Wildcard (Java): Difference between revisions

Revision as of 20:47, 14 October 2018

Revision as of 11:14, 16 December 2019

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<source lang="java5">

class Generic <T extends UpperBound> {

private T t;

void write(T t) {

this.t = t;

}

}

T read() {

return t;

}

}

}

...

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</source>

== Bounded Wildcardswildcards ==

A bounded wildcard is one with either an upper or a lower [[Inheritance (object-oriented programming)|inheritance]] constraint. The bounds can be both class and interface types. Upper bounds are expressed using the '''extends''' keyword and lower bounds using the '''super''' keyword. An upper bound on a wildcard must be a subtype of the upper bound on the type parameter it is assigned.

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can hold instantiations of <code>Generic</code> with any type that is both a subtype of <code>UpperBound</code> and a supertype of <code>SubtypeOfUpperBound</code>. The type bounds are trivial examples that conform.

== Object Creationcreation with Wildcardwildcard ==

No objects may be created with a wildcard type parameter: <code>'''new''' Generic<?>()</code> is forbidden because <code>Generic<?></code> is abstract.<ref>{{citation|title=The Java Language Specification|section=Class Instance Creation Expressions|url=http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.9|publisher=Oracle}}</ref> In practice, this is unnecessary because if one wanted to create an object that was assignable to a variable of type <code>Generic<?></code>, one could simply use any arbitrary type (that falls within the constraints of the wildcard, if any) as the type parameter.

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For both cases, using no parameters is another option. This will generate a warning since it is less type-safe (see [[Raw type]]).

== Example: Listslists ==

In the Java Collections Framework, the class <code>List<MyClass></code> represents an ordered collection of objects of type <code>MyClass</code>.

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<source lang="java5">

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

for (MyClass object : list) { // OK

// do something

}

}

</source>

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<source lang="java5">

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

MyClass m = new MyClass();

list.add(m); // Compile error

}

</source>

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<source lang="java5">

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

MyClass m = new MyClass();

list.add(m); // OK

}

</source>

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<source lang="java5">

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

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

// do something

}

}

</source>