Enthuware. Surprised static methods don't inherit across interfaces but they inherit across classes!

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posted 3 weeks ago

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I was surprised to learn this. Static methods don't inherit across interfaces but they inherit across classes!
Adapted from an Enthuware mock test:

package enthuware.inheritance;

interface BaseI {
    int intMemberBaseInterface = 4;
    static int staticMethodInBaseInterface() {return 44;}
}

interface DerivedI extends BaseI {}
class BaseClass {
    static int intMemberBaseClass = 1;
    static int staticMethodInBaseClass() {return 11;}
}
class DerivedClass extends BaseClass implements DerivedI {

}
public class Static_Inheritance {
    public static void main(String[] args) {
        DerivedClass derivedClass = new DerivedClass();
        System.out.println(derivedClass.intMemberBaseClass); // OK
        System.out.println(derivedClass.staticMethodInBaseClass()); // OK

DerivedI derivedI = derivedClass;
        System.out.println(DerivedI.staticMethodInBaseInterface()); // java: cannot find symbol
        System.out.println(derivedClass.staticMethodInBaseInterface()); // java: cannot find symbol
        System.out.println(BaseI.staticMethodInBaseInterface()); // OK

System.out.println(derivedI.intMemberBaseInterface); // OK

}
}

Stephan van Hulst

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posted 3 weeks ago

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The reason for this is that inheritance of static methods is a bad idea. The Java designers got it wrong the first time.

By the time they added static methods to interfaces, they had learned from their mistakes and they made it so that the methods aren't inherited. But for classes it was too late.

Anil Philip

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posted 3 weeks ago

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Stephan van Hulst wrote:The reason for this is that inheritance of static methods is a bad idea. The Java designers got it wrong the first .

Thanks for the information

Campbell Ritchie

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posted 2 weeks ago

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Stephan van Hulst wrote:The reason for this is that inheritance of static methods is a bad idea. The Java designers got it wrong the first time. . . .

I think that the static keyword had been used to mean something different in C and C++, so they didn't know about that problem. Eiffel has a concept of non‑object functions, and I can't remember whether they were inherited or not. I don't remember there being such restrictions however.

Of course, we have to remember that Eiffel used a dreadful inheritance structure with multiple inheritance and a “bottom” class in every inheritance tree as well as a two‑stage “top”. The two‑stage “top” is all right; only everything else is bad.

Stephan van Hulst

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posted 2 weeks ago

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If I'm honest, I really believe that when they added static methods to classes, there was no other reason to add inheritance other than that somebody proposed "wouldn't it be neat if you could call the method through a subclass?" and the answer was "well why not?".

Campbell Ritchie

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posted 2 weeks ago

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Stephan van Hulst wrote:. . . I really believe . . .

But we are speculating, both of us; we can probably never know.

wouldn't it be neat . . . .

I have heard that many times on this website
If only they had banned calling static methods on object references!