Why is this covariant return type declaration not warning?

Question

Why the code below does not display a warning without compilation:

class Parent {
    public List method(){
        return null;
    }
}

class Child extends Parent {

    public List<String> method() {
        return null;
    }
}

and the following:

class Parent {
    public List<String> method(){
        return null;
    }
}

class Child extends Parent {

    public List method() {
        return null;
    }
}

I am really looking for a link for this behavior in JLS.

Answer 1

Because it is Listnot a subtype List<String>, but (as you know) List<String>is a subtype List.

Examples given in JLS:

class C implements Cloneable { 
    C copy() throws CloneNotSupportedException {
        return (C)clone();
    } 
}
class D extends C implements Cloneable { 
    D copy() throws CloneNotSupportedException {
        return (D)clone();
    } 
}

This is your first example in which the return type of the child is a subtype of the parent. Similarly, in your example List<String>is a subtype List.

4.10.2. Subtyping between classes and interface types

C<F1,...,Fn> (n > 0), C<F1,...,Fn> - :

• C<F1,...,Fn>.
• C<F1,...,Fn>.
• Object, C<F1,...,Fn> - .
• C. ( )

:

class StringSorter {
    // turns a collection of strings into a sorted list
    List<String> toList(Collection<String> c) {...}
}
class Overrider extends StringSorter {
    List toList(Collection c) {...}
}

.

Overrider StringSorter, Overrider.toList List, , List<String>. ( )

JLS 8.4.8.3-1 8.4.8.3-2 :

8.4.8.3.

d1 R1 d2 R2, d1 return-type-substitutable (§8.4.5) d2 .

- .

R1 R2, SuppressWarnings (§9.6.4.5).