Wildcard (Java): Difference between revisions

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== Covariance for generic types ==

Unlike arrays (which are [[Covariance and contravariance (computer science)#Covariant arrays in Java and C#|covariant]] in Java{{sfn|Bloch|2018|loc=Chapter §5 Item 26: Don't use raw types|pp=117-122}}), different instantiations of a generic type are not compatible with each other, not even explicitly.{{sfn|Bloch|2018|loc=Chapter §5 Item 26: WithDon't use raw types|pp=117-122}} For example, the declarationdeclarations <code>Generic<Supertype> superGeneric; Generic<Subtype> subGeneric;</code> will cause the compiler wouldto report a conversion errorerrors for both castings <code>(Generic<Subtype>)superGeneric</code> and <code>(Generic<Supertype>)subGeneric</code>.{{sfn|Bloch|2018|loc=Chapter §5 Item 26: Don't use raw types|pp=117-122}}

This incompatibility maycan be softened by the wildcard if <code>?</code> is used as an actual type parameter.{{sfn|Bloch|2018|loc=Chapter §5 Item 26: Don't use raw types|pp=117-122}} <code>Generic<?></code> is a supertype of all parameterizarions of the generic type <code>Generic</code>.{{sfn|Bloch|2018|loc=Chapter §5 Item 31: Use bounded wildcards to increase API flexibility|pp=139-145}} This allows objects of type <code>Generic<Supertype></code> and <code>Generic<Subtype></code> to be safely assigned to a variable or method parameter of type <code>Generic<?></code>.{{sfn|Bloch|2018|loc=Chapter §5 Item 3126: Use bounded wildcards toDon't increaseuse APIraw flexibilitytypes|pp=139117-145122}} Using <code>Generic<? extends Supertype></code> allows the same, restricting compatibility to <code>Supertype</code> and its children.{{sfn|Bloch|2018|loc=Chapter §5 Item 31: Use bounded wildcards to increase API flexibility|pp=139-145}} Another possibility is <code>Generic<? super Subtype></code>, which also accepts both objects and restricts compatibility to <code>Subtype</code> and all its parents.{{sfn|Bloch|2018|loc=Chapter §5 Item 31: Use bounded wildcards to increase API flexibility|pp=139-145}}

== Wildcard as parameter type ==

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A <code>List<? '''extends''' MyClass></code> is a list of objects of some subclass of <code>MyClass</code>, i.e. any object in the list is guaranteed to be of type <code>MyClass</code>, so one can iterate over it using a variable of type <code>MyClass</code><ref>[[Inheritance (object-oriented programming)]]</ref>

<syntaxhighlight lang="java">

public void doSomething(final List<? extends MyClass> list) {

for (final MyClass object : list) { // OK

// do something

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However, it is not guaranteed that one can add any object of type <code>MyClass</code> to that list:

<syntaxhighlight lang="java">

public void doSomething(final List<? extends MyClass> list) {

final MyClass m = new MyClass();

list.add(m); // Compile error

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A <code>List<? '''super''' MyClass></code> is a list of objects of some superclass of <code>MyClass</code>, i.e. the list is guaranteed to be able to contain any object of type <code>MyClass</code>, so one can add any object of type <code>MyClass</code>:

<syntaxhighlight lang="java">

public void doSomething(final List<? super MyClass> list) {

final MyClass m = new MyClass();

list.add(m); // OK

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However, it is not guaranteed that one can iterate over that list using a variable of type <code>MyClass</code>:

<syntaxhighlight lang="java">

public void doSomething(final List<? super MyClass> list) {

for (final MyClass object : list) { // Compile error

// do something