Wildcards The Java Tutorials > Bonus > Generics

Neil said…Generic’s Bounded wildcards if used as method arguments definitely increase flexibility of code. Formal parameters which accept Bounded argument are like coding for interface than implementation. We have a method https://forexaggregator.com/ that prints a list of cards. This method takes in an unknown type whose supertype is Integer. If the call printTitles is valid, so is the call printTitlesAndAddMagazine, since the two methods have the same signature.

Super T specifies a type lower bound — the type must be a supertype of T. Java does not allow the specification of both an upper bound and a lower bound (though some languages do, e.g., Scala). Method printTitlesAndAddMagazine must use List exactly in its signature. For this reason, wildcard type instantiations are valid types for referencing an object, but they cannot be used as the type to create an instance of an object. In general, you cannot use a wildcard type with the new keyword to allocate an object instance because the wildcard denotes one or a possible set of objects. Since there are two kinds of bounded wildcards in generics, super and extends, When should you use the super wildcard and when should you extend wildcards.

wildcard java

In Java, this takes the form of so-called generics, introduced in Java 5. Lower bound wildcard − If a variable is of out category, use super keyword with wildcard. Upper bound wildcard − If a variable is of in category, use extends keyword with wildcard. Generics in Java are invariant by default, for a good reason of staying type- safe.


The arguments which are declared like this can hold any type of objects. For example, Collection or ArrayList can hold any type of objects like String, Integer, Double etc. Will accept only MyObject or children of MyObject(i.e. any object of type OurObject or YourObject or MyObject, but not any object of superclass of MyObject). This is as flexible as the Java method with type bounds because, in Scala, List is covariant and functions are covariant in their return type. Here, the type of the list elements becomes a type argument of the method, named A .

wildcard java

But bear with me and really try to imagine an upside down world where GenericType is a subtype of GenericType. On the other hand, bounded wildcards provide limited flexibility within bound. Any Type with bounded wildcards can only be instantiated within bound and any instantiation outside bound will result in a compiler error.

degenerate sense. The elements of List are actually all of some unknown

Can hold any parameterization of Generic whose any type argument is both a subtype of the corresponding type parameter’s upper bound and a supertype of SubtypeOfUpperBound. This reference can hold any parameterization of Generic whose type argument is a subtype of SubtypeOfUpperBound. Bounded wildcards are just what one needs to handle the example of the DMV passing its data to the census bureau. Our example assumes that the data is represented by mapping from names to people .

In the code above, the list variable can store objects of any type. If we want to print the name of all the cats in a list, we really don’t need a Consumer. A Consumer will do, since the name of the animal is kept in the base class. We have a method that prints the list of cards. The only information that we know when we create the method is that ? Can be replaced with a subclass of the Object class.

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It has to be a type that extends the java.lang package for it to work, otherwise, you’ll get a compilation error. It would seem natural to use method printTitles to print the titles of the books in the library, since books are publications. Out variable − An out variable holds data updated by the code.

As we hinted in the example, it may help to read ? We’ll show a good example of this when we talk about generic methods later. For now, just try to digest this as complementary to upper bounds. In the Java programming language, the wildcard ? Is a special kind of type argument that controls the type safety of the use of generic types.

But the latter would add a magazine to a list of books! Since Magazine is not a subtype of Book, this would break type safety. Adding a magazine is a service that is supported by List but not by List.

  • The “worst” that can happen is that our base list of type List points at List, but that is no problem because we can add Cats, or Animals to List.
  • Since we don’t know what type that is, we cannot pass anything in.
  • But the latter would add a magazine to a list of books!
  • However, if we pass a list of types Double then we will get a compilation error.
  • The whole point of variance is about what happens when we deal with complex types .

Whoever creates the consumer or supplier, can decide the specific implementation of the InputStream interface they are writing it for . HOWEVER, when somebody uses their work , he/she has the same freedom. He/She can decide to use a specific supplier and a specific How To Become a Blockchain Developer A Comprehensive Step-by-Step Guide consumer, or a specific supplier and a generic consumer, or a generic supplier and a generic consumer… you get my drift. In all those cases, type-safety is preserved. B) We want to add elements of a specific type but don’t want to read the elements.

It can be used in variable declarations and instantiations as well as in method definitions, but not in the definition of a generic type. This is a form of use-site variance annotation, in contrast with the definition-site variance annotations found in C# and Scala. Bounded wildcards in generics also increase the flexibility of any API. To me, it’s a question of requirement, if a method also needs to accept any implementation of T then use extends wildcards.

List can be read and

We have a method that takes in a list of unknown types. That is, the type is unknown but a „bound” can be placed on it. In this case, it is bounded by some class, which is a subclass of B. But in this case, we’re telling the compiler that this is OK.

wildcard java

A much more useful and natural example of contravariance is the Consumer interface. A) We want to read/delete/sort the elements, but not add any new ones, in which case we need to use a base list pointer of type List. This is explored in detail in the previous post. This means that the worst case scenario, is that this list contains Animals. Note how we can now be sure that we are getting an animal, but we cannot be sure if it is an instance of the Animal.class, the Cat.class or the Dog.class.

Java Generics – Wildcards

The wildcard can be used in a variety of situations such as the type of a parameter, field, or local variable; sometimes as a return type. Unlike arrays, different instantiations of a generic type are not compatible with each other, not even explicitly. This incompatibility may be softened by the wildcard if ? We are reminded in this example that List is not a List; polymorphism doesn’t flow that way with generic instantiations of concrete types. But List, the unbounded wildcard instantiation, can be assigned any instantiation of List.

A bounded wildcard places a restriction on the type by saying that it either has to extend a specific type , or has to be an ancestor of a specific type . But a List is actually a list that holds concrete Object types. The List can only be read and only read as Object in a degenerate sense. The elements of List are actually all of some unknown type. The elements of the unknown type list all have a common supertype that could be Object or some other common type that is more restrictive than Object.

He started programming with Java in the time of Java 1.4 and has been falling in love with Java since then. Make friend with him on Facebook and watch his Java videos you YouTube. How to compare two lists of values in Microsoft Ex… There are a lot of other things you might have told about wildcards. Right now, this tutorial does not have any value. It provides the highest level of flexibility on passing method argument.

List is the type of a list in which the elements have an unspecified type that is a subtype of Publication . In particular, List is a subtype of List and the call printTitles is now valid. Since an element p of the list must be of a type that is a subtype of Publication, it also has type Publication and thus a title method. Ver wondered what the syntax or in Java was for? It will also briefly discuss alternatives in languages that support covariant and contravariant types.

Even though logically, Consumer SHOULD BE a base class of Consumer, they are invariant. We can generalize the code for adding Cats to any roster by using List . With Object because Object is the parent of all classes, Integer included. In the end, we populate the list and print out the result. Asking for help, clarification, or responding to other answers.