Interfaces and polymorphism

The idea

An interface describes operations that implementing objects promise to provide. A variable with the interface type can refer to any compatible implementation. This lets a client work with a capability, such as producing a charge, without depending on the implementation details. The variable type determines which members can be requested at compile time; the actual object determines the implementation of an overridden instance method at runtime.

Use polymorphism to remove repeated type checks from client code. Each implementation can calculate its answer differently while honoring the same contract. Specify meaning as well as syntax: a method returning an int does not by itself say whether negative values are valid or which units are used.

interface Charge { int cents(); }
class FlatCharge implements Charge {
  public int cents() { return 250; }
}
class TicketCharge implements Charge {
  public int cents() { return 400; }
}
public class Main {
  public static void main(String[] args) {
    Charge[] charges = {new FlatCharge(), new TicketCharge()};
    int total = 0;
    for (Charge c : charges) total += c.cents();
    System.out.println(total);
  }
}

650

The loop has one interface-based call site. Each object supplies its own cents implementation, and both results contribute to the sum.