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.
錯(cuò)誤消息的第二行表明，您嘗試將一個(gè)JLabel添加到第三個(gè)錯(cuò)誤行報(bào)告的JButton對(duì)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，如果想要將JLabel添加到集合，則編譯器會(huì)在編譯時(shí)發(fā)現(xiàn)差異和并拋出異常。