Github user kiszk commented on the issue:
https://github.com/apache/spark/pull/22241
Adding tests looks good.
Probably beyond the purpose of this PR, is the following difference
intentionally designed?
```
// If the data type is in @specialized annotation, and
// the `key` is not be contained, the `map(key)` will return 0
```
```
// If the data type is not in @specialized annotation, and
// the `key` is not be contained, the `map(key)` will return null
```
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