The Expert Object Design

Class StampDispenser illustrates the basic form of a service-oriented object. Its instance variables are private, so its accessible methods are the only way to manipulate the state of the object. Although service-oriented objects like StampDispenser have state, they use their state to decide how to behave when their methods are invoked. Consider the code of StampDispenser's add method, shown in Listing 6-1.

Listing 6-1. The add method of StampDispenser

package com.artima.examples.stampdispenser.ex1;

//...

public class StampDispenser {

    private final static int STAMP_VALUE = 20;
    private int balance;

    //...

    public synchronized void add(int amount) {

        if ((amount != 5) && (amount != 10)) {
            throw new IllegalArgumentException();
        }

        balance += amount;

        if (balance >= STAMP_VALUE) {

            // Dispense a stamp and return any change
            // balance - STAMP_VALUE is amount in excess of twenty cents
            // (the stamp price) to return as change. After dispensing the
            // stamp and returning any change, the new balance will be zero.
            StampDispenserEvent event = new StampDispenserEvent(this,
                balance - STAMP_VALUE, 0);
            balance = 0;
            fireStampDispensed(event, listenersClone);
        }
        else {

            // Fire an event to indicate the balance has increased
            StampDispenserEvent event = new StampDispenserEvent(this,
                amount, balance);
            fireCoinAccepted(event, listenersClone);
        }
    }

    //...
}

StampDispenser's balance variable keeps track of the amount of money inserted but not returned or exchanged for stamps. StampDispenser offers no data-oriented methods to set and get the balance. Rather, it uses the balance to decide how to behave when its service-oriented methods add and returnCoins are invoked. As shown by the highlighted portions of Listing 6-1, when the StampDispenser's add method is invoked, it uses its current balance to decide whether or not to dispense a stamp, whether or not to return any change, and what new value to give to balance.

This guideline's main point is that in the basic, expert object design, objects keep their state private and expose only their behavior. The state can be either mutable or immutable. The reason such objects have state is to help them decide how to behave when called upon to perform a service. Thus, even though such objects have both state and behavior, they are service-oriented, not data-oriented.