Java: How can I access to members of each class???

class Dog {
    private name = null;
    private weight  = null;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    public String getWeight() {
        return weight;
    }
    public void setWeight(String weight) {
        this.weight = weight;
    }
}


class Cat {
    private age = null;
    public String getAge() {
        return age;
    }
    public void setAge(String age) {
        this.age = age;
    }

}


class commUtil {
    //TODO: output members of class
    public void display (Object obj){
        //Question: How to access members of each classes?
        //code here...






    }

}


class mainClass {
    public static void main(String[] args) {

        Dog d = new Dog();
        commUtil.display(d);

        or 

        Cat c = new Cat();
        commUtil.display(c);
    }
}

In case 1: 
Dog d = new Dog();
d.setName("Lion");
d.setWeight("2Kg");
commUtil.display(d);

It will display Name and Weight of Dog

In case 2: 
Cat c = new Cat();
c.setName("MiMi");
c.setAge("1");
commUtil.display(c);

It will display Name and Age of Cat