4.1 Class Set

A collection is a set, which is an object that may hold a set of objects. The Set collection is defined as follows:

class Set(class ElmType:< Object): 
   insert(e: ref ElmType): :::
   has(e: ref ElmType): :::
   remove(e: ref ElmType): :::
   :::

Example 4.1 Class Set

The description of class Set as shown here is a simplified description of the class. In a later chapter, we show a more complete description of the Set class.

Class Set has a parameter, ElmtType, which is a class name. The ElmType parameter specifies the type of the elements that may be inserted into the Set.

Class Set has three methods, insert, has and remove, each having a parameter e being a reference to an object of type ElmType – the class parameter of class Set.

Note! We have not specified the details of the three methods — the colons (:::) represents the details not shown. The three methods actually define what is called the interface of class Set — the interface of a class (or object) are the attributes (here methods) that may be used to access the attributes of a given instance of the class. The details are often referred to as the implementation of the class.

It is a fundamental principle of programming to define classes and objects and only expose their interfaces. The advantage of this is that as long as the interface of a given class is not changed, it is possible to change the implementation of the class without affecting the use of the class, which may appear in many places in the code.

We may use class Set to represent our Account-file in the following way:

theAccountsFile: obj Set(Account)

We may insert a new account into theAccountsFile as follows:

aCustomer: ref Customer
anAccount: ref Account
aCustomer:= Customer("Linda Berry","England","linda@google.com")
anAccount := Account(aCustomer)
theAccountsFile.insert(anAccount)

We do not need the two variables aCustomer, and anAccount, since we may inline the expressions for Customer and Account in the insert expression as in:

theAccountsFile.insert(
   Account( 
      Customer("Linda Berry","England","linda@google.com")))