Generics Help please !

[jose chiramal]

The below gives me error :

public void foo() {
  Cat[] cats = {new Cat(), new Cat()};
  addAnimal(cats);  // no problem, send the Cat[] to the method
}


and the original method stays the same:

public void addAnimal(Animal[] animals) {
   animals[0] = new Dog();  // Eeek! We just put a Dog
                            // in a Cat array!  // isn't this similar to adding a Dog object to the animal array ?
}
   

However , this doesnt give me error :

    

List<Animal> animals = new ArrayList<Animal>();
animals.add(new Cat());  // OK
animals.add(new Dog());  // OK

Why so, could someone please explain ?

[Ryan Hamilton]

  public class Animal {}
class Dog extends Animal {}


class A {
    public static void main(String[] args) {
        List<Animal> animalsL = new ArrayList<Animal>();
        List<Dog> dogsL = new ArrayList<Dog>();
        Animal[] animals = new Animal[]{ new Animal() };
        Dog[] dogs = new Dog[]{ new Dog() };
        Animal animal = new Animal();
        Dog dog = new Dog();

        list(animalsL);
        array(animals);
        individual(animal);

        // list(dogsL); // doesn't work
        array(dogs);
        individual(dog);
    }

    static void array(Animal[] animals) {    }
    static void list(List<Animal> animals) {    }
    static void individual(Animal animal) {  }
}  

For primitives, the compiler knows that Dog is a subtype of Animal, and that Dog[] is a subtype of Animal[], for generics this is not the case.

As to why java allows implicit casting of array subtypes, which can then cause the error shown, I am not sure.

[Rajeev Rnair]

jose chiramal wrote:The below gives me error :

public void foo() {
  Cat[] cats = {new Cat(), new Cat()};
  addAnimal(cats);  // no problem, send the Cat[] to the method
}


and the original method stays the same:

public void addAnimal(Animal[] animals) {
   animals[0] = new Dog();  // Eeek! We just put a Dog
                            // in a Cat array!  // isn't this similar to adding a Dog object to the animal array ?
}
   

Hi Jose,

First of all your code (1st) will NOT give compile error, it gives a RuntimeException though!
The reason is you are trying to put a Dog() into a Cat[]. Dog is NOT a Cat , so that gives an Exception. It compiles fine because both are sub classes of Animal

public class Test2 {
	public static void main(String... args) {
		new Test2().foo();
	}
	
	public void foo() {   
		Cat[] cats = {new Cat(), new Cat()};   
		addAnimal(cats);  // no problem, send the Cat[] to the method   
	} 
	
	public void addAnimal(Animal[] animals) {   
		animals[0] = new Cat();   // compiles ok, runs fine!
		animals[0] = new Dog();  // compiles ok, RuntimeException
 	}
}

class Animal {}
class Dog extends Animal {}
class Cat extends Animal{}

[Flavio Luiz Maria]

You also can to do:

static void List(List<? extends Animal> animals) {}

In this method you can add all objects that extends Animal

[Rajeev Rnair]

Flavio Luiz Maria wrote:You also can to do:

static void List(List<? extends Animal> animals) {}

In this method you can add all objects that extends Animal

Hi Flavio, welcome to the ranch!
yes, you are right! you can add all subclasses of Animal to the above method. So you can add a Dog() or a Cat()

hope this helps!

[Ryan Hamilton]

Why does java allow implicit casting of array subtypes?
Particularly when it doesn't allow similar behavior for collections?

[ujjawal rohra]

This is because there is an Exception for arrays--ArrayStore Exception while Generics are only a compile time protection with no run time exception.

That is, the line

List<Number> l=new ArrayList<Number>();
is only checked at compile time. At run time it becomes

List l=new ArrayList();

And if you are allowed to add a Float into Integer list at compile time just imagine what will happen to your code at run time