names.length是names数组的长度(大)
names[i].length是names数组中包含数组的长度（小）
String[][] names={{"tom","jack","mike"},{"zhangsan","lisi","wangwu"}}; names.length 长度有2, {"tom","jack","mike"}和{"zhangsan","lisi","wangwu"}
names[1].length 其中 names[1]是{"tom","jack","mike"}，长度有3.