java - 找不到变量JOptionPane

Question

private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {
    int a=JoptionPane.showConfirmDialog(null,"Do you really want to close the application","Select",JOptionPane.YES_NO_OPTION);
    if(a==0)
    {
        System.exit(0);
    }

Answer 1

int a=JoptionPane.show..

应该:

int a=JOptionPane.show..

Java区分大小写，选项 Pane 的第一个“o”为大写字母“O”。