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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In the body of a generic unit, the (formal) type parameter is handled like its [[bounded quantification|upper bound]] (expressed with <code>'''extends'''</code>; <code>Object</code> if not constrained).{{sfn|Bloch|2018|loc=Chapter §5 Item 31: Use bounded wildcards to increase API flexibility|pp=139-145}} If the return type of a method is the type parameter, the result (e.g. of type <code>?</code>) can be referenced by a variable of the type of the upper bound (or <code>Object</code>). In the other direction, the wildcard fits no other type, not even <code>Object</code>: If <code>?</code> has been applied as the formal type parameter of a method, no actual parameters can be passed to it. However, objects of the unknown type can be read from the generic object and assigned to a variable of a supertype of the upperbound.

Sample code for the <code>Generic<T '''extends''' UpperBound></code> class:

<syntaxhighlight lang="java">

class Generic <T extends UpperBound> {

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}

}

</syntaxhighlight>

Sample code that uses the <code>Generic<T '''extends''' UpperBound></code> class:

<syntaxhighlight lang="java">

...

final Generic<UpperBound> concreteTypeReference = new Generic<UpperBound>();

final Generic<?> wildcardReference = concreteTypeReference;

final UpperBound ub = wildcardReference.read(); // Object would also be OK

wildcardReference.write(new Object()); // type error

wildcardReference.write(new UpperBound()); // type error

concreteTypeReference.write(new UpperBound()); // OK

...

</syntaxhighlight>

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<syntaxhighlight lang="java">

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

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

// do something

}

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<syntaxhighlight lang="java">

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

final MyClass m = new MyClass();

list.add(m); // Compile error

}

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<syntaxhighlight lang="java">

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

final MyClass m = new MyClass();

list.add(m); // OK

}

Line 90 ⟶ 96:

<syntaxhighlight lang="java">

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

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

// do something

}