求助：我照着教材上打的代码，为啥运行不了？下面写着：
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 12
at PanelGridLayout.<init>(PanelGridLayout.java:10)
at ShowLayout.<init>(ShowLayout.java:8)
at Example9_4.main(Example9_4.java:3)
还有这种左边为啥会有个黄色的感叹号？
public class PanelGridLayout extends JPanel{
这是啥意思？要咋整？感叹号内写着：
Adds a default serial version ID to the selected type.
Use this option to add a user-defined ID in combination with custom
serialization code if the type did undergo structural changes since its
first release.
对了，还有一个问题，ShowLayout.Java里最后一句：volidate（）是什么意思呀？有什么用？
