Компоненты отображаются через раз

JFrame frame = new JFrame("Calculate");
  frame.setSize(400, 200);
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.setLocationRelativeTo(null);
  frame.setVisible(true);
  frame.setLayout(new FlowLayout());
  
  JPanel mainPanel = new JPanel();
  mainPanel.setLayout(new BoxLayout(mainPanel, BoxLayout.Y_AXIS));
  mainPanel.setBorder(BorderFactory.createEtchedBorder());
  frame.add(mainPanel);
  
  
  JPanel panel1 = new JPanel(new FlowLayout());
  JPanel panel2 = new JPanel(new FlowLayout());
  JPanel panel3 = new JPanel(new FlowLayout());
  panel1.setBorder(BorderFactory.createEtchedBorder());
  panel2.setBorder(BorderFactory.createEtchedBorder());
  panel3.setBorder(BorderFactory.createEtchedBorder());
  mainPanel.add(panel1);
  mainPanel.add(panel2);
  mainPanel.add(panel3);
  
  JLabel label1 = new JLabel("Число 1:");
  panel1.add(label1);
  JTextField  number1 = new JTextField(10);
  panel1.add(number1);
  JLabel label2 = new JLabel("Число 2:");
  panel1.add(label2);
  JTextField  number2 = new JTextField(10);
  panel1.add(number2);
  
  JButton button1 = new JButton("+");
  JButton button2 = new JButton("-");
  JButton button3 = new JButton("*");
  JButton button4 = new JButton("/");
  panel2.add(button1);
  panel2.add(button2);
  panel2.add(button3);
  panel2.add(button4);
  
  JLabel label3 = new JLabel("Результат:");
  JLabel label4 = new JLabel("");
  panel3.add(label3);
  panel3.add(label4);