Question

It’s obvious that a parent class’s object can hold a reference to a child, but does this not hold true in case of parameterised collection ??

eg:

Car class is parent of Sedan

So

public void doSomething(Car c){
    ...
}

public void caller(){
    Sedan s = new Sedan();
    doSomething(s);
}

is obviously valid

But

public void doSomething(Collection<Car> c){
    ...
}

public void caller(){
    Collection<Sedan> s = new ArrayList<Sedan>();
    doSomething(s);
}

Fails to compile

Can someone please point out why? and also, how to implement such a scenario where a function needs to iterate through a Collection of parent objects, modifying only the fields present in parent class, using parent class methods, but the calling methods (say 3 different methods) pass the collection of three different subtypes..

Ofcourse it compiles fine if I do as below:

public void doSomething(Collection<Car> c){
    ...
}

public void caller(){
    Collection s = new ArrayList<Sedan>();
    doSomething(s);
}

Answer 1

Use

public void doSomething(Collection<? extends Car> c){}

or (as suggested)

public <T extends Car> void doSomething(Collection<T> c){}

This would mean that the Collection is of any subclass of Car (or Car itself), rather than “It’s a collection of Car instances only”

This is because collections are invariant, unlike arrays, which are covariant. To quote Effective Java:

Covariant [..] means that if Sub is a subtype of Super, then the array type Sub[] is a subtype of Super[]. Generics, by contrast, are invariant: for any two distinct types Type1 and Type2, List<Type1> is neigther a subtype nor a supertype of List<Type2>.