title: How to handle exception on empty textfields?
body: <p>I want to calculate logarithm values but error occurs on empty text fields.</p>
  
  <pre><code>//Libraries
  import javax.swing.JButton;
  import javax.swing.JFrame;
  import javax.swing.JLabel;
  import javax.swing.JTextField;
  import javax.swing.JOptionPane;
  import java.awt.Font; //For Font And Size
  import java.awt.Color; //For Color Change
  import java.awt.event.ActionListener;
  import java.awt.event.ActionEvent;
  
  public class ChildClass extends JFrame {
  
      private JTextField textfield1;
      private JTextField textfield3;
      private JTextField textfield4;
      private JButton button1;
  
      public ChildClass() {
          super("Logrithm");
          getContentPane().setBackground(new Color(153, 204, 153));
  
          getContentPane().setLayout(null);
  
          textfield1 = new JTextField();
          textfield1.setBounds(62, 77, 62, 26);
          getContentPane().add(textfield1);
  
          textfield3 = new JTextField();
          textfield3.setBounds(127, 224, 153, 26);
          getContentPane().add(textfield3);
  
          textfield4 = new JTextField();
          textfield4.setBounds(175, 124, 62, 26);
          getContentPane().add(textfield4);
  
          button1 = new JButton();
          button1.addActionListener(new ActionListener() {
              public void actionPerformed(ActionEvent e) {
                  double number1,baseE,result;
                  String text1 = textfield1.getText();
                  String text3 = textfield4.getText();
  
                  if (text1.isEmpty() &amp;&amp; text3.isEmpty()) {
                      JOptionPane.showMessageDialog(null, "Enter Values In TextField", "Invalid TextFields", JOptionPane.ERROR_MESSAGE);
                  }
  
                  if (text3.isEmpty()) {
                      number1 = Double.parseDouble(text1);
                      result = Math.log(number1);
                      textfield3.setText("" + result);
                  }
                  else if (!text3.isEmpty()) {
                      number1 = Double.parseDouble(text1);
                      baseE = Double.parseDouble(text3);
                      result = Math.log(number1) / Math.log(baseE);
                      textfield3.setText("" + result);
                  }
              }
          });
          button1.setFont(new Font("Bullet", Font.PLAIN, 16));
          button1.setText("Click It");
          button1.setBounds(161, 172, 87, 26);
          getContentPane().add(button1);
      }
  }
  </code></pre>
  
  <p>This is Main Method:</p>
  
  <pre><code>public class MainMethod {
  
      public static void main(String[] args) {
  
          ChildClass obj=new ChildClass();
  
          obj.setSize(450, 400);
          obj.setResizable(false);
  
          obj.setDefaultCloseOperation(obj.EXIT_ON_CLOSE);
          obj.setLocationRelativeTo(null);
          obj.setVisible(true);
      }
  }
  </code></pre>
  
  <p>Problem I am facing when I click the jbutton with empty textfields. Popup MessageDialog comes after I click ok and then:</p>
  
  <pre><code>Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: empty String
  </code></pre>
  
  <p>How to handle this error?</p>