Designing a class

Kinds of classes Utility classes – Math (Do something) Object classes – ExpressionParser, Scanner (Are something) Hybrid classes – Integer (Are something but contain helpful utilities)

Steps Write a description (or use one provided) Create an initial encapsulation Refine the encapsulation Identify helpful constructors Identify helpful overloaded constructors Think about the need for private methods Identify helpful overloaded methods Identify class level attributes Identify class level behaviors

Description Should include what we know about the object data Should include what the object should be able to do If the description is not complete, you can’t create the class.

Initial Encapsulation Identify nouns and verb phrases in the description. Nouns are data. What data types? Which are the important nouns. Which ones are pertinent to the problem? Verbs are the methods. What parameters do they need? What do they need to return?

Money The money class will represent US Money amounts. Once created a particular Money object cannot change. We need to be able to add two Money objects to get a third, subtract two Money objects, multiply a money object by a value (such as .05) to get a new Money amount and divide a Money object by a value (such as 2.5). We must also be able to compare Money objects to determine which is larger, small or if they are the same. We must be able to display a Money object as normal dollars and cents. Money may only be a whole number of cents. In other words, Money object may be 3 dollars and 12 cents, but not 3 dollars and 12.5 cents.

Initial encapsulation On your whiteboards, begin writing the class. You should carefully think about the instance variables that you need. How will you internally store money values? (What does a Money object look like?) What methods are directly suggested by the description? (Don’t worry about constructors at this point). (What should a Money object do?)

Refinement Do the attributes really reflect the state of the object? Are there any that would be used in only one method? Are there any that represent derivations? If any of the data changes, would we need another method. For example, rather than storing derived data, perhaps we need a method for that.

Identify constructors How will we want to build new Money objects? What makes sense for the user of this class to do?

Identify any helpful overloaded constructors Should we have a default constructor? Not automatic if any other constructor is defined Should we have multiple explicit value constructors? Are there multiple ways of initializing data Are there methods that can help the constructor do it’s job? “Check” methods to check for validity Constructors can call other constructors. Stock.java CopyConDemo.java

Do we need any helper methods? These can be methods that would be “utility” methods to users of the class or could be a helper method for other methods in the class. Private if only used inside of the class; public if a utility.

Identify overloaded methods Different parameter types may have subtle differences. We may want to provide different interfaces if, for example, we have a similar function but want to pass in different parameters. Or, if we want to allow the possibility of different numbers of parameters (min method).

Variable length parameter lists May obviate the need to have overloaded methods in the case of cardinality (handle 2, handle 3, handle more). See VarargsDemo1.java

Class attributes? Helpful constants, special values Common values used across all objects Object counters What might we want to include that are “constant” money amounts?