Penanganan ActionEvent Di Java (actionPerformed)

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// Nama file :LingkaranGUI.java
// Membuat aplikasi lingkaran berpenampilan grafis

// Mengimpor kelas
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.TitledBorder;
import java.text.DecimalFormat;
import bangun.datar.Lingkaran;

// Mendeklarasikan kelas
public class LingkaranGUI extends JFrame implements ActionListener{

   private JLabel label1, label2, label3, label4;
   private JTextField text1, text2, text3, text4;
   private JButton tombol;
   private JPanel panel1, panel2, panel3, panel4;
   private Font font;
   private DecimalFormat formatAngka;
   private Lingkaran lingkaran;

   // Mendesain intefer grafis
   public LingkaranGUI() {

      label1 = new JLabel("Jari-Jari");
      label2 = new JLabel("Diameter");
      label3 = new JLabel("Luas");
      label4 = new JLabel("Keliling");

      panel1 = new JPanel();
      panel1.setLayout(new GridLayout(4, 1));
      panel1.add(label1);
      panel1.add(label2);
      panel1.add(label3);
      panel1.add(label4);
     
      font = new Font("Tahoma", Font.BOLD, 14);  

      text1 = new JTextField(11);
      text1.setHorizontalAlignment(JTextField.RIGHT);  
      text1.setFont(font);
      text2 = new JTextField(11); 
      text2.setHorizontalAlignment(JTextField.RIGHT);     
      text2.setFont(font);
      text2.setForeground(Color.RED);
      text2.setEditable(false);
      text3 = new JTextField(11);  
      text3.setHorizontalAlignment(JTextField.RIGHT);    
      text3.setFont(font);
      text3.setForeground(Color.RED);
      text3.setEditable(false);
      text4 = new JTextField(11); 
      text4.setHorizontalAlignment(JTextField.RIGHT);     
      text4.setFont(font);
      text4.setForeground(Color.RED);
      text4.setEditable(false);

      panel2 = new JPanel();
      panel2.setLayout(new GridLayout(4, 1));
      panel2.add(text1);
      panel2.add(text2);
      panel2.add(text3);
      panel2.add(text4);

      tombol = new JButton("Hitung");
      panel3 = new JPanel();
      panel3.setLayout(new FlowLayout(FlowLayout.RIGHT));
      panel3.add(tombol);

      panel4 = new JPanel();
      panel4.setBorder(new TitledBorder("Form Lingkaran"));
      panel4.setLayout(new BorderLayout());
      panel4.add(panel1, BorderLayout.LINE_START);
      panel4.add(panel2, BorderLayout.LINE_END);

      getContentPane().add(panel4, BorderLayout.CENTER);
      getContentPane().add(panel3, BorderLayout.PAGE_END);

      // Registrasi obyek pendengar
      tombol.addActionListener(this);
   }

   // Mendeklarasikan ulang metoda actionPerformed
   public void actionPerformed(ActionEvent ae) {

      if (ae.getSource() == tombol) {

         double r = Double.parseDouble(text1.getText());
         lingkaran = new Lingkaran(r);

         formatAngka = new DecimalFormat("0.00");
         text2.setText("" + formatAngka.format(lingkaran.getJari2() * 2));
         text3.setText("" + formatAngka.format(lingkaran.hitungLuas()));
         text4.setText("" + formatAngka.format(lingkaran.hitungKeliling()));
      }
   }

   // Metoda main
   public static void main(String[] args) {
   
      LingkaranGUI frame = new LingkaranGUI();
      frame.setTitle("Kelas LingkaranGUI");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setSize(250, 180);
      frame.setVisible(true);
   }
}