Generic return type questions

[Sergej Smoljanov]

When i try some code, i found strange (for me) behaviour:

class A{
    public List<String> ment(List<String> a){
     return null;
    }
}

class B extends A{
   
    public List<Integer> ment(List<String> a){
        return null;
    }
}

compiler error wrote: error: ment(List<String>) in B cannot override ment(List<String>) in A public List<Integer> ment(List<String> a){ return type List<Integer> is not compatible with List<String>

thats looks not strange, next i edit class B ment parameter to List a

class A{
    public List<String> ment(List<String> a){
     return null;
    }
}

class B extends A{
   
    public List<Integer> ment(List a){
        return null;
    }
}

unchecked warning wrote:uses unchecked or unsafe operations.

but code compile ok - and i think this is strange but my first opinion than if i also change class A ment parament to List a code will compile to, but i was wrong

class A{
    public List<String> ment(List a){
     return null;
    }
}

class B extends A{
   
    public List<Integer> ment(List a){
        return null;
    }
}

compiler error wrote:ment(List) in B cannot override ment(List) in A public List<Integer> ment(List a){ return type List<Integer> is not compatible with List<String>

_____________________
My question is why second example compile ok and why third not compile?

[Gajendra Kangokar]

In second example you have used argument List without generics which is equals to List<?>.
That means you have overloaded method instead overriding,so it compiles.

Where as in other two example you are overriding where return type must match to parent method.

[Ankit Garg]

I agree Sergej, this does seem a bit weird. If you mix typed and raw parameters, then compiler is ignoring the incompatible typed return types. Don't know what the reason is, but it's not intuitive...

[Ankit Garg]

Gajendra Kangokar wrote:That means you have overloaded method instead overriding,so it compiles.

Gajendra, that's what I initially thought as well. But I tried adding Override annotation to be sure and it is indeed overriding.

import java.util.List;
class A{
    public List<String> ment(List<String> a){
     return null;
    }
}
 
class B extends A{
    @Override
    public List<Integer> ment(List a){
        return null;
    }
}

Since both methods have the same erasure, they can't be overloaded. Following gives error:

import java.util.List;
class A{
    public List<String> ment(List<String> a){
     return null;
    }
    public List<Integer> ment(List a){
        return null;
    }
}
--------
javac A.java
A.java:6: ment(java.util.List<java.lang.String>) is already defined in A
    public List<Integer> ment(List a){
                         ^
1 error

[Gajendra Kangokar]

Yes right

import java.util.List;
class A{
    public List<String> ment(List<String> a){
     return null;
    }
   public List<Integer> ment(List a){
        return null;
    }
}
 

In rum time the above code becomes

import java.util.List;
class A{
    public List ment(List a){
     return null;
    }
   public List ment(List a){
        return null;
    }
}

we dont want code that compiles fine but gives error at runtime so compiler does not allow to have such methods