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o Code that implements has the responsibility of throwing exceptions。 Code that imple
ments does not try to fix or swallow the exception。 This means code that implements will
implement a Finally block to reset state； but usually not a Catch block。
o Code that organizes has the responsibility of being aware that exceptions might occur。
This means code that organizes will implement Catch blocks to catch exceptions and
process them。 Usually； this code will not have a Finally implementation； but it may
filter exceptions。
o Exceptions can be filtered to determine which exceptions will be captured and which
cannot be captured。
o You can help make your code exception…safe by implementing sandboxing
functionality。
o Implement default state so that your code is easier to read and maintain。
Some Things for You to Do
To apply what you’ve learned in this chapter； here’s what you can do； summed up in a single step：
1。 Rewrite all of the code examples in Chapter 4 to be exception…safe。
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C H A P T E R 6
■ ■ ■
Learning the Basics of Object
Oriented Programming
At this point； you should be fortable with writing basic Visual Basic code； but your coding
style would solve problems directly without considering reusability or other more advanced
concepts。 You could code yourself out of a wet paper bag； but once you were out of the bag； you
would have no idea what to do next。 （In case you’re interested； the phrase； “You couldn’t do ＇fill
in topic＇ out of a wet paper bag” implies that your skills in a certain area are so weak that a wet
paper bag is more overpowering。 The reference to wet paper bag es from the fact that wet
paper bags have no holding power and will tear at the slightest nudging。）
This chapter focuses on reusing base functionality； where two classes share methods and
properties in mon to solve a particular problem。 To demonstrate the concepts； we will create
a simple currency exchange application。 This application will use object…oriented programming；
a powerful set of techniques mon in modern programming languages such as Visual Basic。
In this chapter； you’ll learn about the following topics：
Object…oriented programming： Object…oriented； often abbreviated as OO； refers to a way of
building applications using instantiated types。 First； you define the type and its behavior。
Then； when instantiated； the type has a state。 When you are developing； you don’t know
the state of the object； so you can only guess at it。
Data member scopes： Types have methods； and methods can be called by other types。 But
do you want to let all types call all of a type’s methods？ Think of it as follows。 When you are
at a party； do you want to allow everyone to grab into your pockets？ No； you want to control
access to your pockets； just as you want to control access to your type’s methods。
Properties： There are methods； and there are properties。 Methods are supposed to do things
to a type。 Properties are external state representations of a type。
Base classes： The term base class denotes mon functionality。 We use the word base；
because in object…oriented programming； hierarchy is defined from the base upwards。 We
use the word class； because class is the basic type that contains functionality。
Let’s begin with some background on how currency exchange works； which applies to the
currency converter application we’ll develop in this chapter to demonstrate object…oriented
programming concepts。
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136 CH AP T E R 6 ■ L E A R N IN G T HE B AS IC S O F OB J E CT OR I E N T E D P R O G R AM M IN G
Understanding Currency Spreads
This chapter’s sample application deals with exchanging different currencies。 Did you know
that when you exchange one currency for another； you never need to pay a processing fee？
Whether you are exchanging currencies at some airport or on an exchange； you should not be
charged any processing fees。 It makes you wonder how people earn money by accepting one
currency and then giving out another currency。 The answer lies in how currency exchange works。
When dealing with currencies； you are always dealing with currency pairs。 This is different
from something like buying a stock； when you are dealing with only that stock。 A currency by
itself is actually quite useless。 Suppose you have a 100 in your hand。 What will it be worth one
year from now？ The answer is simple： 100。 Yes； you can argue that the 100 will not buy as
much next year； but that would be plicating the issue with concerns not related to this
discussion。 The fact is that if you have 100 dollars； 100 yen； 100 euros； and so on into infinite
time； you will always have 100 currency units。
Currency exchange involves paring one currency to another currency。 The value of a
currency is what one person would give in parison to another currency。
Foreign exchange （forex） market traders make their money by dealing in spreads。 Spreads
are tricky because； as the exchange rate moves up or down； so does the spread。 For example；
hotels always give you horrible rates； and you may wonder why they are ripping you off。 But