Calling common methods without a common interface

I have some generated code (i.e. it cannot be changed) that looks something like this.

class Generated1 {
    public String getA() {
        return "1";
    }

    public void setB(String b) {
    }

    public void setC(String c) {
    }

    public void setD(String d) {
    }
}

class Generated2 {
    public String getA() {
        return "2";
    }

    public void setB(String b) {
    }

    public void setC(String c) {
    }

    public void setD(String d) {
    }
}

I am exploring these objects by reflection. None of them implement any common interface but there's many of them and I want to treat them as if they implement:

interface CommonInterface {
    String getA();

    void setB(String b);

    void setC(String c);

    void setD(String d);
}

It certainly should be possible. This is considered perfectly good code

class CommonInterface1 extends Generated1 implements CommonInterface {
    // These are perfectly good classes.
}

class CommonInterface2 extends Generated2 implements CommonInterface {
    // These are perfectly good classes.
}

I suppose what I'm looking for is something like:

private void doCommon(CommonInterface c) {
    String a = c.getA();
    c.setB(a);
    c.setC(a);
    c.setD(a);
}

private void test() {
    // Simulate getting by reflection.
    List<Object> objects = Arrays.asList(new Generated1(), new Generated2());
    for (Object object : objects) {
        // What is the simplest way to call `doCommon` with object here?
        doCommon(object);
    }
}