Java Inheritance issue

While exploring for scjp questions, I came across this behaviour which I found strange.

I have declared two classes Item and Bolt as follows:

class Item {
    int cost = 20;

    public int getCost() {
        return cost;
    }
}

class Bolt extends Item {
    int cost = 10;

    public int getCost() {
        return cost;
    }
}

and tried to access the value of cost twice

public class Test {
    public static void main(String[] args) {
        Item obj = new Bolt();
        System.out.println(obj.cost);
        System.out.println(obj.getCost());
    }
}

The output I get is 20 10. I can't understand how this happens.