covariance

Give examples of functions which demonstrate covariance and contravariance in the cases of both overloading and overriding in Java? [closed]

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Covariance: class Super { Object getSomething(){} } class Sub extends Super { String getSomething() {} } Sub#getSomething is covariant because it returns a subclass of the return type of Super#getSomething (but fullfills the contract of Super.getSomething()) Contravariance class Super{ void doSomething(String parameter) } class Sub extends Super{ void doSomething(Object parameter) } Sub#doSomething is contravariant because … Read more

What is a covariant return type?

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Covariant return, means that when one overrides a method, the return type of the overriding method is allowed to be a subtype of the overridden method’s return type. To clarify this with an example, a common case is Object.clone() – which is declared to return a type of Object. You could override this in your … Read more

Why doesn’t the example compile, aka how does (co-, contra-, and in-) variance work?

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Generically, a covariant type parameter is one which is allowed to vary down as the class is subtyped (alternatively, vary with subtyping, hence the “co-” prefix). More concretely: trait List[+A] List[Int] is a subtype of List[AnyVal] because Int is a subtype of AnyVal. This means that you may provide an instance of List[Int] when a … Read more

Co-variant array conversion from x to y may cause run-time exception

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What it means is this Control[] controls = new LinkLabel[10]; // compile time legal controls[0] = new TextBox(); // compile time legal, runtime exception And in more general terms string[] array = new string[10]; object[] objs = array; // legal at compile time objs[0] = new Foo(); // again legal, with runtime exception In C#, … Read more

Why covariance and contravariance do not support value type

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Basically, variance applies when the CLR can ensure that it doesn’t need to make any representational change to the values. References all look the same – so you can use an IEnumerable<string> as an IEnumerable<object> without any change in representation; the native code itself doesn’t need to know what you’re doing with the values at … Read more

Difference between Covariance & Contra-variance

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The question is “what is the difference between covariance and contravariance?” Covariance and contravariance are properties of a mapping function that associates one member of a set with another. More specifically, a mapping can be covariant or contravariant with respect to a relation on that set. Consider the following two subsets of the set of … Read more

Covariance and contravariance real world example

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// Contravariance interface IGobbler<in T> { void gobble(T t); } // Since a QuadrupedGobbler can gobble any four-footed // creature, it is OK to treat it as a donkey gobbler. IGobbler<Donkey> dg = new QuadrupedGobbler(); dg.gobble(MyDonkey()); // Covariance interface ISpewer<out T> { T spew(); } // A MouseSpewer obviously spews rodents (all mice are // … Read more

Why are arrays covariant but generics are invariant?

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Via wikipedia: Early versions of Java and C# did not include generics (a.k.a. parametric polymorphism). In such a setting, making arrays invariant rules out useful polymorphic programs. For example, consider writing a function to shuffle an array, or a function that tests two arrays for equality using the Object.equals method on the elements. The implementation … Read more