CSC/ECE 517 Fall 2009/wiki2 4 dn

A large fraction of the if-statements in o-o programs could be replaced by uses of polymorphism, which would lead to more modular and maintainable programs. Explain why if-statements are error prone if there is any chance that future development may require them to change. Give examples of if-statements that can be replaced by elegant uses of polymorphism, trying to find some useful replacements that are not obvious. Concentrate on polymorphism, not other reasons for avoiding use of ifs.

The If Statement

The if-then statement is the most basic of all the control flow statements. It tells a program to execute a certain section of code only if a particular test evaluates to true.

if (daytime is morning)
{
  print "good morning"
}
else if (daytime is afternoon)
{
  print "good afternoon"
}
....
....

When and why are if-statements error prone?

Maintainability and Extensibility

Methods that use conditional if-statements can be hard to maintain and extend.

Public class  
if (operator = "+")
{
  result = a+b
}
else if(operator = "-")
{
  result = a-b
}

In the above example if the programmer wanted to extend her code to handle multiplication, division and the mod operation then she would have to write an explicit conditional statement to handle each of these. If the condition is checked in multiple places then the programmer will have to add the additional conditional statement in each of these places. This not only results in duplication of code but is also highly prone.

Testing

Sometimes in a program the same condition may have to be checked in multiple places.

if(flag is enabled)
{
  read a
}
else if(flag is not enabled)
{
  read b
}

.....
.....

if(flag is enabled)
{
 write a 
}
else if(flag is not enabled)
{
 write b
}

In the above example, flag is checked multiple times. This scenario can be particularly difficult for testing and debugging because, the progaram needs to read a and write a or read b and write b. The program should not read a and write b or read b and write a. If the condition needs to be checked more than twice then testing and debugging becomes more complicated.

If statements and Polymorphism

Most if-statements such as the ones shown above can be replaced with polymorphism.

In object-oriented programming, polymorphism refers to a programming language's ability to process objects differently depending on their data type or class. More specifically, it is the ability to redefine methods for derived classes.

When to use plymorphism instead of if-statement

• If an object should behave differenty based on its state.
• If the same condition has to be checked in multiple places.