Overview
Type wildcard is the most tricky part of Java Generics. The readers are assumed having the basic knowledge of Java Generics. For a quick review of the basics of Java Generics, see my other post,
The Basics of Java Generics.
Type wildcard is only used in
parameterized types, as the
type arguments. A
type wildcard is never used in a definition of
generic type or
generic method . In practice, such
parameterized types are commonly used in method definitions, serving as the type of the formal parameters of the methods.
A
parameterized type may take one of four forms:
- List<String>, here String is a concrete type, serving as the type argument for the parameterized type List<String>
- List<?>, where ? is a type wildcard without a bound, serving as the type argument for the parameterized type List<?>
- List<? extends Number>, where ? is a type wildcard with a upper bound, serving as the type argument for the parameterized type List<? extends Number>, and Number is the upper bound of the wildcard.
- List<? supper Number>, where ? is a type wildcard with a lower bound, serving as the type argument for the parameterized type List<? super Number>, and Number is the lower bound of the wildcard.
For a
parameterized type with a
type wildcard as a
type argument, the wildcard may have either a single upper or lower bound, but not both.
In a World without Type Wildcard
To understand why Java introduces the
type wildcard for parameterized type, let’s look at an example. This example is about a circus. In the circus, there are two kinds of animals, birds and dogs. Brids can fly. Dogs can bark. A special kind of birds, nightingares, can also sing. Class hierarchy of animals used in this example are shown in Figure 1. Code for all kinds of animals are shown in Listing 1-1 to 1-4.
 |
| Figure 1 - Animal Hierarchy |
Listing 1-1 - Animal
package animals;
public class Animal {
private String name;
public Animal(String name) {
this.name = name;
}
public void jump() {
System.out.println(getName() + " is jumping.");
}
public String getName() {
return name;
}
}
Listing 1-2 -Brid
package animals;
public class Bird extends Animal {
public Bird(String name) {
super(name);
}
public void fly() {
System.out.println(getName() + " is flying.");
}
}
Listing 1-3 - Nightingale
package animals;
public class Nightingale extends Bird {
public Nightingale(String name) {
super(name);
}
public void sing() {
System.out.println(getName() + " is singing.");
}
}
Listing 1-4 - Dog
package animals;
public class Dog extends Animal {
public Dog(String name) {
super(name);
}
public void bark() {
System.out.println(getName() + " is barking.");
}
}
There is a kind of actors in the circus called
AnimalTrainer. When given a collection of animals, the
AnimalTrainer commands them to jump. The first version of
AnimalTrainer is shown in Listing 2-1.
Listing 2-1 - AnimalTrainer Version 1 - Without Type Wildcard
package nowildcard;
import java.util.List;
import animals.Animal;
public class AnimalTrainer {
public void act(List<Animal> animalList) {
for (Animal animal : animalList) {
animal.jump();
}
}
}
The first version of the main class,
Circus, is shown in Listing 2-2. When the Circus program is launched, it will at first create an
AnimalTrainer. Then a list of Animals is created, and two animals are added into the list. It finally invokes the act method, passing the animal list, on the
AnimalTrainer. The
AnimalTrainer acts by command every animal to jump. So far so good.
Listing 2-2 - Circus Version 1
package nowildcard;
import java.util.ArrayList;
import java.util.List;
import animals.Animal;
import animals.Bird;
import animals.Dog;
public class Circus {
public static void main(String[] args) {
AnimalTrainer animalTrainer = new AnimalTrainer();
List<Animal> animalList = new ArrayList<Animal>();
animalList.add(new Dog("Bob"));
animalList.add(new Dog("Amy"));
animalTrainer.act(animalList);
}
}
The second version of the main class,
Circus, is shown in Listing 2-3. The
AnimalTrainer commands a list of birds to jump. We reason that it will be fine since a bird is actually an animal and can jump. This version of the
Circus class, however, does not compile. The line with trouble is
animalTrainer.act(birdList); (Line 15) The error message is:
The method act(List<Animal>) in the type AnimalTrainer is not applicable for the arguments (List<Bird>)
Listing 2-3 - Circus Version 2
package nowildcard;
import java.util.ArrayList;
import java.util.List;
import animals.Bird;
public class Circus {
public static void main(String[] args) {
AnimalTrainer animalTrainer = new AnimalTrainer();
List<Bird> birdList = new ArrayList<Bird>();
birdList.add(new Bird("Tim"));
birdList.add(new Bird("Nancy"));
animalTrainer.act(birdList);
}
}
In fact, Java does not regard
List<Bird> as a subtype of
List<Animal>. So the
act method only accepts
List<Animal> as argument, rejecting
List<Bird>. You might wonder why Java has such an “unreasonable” rule. Actually, this rule exists for a very good reason. Let’s see why.
What if List<Bird> Were Regarded as a Subtype of List<Animal>?
If Java regarded
List<Bird> as a subtype of
List<Animal>, we were going to have trouble. To show the trouble, let’s update the
Circus class as in Listing 3-3. At first lets look at the two new classes newly added,
Magician as in Listing 3-1, and
BirdTrainer as in Listing 3-2. The
act method of
Magician takes a list of
Animals, and replaces all
Animals in the list by two
Dogs. The
Magician class compiles perfectly. The
act method of BirdTrainer takes a list of
Birds and commands each of them to fly. The
BirdTrainer class as well compiles perfectly. The
Circus class (Version 3, Listing 3-3) does not compile for the same reason as in the Version 2 (Listing 2-3). Line 16 fails. The error message is:
The method act(List<Animal>) in the type Magician is not applicable for the arguments (List<Bird>)
The Java compiler rejects Line 16 for a good reason. If Java regarded
List<Bird> as a subtype of
List<Animal> and allowed Line 16 compile, we were going to have run time exception because at run time, when the
BirdTrainer got a list, the list actually would contain two
Dogs. The
Dogs would be commanded to fly but they cannot (a
Dog object does not have a
fly method). In other words, the Java type integrity would be broken.
To prevent trouble of this nature, Java does not regard
List<Bird> as a subtype of
List<Animal>. In a generic term of generics, it is said that the type parameter
T in
List<T> is
non-variant.
Listing 3-1 - Magician Version 1
package whatif;
import java.util.List;
import animals.Animal;
import animals.Dog;
public class Magician {
public void act(List<Animal> animalList) {
for (int i = 0; i < animalList.size(); i++) {
animalList.remove(i);
}
animalList.add(new Dog("Green"));
animalList.add(new Dog("Red"));
}
}
Listing 3-2 - BirdTrainer Version 1
package whatif;
import java.util.List;
import animals.Bird;
public class BirdTrainer {
public void act(List<Bird> birdList) {
for (Bird bird : birdList) {
bird.fly();
}
}
}
Listing 3-3 - Circus Version 3
package whatif;
import java.util.ArrayList;
import java.util.List;
import animals.Bird;
public class Circus {
public static void main(String[] args) {
BirdTrainer birdTrainer = new BirdTrainer();
Magician magician = new Magician();
List<Bird> birdList = new ArrayList<Bird>();
birdList.add(new Bird("Tim"));
birdList.add(new Bird("Nancy"));
magician.act(birdList); // this line won't compile.
birdTrainer.act(birdList);
}
}
Type Wildcard with a Upper Bound
Now since we understand why List<Bird> is not regarded as a subtype of List<Animal>, we can come back to seek a solution for our situation: How to enable an AnimalTrainer to accept a List<Bird> as well as a List<Animal>. Java’s solution is type wildcard. Specifically, we can change the AnimalTrainer class to be like in Listing 4-1 (Line 7:
act(List<? extends Animal> animalList)). The
? in Line 7 is called a
type wildcard, and
? extends Animal says
Animal is the
upper bound of the
type wildcard.
With this version of
AnimalTrainer, the main class,
Circus, as in Listing 4-2 (it is essentially the same as in Listing 2-3) compiles and runs successfully.
Listing 4-1 - AnimalTrainer Version 2, With Type Wildcard With Upper Bound
package upperbound;
import java.util.List;
import animals.Animal;
public class AnimalTrainer {
public void act(List<? extends Animal> animalList) {
for (Animal animal : animalList) {
animal.jump();
}
}
}
Listing 4-2 - Circus Version 4
package upperbound;
import java.util.ArrayList;
import java.util.List;
import animals.Bird;
public class Circus {
public static void main(String[] args) {
AnimalTrainer animalTrainer = new AnimalTrainer();
List<Bird> birdList = new ArrayList<Bird>();
birdList.add(new Bird("Tim"));
birdList.add(new Bird("Nancy"));
animalTrainer.act(birdList);
}
}
In general, there are three rules about
type wildcard with a upper bound, given a generic type G (e.g.
List<E>)
, and two concrete types X and Y where Y is a subtype of X (e.g. Bird is a subtype of Animal):
- G<? extends Y> is subtype of G<? extends X> (e.g. List<? extends Bird> is a subtype of List<? extends Animal>
- G<X> is subtype of G<? extends X> (e.g. List<Bird> is a subtype of List<? extends Bird>)
- G<?> is just a shorthand for G<? extends Object> (e.g. List<?> is a shorthand for List<? extends Object>)
(For the formal specification, see 4.5.1.1 Type Argument Containment and Equivalence, The Java Language Specification, Third Edition)
(Those rules have something to do with the idea of covariance in the theory of generics. They however do not align very well with the theory.)
In the context of our example, Bird is a subtype of Animal, so List<? extends Bird> is a subtype of List<? extends Animal>. Meanwhile, List<Bird> is a subtype of List<? extends Bird>. Therefore List<Bird> is a subtype of List<? extends Animal>. That
is the reason why the act method has a formal parameter of the type List<? extends Animal> and can accept List<Bird> and List<Animal> as argument.
You might wonder whether the Magician class can also be updated with type wildcard and bring back the trouble of messing up birds by dogs. It cannot. The catch is that when the act method of the Magician is updated to take a parameter of the type List<? extends Animal>, as in Listing 4-3 (Line 8), it is no longer allowed to call the add method of the List class. The compile error message is "The method add(capture#3-of ? extends Animal) in the type List<capture#3-of ? extends Animal> is not applicable for the arguments (Dog)". Therefore, it cannot add dogs into the list. It fails because the add method here expects an argument of the type “? extends Animal” but Dog is not a subtype of it (Note: even though List<Dog> is a subtype of List<? extends Animal>, Dog is not a subtype of "? Extends Animal". More generally, no class is regard as a subtype of “? Extends Animal”. It is said that “? Extends Animal” is an undefined type (Note: meanwhile, List<? Extends Animal> is a well defined
parameterized type.)
Listing 4-3 Magician Version 2
package upperbound;
import java.util.List;
import animals.Animal;
import animals.Dog;
public class Magician {
public void act(List<? extends Animal> animalList) {
for (int i = 0; i < animalList.size(); i++) {
animalList.remove(i);
}
animalList.add(new Dog("Green")); // now these two lines do not compile
animalList.add(new Dog("Red"));
}
}
Based on my experience, the general rule is like this: When a method with a parameter of a
parameterized type that is one with
type wildcard with a upper bound (e.g.
act(List<? extends Animal>)), inside the body of the method, it is not allowed to call any method on the parameter object unless the method is parameterless (e.g.
add(E element) on
List<E>). I tried to find something about it in the Java Language Specification but found nothing. However, it seems that all Java compilers that I saw are implemented in this way. I could not figure out how to derive this rule from other more basic rules in the Java Specification.
Type Wildcard with a Lower Bound
Similar to
type wildcard with a upper bound, a
type wildcard may have a
lower bound. (However, a
type wildcard cannot have both upper and lower bound, nor have more than one upper or lower bounds). The syntax is like this:
List<? super Bird> where Bird is the
lower bound of the
type wildcard.
In general, there are also three rules about
type wildcard with a lower bound, given a
generic type G (e.g. List<E>), and two
concrete types X and Y where X is a subtype of Y (e.g. Bird is a subtype of Animal):
- G<? super Y> is a subtype of G<? super X> (e.g. List<? super Animal> is a subtype of List<? super Bird>
- G<X> is subtype of G<? super X> (e.g. List<Bird> is a subtype of List<? super Bird>)
(Again, for the formal specification, see 4.5.1.1 Type Argument Containment and Equivalence, The Java Language Specification, Third Edition)
(Those rules have something to do with the idea of contra-variance in the theory of generics. They however do not align very well with the theory.)
Let's illustrate the uses of
type wildcard with a lower bound with another version of our Circus program as in Listing 5-1. In this version of the
Circus class, an
animalList (of type
List<Animal>) and a
birdList (of type
List<Bird>) are created in the main method. Instead of adding animals or birds to the lists in the main method, the
animalList is passed to the
addBirds method of a
BirdKeeper object, and the
addDogs method of a
DogKeeper object, to add birds and dogs into it. Then an
AnimalTrainer acts on the
animalList by commanding the animals (birds and dogs) in the list to jump. Also, the
birdList is passed to the
addBirds method of the
BirdKeeper object to add birds to it. And a
BirdTrainer acts on the list by commanding the birds in the
birdList to fly. The pertinent version of
AnimalTrainer and
BirdTrainer are shown in Listing 5-2 and 5-3.
Notice the following points in the above example. For the
animalList to contain both birds and dogs, it must be defined as of type
List<Animal>, instead of
List<Bird> or
List<Dog>. For the
BirdTrainer to command birds in the
birdList to fly, the
birdList must be defined as of
type List<Bird>, instead of
List<Animal>.
For the program to work, the
addBirds method of the
BirdKeeper class must be able to:
- accept a List<Animal> as argument (Line 21, the Circus class)
- accept a List<Bird> as argument (Line 26, the Circus class)
- add birds into the List<Animal> and the List<Bird> (Line 7-8, the BirdKeeper class)
The solution is shown in Listing 5-4. The type of the formal parameter to the
addBirds method is
List<? super Bird>. The
parameterized type that servers as the type of the formal parameter is a
type wildcard with a lower bound. Other choices won't work. If we had the method header as
public void addBirds(List<Animal> animalList), the method would not accept
List<Bird> as argument; if we If we had the method header as
public void addBirds(List<Bird> animalList), the method would not accept
List<Animal> as argument; If we had the method header as
public void addBirds(List<? extends Animal> animalList), the method would not be able to add anything to the list (i.e.
animalList). A
type wildcard with a lower bound is the necessary solution to this case.
Notice that it is allowed to call a method with parameters (e.g the add method of List<E>) on an object referenced by a formal parameter of type wildcard with a lower bound (e.g. the parameter named animalList in the addBirds method of the BirdKeeper class), while it is not allowed if the wildcard has a upper bound.
Similarly, the DogKeeper class is shown in Listing 5-5.
Listing 5-1 - Circus Version 5
package lowerbound;
import java.util.ArrayList;
import java.util.List;
import animals.Animal;
import animals.Bird;
public class Circus {
public static void main(String[] args) {
AnimalTrainer animalTrainer = new AnimalTrainer();
BirdTrainer birdTrainer = new BirdTrainer();
List<Animal> animalList = new ArrayList<Animal>();
BirdKeeper birdKeeper = new BirdKeeper();
birdKeeper.addBirds(animalList);
DogKeeper dogKeeper = new DogKeeper();
dogKeeper.addDogs(animalList);
animalTrainer.act(animalList);
List<Bird> birdList = new ArrayList<Bird>();
birdKeeper.addBirds(birdList);
birdTrainer.act(birdList);
}
}
Listing 5-2 - AnimalTrainer Version 3
package lowerbound;
import java.util.List;
import animals.Animal;
public class AnimalTrainer {
public void act(List<Animal> animalList) {
for (Animal animal : animalList) {
animal.jump();
}
}
}
Listing 5-3 - BirdTrainer Version 2
package lowerbound;
import java.util.List;
import animals.Bird;
public class BirdTrainer {
public void act(List<Bird> birdList) {
for (Bird bird : birdList) {
bird.fly();
}
}
}
Listing 5-4 - BirdKeeper
package lowerbound;
import java.util.List;
import animals.Bird;
public class BirdKeeper {
public void addBirds(List<? super Bird> animalList) {
animalList.add(new Bird("Tim"));
animalList.add(new Bird("Nancy"));
}
}
Listing 5-5 - DogKeeper
package lowerbound;
import java.util.List;
import animals.Dog;
public class DogKeeper {
public void addDogs(List<? super Dog> animalList) {
animalList.add(new Dog("Bob"));
animalList.add(new Dog("Amy"));
}
}
Finally, notice that the
addBirds method of the
BirdKeeper class won't accept an argument of the type
List<Nightingale> because
List<Nightingale> is not regarded as a subtype of
List<? super Bird>. The version of the
Circus class as shown in Listing 5-5 won't compile. Line 30,
birdKeeper.addBirds(nightingaleList);, will fail. The compile error is:
The method addBirds(List<? super Bird>) in the type BirdKeeper is not applicable for the arguments (List<nightingale>)
Listing 5-5 - Circus Version 6
package lowerbound;
import java.util.ArrayList;
import java.util.List;
import animals.Animal;
import animals.Bird;
import animals.Nightingale;
public class Circus {
public static void main(String[] args) {
AnimalTrainer animalTrainer = new AnimalTrainer();
BirdTrainer birdTrainer = new BirdTrainer();
List<Animal> animalList = new ArrayList<Animal>();
BirdKeeper birdKeeper = new BirdKeeper();
birdKeeper.addBirds(animalList);
DogKeeper dogKeeper = new DogKeeper();
dogKeeper.addDogs(animalList);
animalTrainer.act(animalList);
List<Bird> birdList = new ArrayList<Bird>();
birdKeeper.addBirds(birdList);
birdTrainer.act(birdList);
List<Nightingale> nightingaleList = new ArrayList<Nightingale>();
birdKeeper.addBirds(nightingaleList); // does not compile
}
}
Conclusion
- Type wildcard is used to increase flexibility of methods that take parameters of parameterized types
- It is not allowed to call a method on an object referenced by a parameter of a parameterized type with type wildcard with a upper bound unless the method is parameter less.
- It is allowed to call any method on an object referenced by a parameter of a parameterized type with type wildcard with a lower bound.
References
- The Java Language Specification, Third Edition, by James Gosling, Bill Joy, Guy Steele, and Gilad Bracha, Addison Wesley Professional, 2005
- The Java Programming Language, 4th Edition, by Ken Arnold, James Gosling, and David Holmes, Prentice Hall PTR, 2005
