Query regarding Application Classes - 3

Question

While going through Application Package EX, I noticed that most of the classes are extending other classes, while some are actually implemeting some classes..
class ExpenseReport extends EX:Business:Transaction
end-class;
class ExpenseReportLine implements EX:Business:TransactionLine
end-class;
Initially when I came accross this, I percieved TransactionLine to be an Interface. But upon opening its code, it came out to be a Class.
Can anybody tell me what is the differencr between extending a class and implementing a class ??

Answer 1

From ExpenseReportLine, you would think that ExpenseReportLine implements the interface EX:Business:TransactionLine. That is the intent of the implements keyword. If a programmer uses implements then you expect the following name to represent an interface. PeopleCode, however, treats implements and extends the same. In PeopleCode, an interface is actually a shorthand method for declaring a class where all methods are abstract. Put another way, a class that declares all methods as abstract is an interface. Perhaps the base class started as an interface and then the programmer converted it to a class?
When writing your own code, use implements for interfaces and extends for inheritance. Likewise, use the interface keyword to create interfaces. Don't create classes where all methods are abstract.
Coding is communication. Word choice is crucial to effective communication. I apologize for the incorrect use of words in these classes. In this case, the computer treats the code the same. Programmers communicate with two audiences: the computer and other programmers. In this case, it appears that the programmer effectively communicated with the computer, but poorly chose words for communicating with other programmers.