获取jcombobox所选项目

Question

我有这个代码,我想从jcombobox获取所选项目,但是当我运行我的项目时,它给我复制打印所选项目的值和java.Lang.NullPointerException这是代码:

 private void jComboBox4ItemStateChanged(java.awt.event.ItemEvent evt) {                                            
        // TODO add your handling code here:
         if (evt.getStateChange()==ItemEvent.SELECTED){

             String a=String.valueOf(jComboBox4.getSelectedItem());
         System.out.print(a);

         try{
        String del2="select distinct PTYPE from Projects inner join project on projects.PNUMBER=(select pro_id from project where pro_name='"+a+"')";
         psst=con.prepareStatement(del2);
        String td2;
          DefaultComboBoxModel mode2 = new DefaultComboBoxModel();
           ResultSet rss=psst.executeQuery();
           while(rss.next()){
            td2=rss.getString("PTYPE");
    mode2.addElement(td2);
       jComboBox7.setModel(mode2);
           }
    }
        catch(SQLException ex){
            JOptionPane.showMessageDialog(null, ex.toString());
 } 
}

Answer 1

我假设您的itemStateChanged()方法中有此代码。您获得两次的原因是选择新值和取消选择旧值都会发生这种情况。

您的代码应该类似于：

    myCombo.addItemListener(new ItemListener() {
        @Override
        public void itemStateChanged(ItemEvent e) {
            if(e.getStateChange() == ItemEvent.SELECTED) {
                String a=jcombobox.getselecteditem().toString();
                System.out.print(a); 
            }
        }
    });