 0 ) {
            try {
                double d1 = Double.parseDouble( field_1.getText() );
                double d2 = Double.parseDouble( field_2.getText() );
                Double result = Math.sqrt( d1*d1 + d2*d2 );
 
                field_result.setText(result.toString());
            } catch (NumberFormatException err) {
                field_result.setText("Error");
            }
        }
    }
}