CODEWITHSUNDEEP
javainheritancerecursionreflectionabstract-class
Karl S.
Karl S.

Reputation: 355

Recursive method call that uses reflection fails due to abstract class in hierarchy

I have simplified the issue I'm facing in the SSCCE at the bottom.

As it is the output is B C D as one would expect. If I make, say, C abstract then D gets printed and after that an exception occurs:

java.lang.InstantiationException
    at sun.reflect.InstantiationExceptionConstructorAccessorImpl.newInstance(InstantiationExceptionConstructorAccessorImpl.java:48)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at A.recursiveCall(A.java:22)
    at A.entryPoint(A.java:10)
    at A.main(A.java:6)

Since some of the classes in the hierarchy will necessarily be abstract then my code breaks.

The only solution I can come up with is to remove the recursiveCall method and make classSpecificMethod call the parent implementation.

To avoid the redundancy and possibility for error that this introduces I think (I've never used it) I could use AspectJ to generate the code at compile time. But it seems overkill to me. At least for now since I don't have other uses for it.

If there aren't other ways to do this in plain Java I also welcome answers that use other JVM languages and tools.

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

abstract class A {
    public static void main(String[] args) {
        new D().entryPoint();
    }

    void entryPoint() {
        System.out.println(recursiveCall());
    }

    private String recursiveCall() {
        String result = "";
        Class<?> parentClass = getClass().getSuperclass();
        if (parentClass != A.class) {
            try {
                Constructor<?> baseConstructor = parentClass.getDeclaredConstructor();
                baseConstructor.setAccessible(true);
                @SuppressWarnings("unchecked")
                A baseInstance = (A) baseConstructor.newInstance();
                result = baseInstance.recursiveCall() + " ";
            }
            catch (IllegalAccessException | NoSuchMethodException | InvocationTargetException | InstantiationException ex) {
                ex.printStackTrace();
            }
        }
        result += classSpecificMethod();
        return result;
    }

    protected abstract String classSpecificMethod();

    static class B extends A {
        protected String classSpecificMethod() { return "B"; }
    }

    static class C extends B {
        protected String classSpecificMethod() { return "C"; }
    }

    static class D extends C {
        protected String classSpecificMethod() { return "D"; }
    }
}

Upvotes: 0

Views: 580

Answers (1)

Francisco P&#233;rez
Francisco P&#233;rez

Reputation: 51

You can't create an instance of an abstract class. The only way I see to solve this is by making the recursiveCall() skip abstract classes:

private String recursiveCall() {
    String result = "";
    Class<?> parentClass=getClass();
    do {
        parentClass = parentClass.getSuperclass();
    } while (Modifier.isAbstract(parentClass.getModifiers()) && parentClass != A.class);
    if (parentClass != A.class) {
        try {
            Constructor<?> baseConstructor = parentClass.getDeclaredConstructor();
            baseConstructor.setAccessible(true);
            @SuppressWarnings("unchecked")
            A baseInstance = (A) baseConstructor.newInstance();
            result = baseInstance.recursiveCall() + " ";
        }
        catch (IllegalAccessException | NoSuchMethodException | InvocationTargetException | InstantiationException ex) {
            ex.printStackTrace();
        }
    }
    result += classSpecificMethod();
    return result;
}

Upvotes: 1

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