The second line of the error message says you are trying to add a JLabel to what the third error line is reporting as a List of JButton objects.
错误消息的第二行表明，您尝试将一个JLabel添加到第三个错误行报告的`JButton对象的List。
Then, if you try to add a JLabel to that collection, the compiler picks up the discrepancy and complains at compile time.
然后，如果想要将JLabel添加到集合，则编译器会在编译时发现差异和并抛出异常。