Mocking method from Java abstract class

RegularPoly.java

public abstract class RegularPoly {
    public double getLength() {
        return Math.abs(System.identityHashCode(this)) % 100;
    }

    public abstract double getArea();
}

Square.java

public class Square extends RegularPoly {
    @Override
    public double getArea() {
        return getLength() * getLength();
    }
}

SquareSpec

import spock.lang.Specification

class SquareSpec extends Specification {
    def "should correctly compute area"() {
        given:
        Square square = Spy()

        when:
        def area = square.area

        then:
        2 * square.getLength() >> 10
        area == 100
    }
}