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fp2025:lab09 [2026/05/16 23:04] silviu |
fp2025:lab09 [2026/05/21 18:08] (current) silviu |
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| * get yourself familiar with covariance and contravariance | * get yourself familiar with covariance and contravariance | ||
| * see the benefits of using these concepts | * see the benefits of using these concepts | ||
| + | * understand bounded generics | ||
| - | *Introduction* | + | **Introduction** |
| When we define a generic type, for example ''Shelter[T]'', and we have a types ''Animal'' and ''Dog'' and ''Dog extends Animal'', the type ''Shelter[Dog]'' does not extend type ''Shelter[Animal]''. However, sometimes we would like to have a conversion in one direction or the other. For example, consider the following code: | When we define a generic type, for example ''Shelter[T]'', and we have a types ''Animal'' and ''Dog'' and ''Dog extends Animal'', the type ''Shelter[Dog]'' does not extend type ''Shelter[Animal]''. However, sometimes we would like to have a conversion in one direction or the other. For example, consider the following code: | ||
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| </code> | </code> | ||
| - | **9.2.** Modify this type to use the correct variance: | + | **9.2.** Modify this type to use the correct variance to the generic parameter: |
| <code scala> | <code scala> | ||
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| println(" -> No animals matched this criteria.") | println(" -> No animals matched this criteria.") | ||
| } else { | } else { | ||
| - | matches.foreach(animal => println(s" -> MATCH: ${animal.name}")) | + | println(s" -> There were ${matches.length} matches.") |
| } | } | ||
| } | } | ||
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| Notice the invariance of the generic parameter, it will come up in the last exercise. | Notice the invariance of the generic parameter, it will come up in the last exercise. | ||
| - | **9.6.** Apply the ''crossJoinAndFilter'' method on every possible shelter-predicate combination from the previous exercises, note which ones compile and which none don't. | + | **9.6.** Apply the ''crossJoinAndFilter'' method on every possible shelter-predicate combination from the previous exercises, note which ones compile and which ones don't. |
| **9.7.** Complete the following code and use the proper bound and use it on some shelter: | **9.7.** Complete the following code and use the proper bound and use it on some shelter: | ||