Wildcards The Java Tutorials > Bonus > Generics

Also, I think you should ask another question on this “what’s the difference between List and List” where you will hopefully get more answers. Josh Bloch also has a good explanation of when to use super and extends in this google io video talk where he mentions the Producer extends Consumer super mnemonic. This method can be used on a List where BookTask is a subtype of Callable and produce a List. Method addMagazine can be called on a List or on a List, or even on a List. Even though Book is a subtype of Publication, List is not a subtype of List. On this Wikipedia the language links are at the top of the page across from the article title.

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.

  • This is the same as to say that a wildcard is bounded, and it means that ?
  • To play by the rules of generics and guarantee that we don’t do anything unsafe.
  • I’ve read Bloch’s book, but I still can’t see the difference between extends and super in this particular case.
  • Generics in Java are invariant by default, for a good reason of staying type- safe.

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.

type. The elements of the unknown type list all have a common supertype

Little does this cat know, that this box is a contravariant cardboard cat box. Well, that kindda depends on the language implementation. Some things that are a super/sub class in theory, may not share that inheritance relationship in a specific language.

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.

For instance, arrays in Scala are non-variant, and there is not relationship between Array and Array. For instance, in Scala, functions are covariant in their output type and 5 Brilliant Benefits of Freelance Life contravariant in their input type. In the above program, list1 and list2 are objects of the List class. List1 is a collection of Integer and list2 is a collection of Double.

wildcard java

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 https://topbitcoinnews.org/ 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 .

Java Generics – Guidelines for Wildcard Use

He’s worked as a developer in diverse areas including Statistical Analytics, Static Analysis, Compilers and Network Protocols. He is a leader in the London Java Community. Richard is also a known conference speaker and has presented at Devoxx, JavaOne, QCon SF, JFokus, Devoxx UK, Geecon, Oredev, JAX London, JEEConf and Codemotion. He obtained a PhD in Computer Science from The University of Warwick.

wildcard java

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 .

A lower bound wildcard makes use of the super keyword. In Java, we use “super” to relate to parent class objects. A wildcard that uses the super keyword means that it can accept any type of super type. For example, the super type Integer can be Number and Object. Their use is often necessary in Java because the language has no support for type variance. Other, more modern languages offer additional features, like variance annotations or combined lower/upper bounds.

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Upper bounds are expressed using the extends keyword and lower bounds using the super keyword. Wildcards can state either an upper bound or a lower bound, but not both. The wildcard in Java can be downright confusing.

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.

asking for a type with elements that can hold any type safely and the

For example, new Generic is correct, while new Generic is not. A bounded wildcard is one with either an upper or a lower inheritance constraint. The bound of a wildcard can be either a class type, interface type, array type, or type variable.

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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.

Are written using bounded wildcards which allow them to operate on either Collection of T or Collection of subclass or superclass of T. The wildcard is never used as a type argument for a generic method invocation, a generic class instance creation, or a supertype. Types that need both usually cannot be covariant or contravariant; they have to remain invariant (or non-variant), like the collections in Java.

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.