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.

　　錯誤消息的第二行表明，您嘗試將一個JLabel添加到第三個錯誤行報告的JButton對象的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，則編譯器會在編譯時發現差異和并拋出異常。