Many programs operate on states. For example, the following Java class introduces a Car object:
class Car {
public String type;
public String color;
public int age;
}
public Car (String type, String color, int age) {
this.type = type;
this.color = color;
this.age = age;
}
Car sedan = new Car("Sedan", "red", 5);
Programs written in many languages, such as Java, C#, Ruby, or Python, are full of similar classes. We are so used to this approach that we rarely consider how problematic it can be.
We created an instance of Car, represented by a particular type, sedan. We might have bought this car when it was already five years old. The car can exist in many states—as a brand new car with an age of 1, or as an antique car with an age of over 25. At each point in time, the car has precisely one valid state. Its state might...
