Why Can\'t We Use \'super\' Keyword for Type Parameter Bounds in Java Generics?

Bounding Generics with 'super' Keyword: A Comprehensive Explanation

Java generics allow for type safety and code reusability. One aspect of generics is type bounding, where a type parameter can be restricted to a specific subtype relationship. However, unlike wildcards, type parameters cannot be bounded using the 'super' keyword.

In the Collection interface, the toArray method takes an argument of type 'S[]' with 'S' being a type parameter. The question arises as to why the method is not declared as '' instead of a wildcard ''.

The answer lies in the behavior of 'super' bounding. While 'super' is typically used with wildcards (<'? super T'>), applying it to type parameters (<'' instead of '>) has an unintended consequence. Object is the superclass of all reference types, meaning that 'Object[]' is a valid argument to '<'S super T>>' at compile-time. This defeats the purpose of the bound, as any reference type argument could be passed, even those that are not subclasses of 'T'.

Therefore, 'super' bounding on type parameters would not provide the expected protection against invalid arguments. To illustrate, the hypothetical method '<> S[] toArray(S[] a)' with '' as 'Integer' would still allow an argument of type 'String[]', despite 'String' not being a superclass of 'Integer'. Since 'String[]' is an 'Object[]', it would satisfy the 'super' bound.

This limitation stems from the interplay between generics and arrays. Generics and arrays have different covariance rules, leading to the observed behavior.

It is important to note that this limitation applies only to type parameters. Wildcards can still be bound using 'super' to express variance, as seen in '<'? super T>' in the toArray method, allowing for greater flexibility.

For further insights on generics and type bounding, refer to the following resources:

• Java Tutorials/Generics
• Java Generics: What is PECS?
• What is the difference between super and extends in Java Generics

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