Java static members of abstract classes

Question

I have following setup: superclass Vehicle with atribute capacity and two subclasses Bus and Tram. All trams have the same capacity as well as all buses (it should be static attribute), but capacity of a tram may be different from capacity of a bus. How to implement such approach in Java?

I have partially solved the problem just ignoring the fact that capacity should be static attribute for all subclasses (no static attributes).

Answer 1

You should encapsulate the capacity in a getCapacity method. That method can then look for an appropriate static variable for the specific class, so for example:

abstract class Vehicle {
    public int getCapacity();
}

class Tram {

    private static final int TRAM_CAPACITY = 200;

    @Override
    public int getCapacity() {
        return TRAM_CAPACITY;
    }
}

class Bus {

    private static final int BUS_CAPACITY = 100;

    @Override
    public int getCapacity() {
        return BUS_CAPACITY;
    }
}

This has the added bonus that if one day you want to calculate the capacity based on some other property (for example the number of seats on a tram given a single cart capacity multiplied by the number of carts) you can modify getCapacity directly without modifying the client code.

class Tram {

    private static final int CART_CAPACITY = 100;

    private int carts;

    public Tram(int carts) { this.carts = carts; }

    @Override
    public int getCapacity() {
        return CART_CAPACITY * carts;
    }
}