title: How to check if the text from a textbox is a number?
body: <p>So i have the following window that has a textbox and a button.</p>
  
  <pre><code>public class Menu extends JFrame {
          private static final long serialVersionUID = 1L;
          private static final int WIDTH = 225;
          private static final int HEIGHT = 270;
          private static final int SCALE = 1;  
          private JTextField txtGenerationSize;
          public Menu(String title){
              setSize(450, 358);
              setPreferredSize(new Dimension(WIDTH*SCALE, HEIGHT*SCALE));
              setMaximumSize(new Dimension(WIDTH*SCALE, HEIGHT*SCALE));
              setMinimumSize(new Dimension(WIDTH*SCALE, HEIGHT*SCALE));
              setResizable(false);
              this.pack();
              setVisible(true);
              setTitle(title);
              setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
              getContentPane().setLayout(new CardLayout(0, 0));
  
              JPanel MenuPanel = new JPanel();
              getContentPane().add(MenuPanel, "name_36840921662905");
              MenuPanel.setLayout(null);
  
              JLabel lblTitle = new JLabel("Evolve!");
              lblTitle.setFont(new Font("Arial", Font.PLAIN, 18));
              lblTitle.setBounds(76, 21, 79, 14);
              MenuPanel.add(lblTitle);
  
              JLabel lblGenerationSize = new JLabel("Generation Size");
              lblGenerationSize.setBounds(10, 108, 79, 14);
              MenuPanel.add(lblGenerationSize);
  
              txtGenerationSize = new JTextField();
              txtGenerationSize.setBounds(99, 105, 56, 20);
              MenuPanel.add(txtGenerationSize);
              txtGenerationSize.setColumns(10);
  
              JButton btnStart = new JButton("Start generation");
              btnStart.setBounds(10, 208, 199, 23);
              MenuPanel.add(btnStart);
  
  
          }
      }
  </code></pre>
  
  <p>When they click the button i want to check if what they introduced in the textbox is a number  </p>
  
  <p>How can i do that?</p>