method return value covariance for primitives. Is it works?

Question

**since java 5;

I know that if in the base class I write:

public Number doSomething(){
...
}

in the child class I can write something like this

@Override
public Integer doSomething(){
    ...
}

But I have a question.

If in base class method returns - primitive - array - or Collection.

How can I use covariant at this case?

Can you give some example for those case? At least the base class methods? — Rohit Jain, Mar 03 '14 at 18:39

Sotirios Delimanolis · Accepted Answer · 2014-06-15T20:25:01.833

8

There's no covariance between primitives. No primitive type is a sub type of any other. So you can't do this

class Parent {
    public int method() {
        return 0;
    }
}

class Child extends Parent {
    public short method() { // compilation error
        return 0;
    }
}

For the same reason, corresponding array types for int and short also are not covariant.

With array types, it's similar to your Number example

class Parent {
    public Number[] method() {
        return null;
    }
}

class Child extends Parent {
    public Integer[] method() {
        return null;
    }
}

Similarly for Collection types

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

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

Note the generic type argument has to be compatible (no covariance in generics, except in bounded wildcards).

edited Jun 15 '14 at 20:25

answered Mar 03 '14 at 18:40

Sotirios Delimanolis

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Is not valid **Collection** in parent and **Collection** in child? – gstackoverflow Mar 03 '14 at 18:48
@gstackoverflow No, because covariance doesn't apply to generic types. See [here](http://stackoverflow.com/questions/2660827/java-generics-covariance) and related questions/answers. – Sotirios Delimanolis Mar 03 '14 at 18:49
@erickson What was illegal? Anyway, cleaner like this. Thanks. – Sotirios Delimanolis Mar 03 '14 at 18:49
@Sotirios Delimanolis I think array of primitives is helpful example for this response – gstackoverflow Mar 06 '14 at 11:56
@gstackoverflow You mean you want me to add an example with primitives that doesn't work? – Sotirios Delimanolis Mar 06 '14 at 16:46
@Sotirios Delimanolis Sorry, I am about **int []** in parent and **short []** in child – gstackoverflow Mar 06 '14 at 17:32
@gstackoverflow That's the same as `int` vs `short`. Because `short` is not a sub type of `int`, neither are their corresponding array types. Added an edit. – Sotirios Delimanolis Mar 06 '14 at 17:32
@JimmyT. Do you mean in the sense of a `List` overriding a return type of `List>`? If that is, that is an unchecked conversion, not covariance. Both are supported by `return-type-substitutability`. – Sotirios Delimanolis Jun 15 '14 at 18:55
I meant that `List extends Number>' can be replaced by `List extends Integer>`. – Jimmy T. Jun 15 '14 at 19:56
@JimmyT. Ah, yes. Thank you. I've edited my answer to reflect that. – Sotirios Delimanolis Jun 15 '14 at 20:25

score 0 · Answer 2 · answered Mar 03 '14 at 18:40

0

primitive : No
array : Only if its a subtype of the parent's array type
or Collection : Same thing as 2

answered Mar 03 '14 at 18:40

rpax

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method return value covariance for primitives. Is it works?

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