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Designing objects for flexibility and robustness
Why should you care about designing good objects? One reason is that a set of robust objects can help contribute to the overall robustness of the program they constitute. In addition, well designed objects are more flexible (easier to understand and easier to change) than poorly designed objects.

A fundamental way to make your object designs robust and flexible is by ensuring that your objects have a valid state, and experience only valid state transitions, from the beginning of their lifetimes to the end. The rest of this article will discuss ways to ensure that classes begin their lifetimes with a valid state.

Introducing class CoffeeCup and the virtual café
The discussion of object design in this and subsequent articles will make use of a class that models coffee cups. You can imagine using this class in a program that implements a "virtual café": a place in cyberspace where guests can sit at small tables, sipping virtual cups of coffee and chatting with one another. The primary function of the café is that of a chat room, where people separated by (potentially vast) physical distances yet connected to the same network come together to converse. To make your chat room more compelling, you want it to look like a café. You want each participant to see graphical representations ("avatars") of the other people in the café. And, to make the participants' experience more real, you want the people to be able to interact with certain items in the café, such as tables, chairs, and cups of coffee.

The CoffeeCup
The basic CoffeeCup class has one instance variable, innerCoffee, which keeps track of the number of milliliters of coffee contained in the cup. This variable maintains your virtual coffee cup's state. The following methods allow you to change its state by:

• Adding coffee to the cup (the add() method)
• Removing one sip of coffee from the cup (the releaseOneSip() method)
• Spilling the entire contents of the cup (the spillEntireContents() method)

Here is a class that represents a simple coffee cup in a virtual café:

// In Source Packet in ex1/CoffeeCup.java
class CoffeeCup {

    private int innerCoffee;

    public void add(int amount) {
        innerCoffee += amount;
    }

    public int releaseOneSip(int sipSize) {
        int sip = sipSize;
        if (innerCoffee < sipSize) {
            sip = innerCoffee;
        }
        innerCoffee -= sip;
        return sip;
    }

    public int spillEntireContents() {
        int all = innerCoffee;
        innerCoffee = 0;
        return all;
    }
}