On covariance of array and wildcard of generic type

1. What is covariance of array ？

In the picture above , One integer Array objects are treated as number Array objects . That is, subclass objects can be regarded as parent objects , It's called covariance . So he can also add Double Object of type .

2. What is a wildcard ？

？ That is, uncertain ｊａｖａ type

When the operation type , When you don't need to use type specific functions , Use only Object Function in class . Then you can use it ? Wildcards to table unknown types

① Upper bound wildcard

##　② Lower bound wildcard
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