I know that there are a lot of questions about this topic, but unfortunately they couldn't help me to eliminate my obscurities. First of all, look at the following example. I don't understand, why the following "add"-method someCage.add(rat1) doesn't work and aborts with the following exception:
Exception in thread "main" java.lang.Error: Unresolved compilation
problem: The method add(capture#2-of ? extends Animal) in the type
Cage is not applicable for the
arguments (Rat)
Is this the same reason why Cage<Rat> is not a Cage<Animal>? If yes, I don't understand it in this example, so I'm not sure what the compiler exactly does. Here is the code example:
package exe;
import cage.Cage;
import animals.Animal;
import animals.Ape;
import animals.Lion;
import animals.Rat;
public class Main {
public static void main(String[] args) {
Lion lion1 = new Lion(true, 4, "Lion King", 8);
Lion lion2 = new Lion(true, 4, "King of Animals", 9);
Ape ape1 = new Ape(true, 2, "Gimpanse", true);
Ape ape2 = new Ape(true, 2, "Orang Utan", true);
Rat rat1 = new Rat(true, 4, "RatBoy", true);
Rat rat2 = new Rat(true, 4, "RatGirl", true);
Rat rat3 = new Rat(true, 4, "RatChild", true);
Cage<Animal> animalCage = new Cage<Animal>();
animalCage.add(rat2);
animalCage.add(lion2);
Cage<Rat> ratCage = new Cage<Rat>();
ratCage.add(rat3);
ratCage.add(rat1);
ratCage.add(rat2);
// ratCage.add(lion1); //Not Possible. A Lion is no rat
Cage<Lion> lionCage = new Cage<Lion>();
lionCage.add(lion2);
lionCage.add(lion1);
Cage<? extends Animal> someCage = new Cage<Animal>(); //? = "unknown type that is a subtype of Animal, possibly Animal itself"
someCage = ratCage; //OK
// someCage = animalCage; //OK
someCage.add(rat1); //Not Possible, but why?
animalCage.showAnimals();
System.out.println("\nRatCage........");
ratCage.showAnimals();
System.out.println("\nLionCage........");
lionCage.showAnimals();
System.out.println("\nSomeCage........");
someCage.showAnimals();
}
}
This is the cage class:
package cage;
import java.util.HashSet;
import java.util.Set;
import animals.Animal;
public class Cage<T extends Animal> { //A cage for some types of animals
private Set<T> cage = new HashSet<T>();
public void add(T animal) {
cage.add(animal);
}
public void showAnimals() {
for (T animal : cage) {
System.out.println(animal.getName());
}
}
}
Moreover, I would be pleased if you could give me a meaningful "super" example with this animal-cage-code. Until now I haven't understood how to use it. There are a lot of theoretical examples and I read about the PECS concept but anyhow I wasn't able to employ it in a meaningful matter yet. What would it mean to have a "consumer" (with super) in this example?
Example of super bound
The introduced transferTo() method accepts Cage<? super T> - a Cage that holds a superclass of T. Because T is an instanceof its superclass, it's OK to put a T in a Cage<? super T>.
public static class Cage<T extends Animal> {
private Set<T> pen = new HashSet<T>();
public void add(T animal) {
pen.add(animal);
}
/* It's OK to put subclasses into a cage of super class */
public void transferTo(Cage<? super T> cage) {
cage.pen.addAll(this.pen);
}
public void showAnimals() {
System.out.println(pen);
}
}
Now let's see <? super T> in action:
public static class Animal {
public String toString() {
return getClass().getSimpleName();
}
}
public static class Rat extends Animal {}
public static class Lion extends Animal {}
public static class Cage<T extends Animal> { /* above */ }
public static void main(String[] args) {
Cage<Animal> animals = new Cage<Animal>();
Cage<Lion> lions = new Cage<Lion>();
animals.add(new Rat()); // OK to put a Rat into a Cage<Animal>
lions.add(new Lion());
lions.transferTo(animals); // invoke the super generic method
animals.showAnimals();
}
Output:
[Rat, Lion]
Another important concept is that while it is true that:
Lion instanceof Animal // true
it is not true that
Cage<Lion> instanceof Cage<Animal> // false
It this were not the case, this code would compile:
Cage<Animal> animals;
Cage<Lion> lions;
animals = lions; // This assignment is not allowed
animals.add(rat); // If this executed, we'd have a Rat in a Cage<Lion>
You can add a Rat to a Cage<Rat> (of course).
You can add a Rat to a Cage<Animal>, because a Rat "is" an Animal (extends Animal).
You cannot add a Rat to a Cage<? extends Animal>, because <? extends Animal> might be <Lion>, which a Rat is not.
In other words:
Cage<? extends Animal> cageA = new Cage<Lion>(); //perfectly correct, but:
cageA.add(new Rat()); // is not, the cage is not guaranteed to be an Animal or Rat cage.
// It might as well be a lion cage (as it is).
// This is the same example as in Kaj's answer, but the reason is not
// that a concrete Cage<Lion> is assigned. This is something, the
// compiler might not know at compile time. It is just that
// <? extends Animal> cannot guarantee that it is a Cage<Rat> and
// NOT a Cage<Lion>
//You cannot:
Cage<Animal> cageB = new Cage<Rat>(); //because a "rat cage" is not an "animal cage".
//This is where java generics depart from reality.
//But you can:
Cage<Animal> cageC = new Cage<Animal>();
cageC.add(new Rat()); // Because a Rat is an animal.
Imagine having your Cage<? extends Animal> created by an abstract factory method, which gets implemented by a subclass. In your abstract base class you cannot tell which type actually gets assigned, neither can the compiler, because maybe the concrete class gets only loaded at runtime.
That means, the compiler cannot rely on Cage<? extends Animal> to not be a Cage of some other concrete subtype, which would make the assignment of a different subtype an error.
Both answers so far have been great. I'd just like to add a tidbit to help your understanding of them.
To further Ron's answer, you may be thinking the following:
"why is it that that someCage.add(rat1) becomes a Cage<? extends Animal>.add(rat1)? Can't someCage point to any Cage of any type which extends Animal (and I've now set it to point to a cage of rats?)"
Totally legitimate question. Thing is, when you do the someCage = ratCage, an element-by-element copy is done from ratCage into someCage. So in fact, you have not simply set someCage to now point to a ratCage. In actuality, someCage is still a Cage<? extends Animal>. You can't do someCage.add(rat1) because you don't know the type of the Cage, only that it's type is bounded above by Animal.
P.S.: You can't add anything to someCage since its type is unknown
I think your question can be answered by the following snippet of code:
Cage<? extends Animal> cage = new Cage<Lion>();
cage.add(rat1);
You can clearly see that the code above shouldn't be valid, since you know that cage currently is a lion cage, and you shouldn't be allowed to add a rat to a lion cage.
The compiler does not what value you have assigned to the cage, so it can't allow cage.add(rat1) even if you assign a rat cage to the cage.
Related
This question already has answers here:
What is PECS (Producer Extends Consumer Super)?
(16 answers)
Closed 12 months ago.
Had seen many examples related to use of super wildcard. Majority of them are with Number and Integer classes. However for my understanding I was trying the below code:
package util;
import java.util.*;
class Animal{
void eat() {
System.out.println("animal eats");
}
}
class Dog extends Animal{
void eat() {
System.out.println("dog eats");
}
}
class Cat extends Animal{
void eat() {
System.out.println("cat eats");
}
}
public class Test {
public void addAnimal(List<? super Dog> list) {
list.add(new Animal());//******* getting error here
list.add(new Dog());
}
public static void main(String[] args) {
List<? super Dog> lsDogs = new ArrayList<Dog>();
List<? super Dog> lsAnimals = new ArrayList<Animal>();
}
}
as per the docs which I have understood super means we can add anything that is on right hand side or its super class. Here I created Dog class which extends Animal class. I can only add Dog type of objects and not Animal. Why is it not possible.. any specific reasons
List<? super Dog> says "this is a List that you can add a Dog to. You might be able to add a Dog because it is a List<Dog>, because it's a List<Animal>, or because it's a List<Object>."
You can't add an Animal to a List<? super Dog>, because your Animal could be any Animal subclass, e.g. a Cat, and then you may have a Cat in a List<Dog>.
class Animal { void sound() {}}
class Cat extends Animal {}
class Dog extends Animal {}
class Test {
public static void main(String[] args) {
List<Object> objects = new ArrayList<>();
List<Animal> animals = new ArrayList<>();
List<Dog> dogs = new ArrayList<>();
List<Cat> cats = new ArrayList<>();
add(objects);
add(animals);
add(dogs);
add(cats); // Error, must be a list we can add Dogs to
addAnimal(objects);
addAnimal(animals);
addAnimal(dogs); // Error, must be a list which can contain any Animal
useList(objects); // Error, must be a List of Animals
useList(animals);
useList(dogs);
useList(cats);
}
public static void add(List<? super Dog> list) {
list.add(new Dog());
}
public static void addAnimal(List<? super Animal> list) {
list.add(new Dog());
list.add(new Cat());
for (Animal a : list) { // error, ? can be Object
a.sound();
}
}
// If we're consuming items from the list we use extends
public static void useList(List<? extends Animal> list) {
list.add(new Animal()); // error, ? is a specific but unknown subclass of Animal
for (Animal a : list) {
a.sound();
}
}
}
The ? means that we don't know what the class is but it is a definite class.
? super of Dog means that ? is something that is a parent of dog. For your example you have { Object, Animal, Dog }. So your ? can be any of those 3 classes.
You can add to the list as long as what every you add can be cast to ?.
Can you add a Dog to the list? Yes because Dog can be cast to any Object, Animal, Dog.
Can you add an Animal to the list? No, because Animal cannot be cast to Dog.
Since Dog is the most specified it is the boundary that we compare against. You can add any Dog or class that extends Dog.
I am trying to create a generic method for sub classes lists which add a new element no matter which of the sub class I choose.
for that matter I made an example that will be easy to understand.
There is A zoo container of giraffes and zebras lists. zebra and giraffe are both Animals. I needed to create a'mate' method that will be able to get List of homo gene type meaning list of giraffe or zebra(but not both), and the mate method will add another animal of the type to the existing list (without copying) if there are more than 2 animals in the list.
so to solve this I thought to used reflection since I can not initiate a generic type in java(it is not allowed in java- new T() is not allowed).
so I created an instance of the first element, and tried to add it to the list with animals.add(newAnimal);.. however the compiler complains about it with the following error:
Error: java: no suitable method found for add(Animal)
method java.util.Collection.add(capture#1 of ? extends Animal) is not applicable
(argument mismatch; Animal cannot be converted to capture#1 of ? extends Animal)
method java.util.List.add(capture#1 of ? extends Animal) is not applicable
(argument mismatch; Animal cannot be converted to capture#1 of ? extends Animal)
I solve it by getting the add method at run-time using again, reflection, and it is working(the line in comment), however I would like to know why the compiler does not allow me to use add for animal, because I could not find the answer myself.
The code:
class Animal implements Comparable<Animal>{
int numLegs;
#Override
public int compareTo(Animal o) {
return this.numLegs-o.numLegs;
}
}
class Giraffe extends Animal{
int neckLength;
}
class Zebra extends Animal{
int numOfStripes;
}
class Zoo{
List<Giraffe> giraffes=new ArrayList<Giraffe>();
List<Zebra> zebras=new ArrayList<Zebra>();
public void printMostAnimals(){
for(Animal a: getMostAnimals()){
System.out.println(a);
}
}
private List<? extends Animal> getMostAnimals() {
if(giraffes.size()>zebras.size())
return giraffes;
return zebras;
}
public void mate(List<? extends Animal>animals) throws IllegalAccessException, InstantiationException {
if(animals.size()>2){
Class<?> firstAnimalInstanceClass=animals.get(0).getClass();
Animal newAnimal=(Animal)firstAnimalInstanceClass.newInstance();
animals.add(newAnimal);
// animals.getClass().getDeclaredMethod("add",Object.class).invoke(animals,newAnimal);
System.out.println("new mate was added");
return;
}
System.out.println("no mate been added");
}
}
class App{
public static void main(String[] args) throws InstantiationException, IllegalAccessException {
Zoo z=new Zoo();
z.zebras.add(new Zebra());
z.zebras.add(new Zebra());
z.zebras.add(new Zebra());
z.mate(z.zebras);
System.out.println("new zebra been added?"+(z.zebras.size()>3));
}
}
Thanks,
Because somebody might write:
List<Giraffe> giraffes = new ArrayList<>();
List<? extends Animal> animals = giraffes;
animals.add(new Zebra());
for (Giraffe g : giraffes) { // but there is a Zebra in there!
g.eatLeavesFrom(tallTree); // Zebras can't do that!
}
To have type safety when mating, you could do:
class Animal<M extends Animal<M>> {
M mate;
}
class Giraffe extends Animal<Giraffe> {}
class Zebra extends Animal<Zebra> {}
which allows you to write:
<A extends Animal<A>> void matingSeason(List<A> animals) {
A x = null;
for (A a : animals) {
if (x != null) {
a.mate = x;
x.mate = a;
x = null;
} else {
x = a;
}
}
}
you cannot add anything other than null to List<? extends Animal>.
Ex:
void foo(List<? extends Animal> animals ){
animals.add(new Animal());// compiler error
}
let's assume for a minute it doesn't raise a compiler error and i call foo(new ArrayList<Dog>()), it will add Animal to a List of dogs. clearly that shouldn't be allowed and thus the above code won't compile.
if this didn't raise an error the whole point of generic compile time safety is gone.
You are not allowed to add an Animal "aa" to a List<Giraffe> because "aa" could be a Zebra.
When you declare a List<? extends Animal>, you tell the compiler that it is a a List<Giraffe>, a List<Zebra> or a List<Animal>. Since it could be a List<Giraffe>, we fall in the case above, which means that you cannot add an Animal.
If you remove the generic, it should work, because you tell the compiler that there could be anything in the list:
((List)animals).add(newAnimal);
However you loose all type safety.
Instead why not do it as follows:
class AnimalList<AnimalType extends Animal> extends List<AnimalType> {
void mate() {
Class<AnimalType> firstAnimalInstanceClass = animals.get(0).getClass();
AnimalType newAnimal = (AnimalType)firstAnimalInstanceClass.newInstance();
add(newAnimal);
}
}
In your Zoo, you would declare:
AnimalList<Zebra>
AnimalList<Giraffe>
and then call mate without arguments:
AnimalList<? extends Animal> list;
list.mate();
I will show you two style of declaration of generics. In part1, I'm using generic upper boundary declaration on List as follows:
List<? extends Animal> totList = new ArrayList<Animal>();
But this will throw error like below if you try to add a Animal object to the list:
Exception in thread "main" java.lang.Error: Unresolved compilation problem:
The method add(capture#1-of ? extends Animal) in the type List<capture#1-of ? extends Animal> is not applicable for the arguments (Animal)
at GenericsType.main(GenericsType.java:39)
But as in Part2, if I declare the list inside a generic class in the format below, no errors are thrown while adding (Animal objects) or (subclass of Animal objects) to the list.
class GenericAnimal<T extends Animal>
{
List<T> genList = new ArrayList<T>();
}
Why in part2, it didn't throw error and what is the difference between two style of declaration.
Example Code:
1.Animal.java
public class Animal {
private String name;
private int height;
public void animalJump()
{
if(height>100)
{
System.out.println(name+" with height-"+height+" can JUMP");
}
else
System.out.println(name+" with height-"+height+" cannot jump");
}
public void setName(String name) {
this.name = name;
}
public void setHeight(int height) {
this.height = height;
}
public Animal(String name, int height) {
setName(name);
setHeight(height);
}
}
2.GenericsType.java
import java.util.*;
import Animal;
import Animal.Cannine;
import Animal.Feline;
import Animal.Feline.Cat;
import Animal.Cannine.Dog;
public class GenericsType {
public static List<? extends Animal> totList = new ArrayList<Animal>();
public static void processAllfunc1()
{
for(Animal a : totList)
{
a.animalJump();
}
}
public static void main(String args[])
{
// Part 1
totList.add(new Animal("Animal1",21)); // Error: Unresolved compilation problem:
processAllfunc1();
// Part 2
GenericAnimal<Animal> genericanimal = new GenericAnimal<Animal>();
genericanimal.genList.add(new Animal("Animal2",22)); // No Error, why?
genericanimal.genList.add(new Cat("Cat4",204)); // No Error for Cat also, why?
genericanimal.processAllfunc2();
}
}
3.GenericAnimal.java
public class GenericAnimal<T extends Animal> {
public List<T> genList = new ArrayList<T>();
public void processAllfunc2() {
for (T a : genList) {
a.animalJump();
}
}
}
In part 2, the type of genericanimal.genList is List<T> = List<Animal>. In part 1, the type of the list is List<? extends Animal>.
The issue is that List<? extends Animal> means "a list of some specific subtype of Animal which is unknown." For example, you could write List<? extends Animal> list = new ArrayList<Cat>(). And you shouldn't be able to add any animal to a list of cats. By writing List<? extends Animal>, you're saying that you want to deliberately lose track of which type of animal is allowed into the list, though you know that whatever's in the list is some type of animal.
Why in part1, it threw error while trying to add an Animal object to the list?
List<? extends Animal> totList = new ArrayList<Animal>();
totList.add(new Animal("Animal1",21)); // Error: Unresolved compilation problem:
Because compiler restricts addition of objects on list declared using upper bounded wildcard( ? extends Animal). Compiler doesnt know for sure if the list is typed to the Animal, or Cat or Dog.
Simple, whenever a variable is declared using Upper bounded Wildcard, then inserting elements is not allowed to that variable.
Advantages of using Upper bounded Wildcard is that:
You can create a method that can just read all elements (but no
insertions) in the List.
You can reuse that same method for the list of Animal or list of Dog
or list of Cat.
Also you can safely apply all methods of Animal on elements.
Why in part2, it didn't throw error while adding an Animal object and also Cat object to the list?
public List<T> genList = new ArrayList<T>();
genericanimal.genList.add(new Animal("Animal2",22)); // No Error, why?
genericanimal.genList.add(new Cat("Cat4",204));
Because Here genList is actually of type Animal like in below format
List<Animal> genList = new ArrayList<Animal>();
Now just like normal list of specific type, it can add elements of that type.
So an Animal object can be added to List
Also Cat object is allowed to be added since it is also an Animal object only (Cat is just a subclass of Animal)
Advantages of using <T extends Animal> is that:
You can read all elements of the List.
You can insert elements to the List, but the element must be of type
T.
You can reuse the methods for Animal objects or Cat objects or Dog
objects.
You can safely apply all the Animal methods on elements.
I'm new to generics. Here are my classes.
public interface Animal {
void eat();
}
public class Dog implements Animal {
public void eat() {
System.out.println("Dog eats biscuits");
}
}
public class Cat implements Animal {
public void eat() {
System.out.println("Cat drinks milk");
}
}
Now I want these classes to be used in a generic way.
public class GenericExample {
public <T extends Animal> T method1() {
//I want to return anything that extends Animal from this method
//using generics, how can I do that
}
public <T extends Animal> T method2(T animal) {
//I want to return anything that extends Animal from this method
//using generics, how can I do that
}
public static void main(String[] args) {
Dog dog = method1(); //Returns a Dog
Cat cat = method2(new Cat()); //Returns a Cat
}
}
How can I return the generic type (may be a Cat or Dog) from the methods "method1" and "method2". I've several such methods that returns "T extends Animal", so is it better to declare the generic type in a method level or class level.
You cannot have a method returning a generic type, and expect to be able to access that type in the caller of the method without a cast, unless that type can be deduced from the arguments to the method. So the examples in your main won't work.
So you either go without generics, and either cast the return type manually
public Animal method1()
Dog dog = (Dog)method1()
or have the method return the subclass type to start with.
public Dog method1()
Dog dog = method1()
Or you can go with generics, and either specify the type when calling the method
public <T extends Animal> T method1()
Dog dog = <Dog>method1()
or pass some argument from which the type can be deduced (which the second method already satisfies):
public <T extends Animal> T method1(Class<T> classOfreturnedType)
Dog dog = method1(Dog.class)
Also nituce that you only can call method1 from static main if it is static itself.
The method1 simply says it returns an Animal. Return any instance of Animal, and the code will compile:
return new Dog();
The second method says that it returns an animal which is of the same type than the animal given as argument. So you could return the argument itself, and it would compile. You've said which type the method must return, but you haven't said what the method must do and return, so it's impossible to implement it. All I can say is that return animal; would compile.
I want to write a method that gets an object of the same type as argument, but to declare it in the base class, but I don't want it to accept other object that derived from same base.
For example, I have the class Animal, and I want to write a method sonOf(Animal a), but I don't want someone to do something like that:
Animal bolt = new Dog();
Animal mittens = new Cat();
bolt.sonOf(mittens);
I think a generics is needed, but I'm not sure how to use it correctly.
Thank You.
Hm. You could try, for example:
abstract class Animal<T extends Animal<T>> {
abstract void sonOf(T son);
}
final class Dog extends Animal<Dog> {
void sonOf(Dog dog) { ... }
}
final class Cat extends Animal<Cat> {
void sonOf(Cat cat) { ... }
}
Then you get
final Cat mom = new Cat();
final Cat child = new Cat();
child.sonOf(mom);
final Dog dad = new Dog();
child.sonOf(dad); // "The method sonOf(Cat) in the type Cat is not applicable for the arguments (Dog)"