IngredientAmountParser: Class and State DiagramsThe IngredientAmountParser class converts input strings - from end-users or files - into
meaningful units like ingredients, measures, and quantities. You can read more about its
implementation in the Parser javadoc documentation. We illustrate
the workings of this class using a class diagram and a
state diagram.
The class diagram shows how the class uses the ElementParser interface to implement the
State pattern. (You may want to
read more about the State pattern
on patterndigest.com.) Each of the six parser classes implements the interface, and the client
class, IngredientAmountParser, has a field called parser that is a reference to an
ElementParser. The parser classes reach back into IngredientAmountParser to change
parser when it is time to move to the next state. (What lets them modify the private member
parser? They are inner classes!) This mechanism is particularly nice because it hides the details
of state transitions from the client class. It can be a little tricky to code, especially if you have multiple
client classes - since we have only one, Java's inner classes make our job easier. (As is common in pattern
class diagrams, the diagram shows only the key class members and methods.)
The state diagram clarifies the dynamic behaviour of the parser. The parser has particularly simple state behaviour: it is almost a linear progression. You can use state diagrams to illustrate much more complex state behaviour, but LargoRecipes doesn't have anything more complex than this.
You may notice that the parsing seems out of order - how can you do the ingredient before the measure, for example? This is due to a preprocessing step that converts "1 tablespoon fresh basil", say, into "basil fresh tablespoon 1", which actually makes the parsing much simpler. See the javadoc for details.
The large box is the superstate Parsing, which the parser leaves when it runs out of
tokens.
Last updated 25 October 2002
LargoRecipes source code, program, and web site copyright 2001-3 D. Squirrel