java - Class that implements an interface with a method returning List of interfaces generates compilation error -

let's have these 4 classes:

public interface {}  public interface b {     public geta(); }  public class c implements {}  public class d implements b {     public c geta() {         return new c();     } } 

this compiles well. if have code compilation error:

public interface {}  public interface b {     public list<a> geta(); }  public class c implements {}  public class d implements b {     public list<c> geta() {         return new arraylist<c>();     } }  public class e implements b {     public list<a> geta() {         return new arraylist<c>();     } } 

what reason allows return c in method geta, in first example, generates error in second example when return list?

it looks me both examples should compile or throw error.

thanks.

edit read post using dog/animal, doubt different. have added class e. if call geta in class e list defined in interface. why fails compiling class e?

this relate java generics. have defined return type of method in interface list<a> trying return list<c> actual implementation. if wanna return list<c> method have change interface method follows

public list<? extends a> geta(); 

explanation why can't return new arraylist<c>();

let valid return u mentioned. then

list<a> lista = geta(); // compile if allowed lista.add(new a()); // valid since reference type a.  

so can see no point of allowing way mentioned