为什么 getSelectedItem 返回 null？

我刚刚学习 OOP，对这篇基本文章感到抱歉。我不知道为什么当我尝试获取所选项目的值时它返回 nullJComboBox.

public class AddEmployee extends javax.swing.JInternalFrame{
   public AddEmployee() 
    {
    initComponents();
    this.setSize(1100,500);
    setMonths();
    setJComboBoxProperties();
    check();
    }

    private void setMonths()
    {
       String[] monthsObj = {"January", "February", "March", "April", "May", "June", "July",
    "August", "September", "October", "November", "December"};

       DefaultComboBoxModel monthsModel = new DefaultComboBoxModel(monthsObj);

       cbMonths.setModel((ComboBoxModel)monthsModel);

    }

    private void setJComboBoxProperties()
    {
      cbMonths.setSelectedIndex(-1);
    }

    private String check()
    {
       String cb = (String)cbMonths.getSelectedItem();
       System.out.println(cb);
       return cb;
    }

}

我投了String cb所以它不会给我一个空值。但我试图检查所选项目，但它给了我空值。

Call setSelectedItem初始化组合框后。请参阅文档 https://docs.oracle.com/javase/7/docs/api/javax/swing/DefaultComboBoxModel.html#setSelectedItem(java.lang.Object).

  DefaultComboBoxModel monthsModel = new DefaultComboBoxModel(monthsObj);
  monthsModel.setSelectedItem('September');