If are bad, if are evil. If are against object-oriented programming. If defeats polymorphism — These popular remarks are easier said than enforced.
Ifs can pop up for various reasons, which would deserve a full study in order to build a decent taxonomy. Here is however a quick one:
- Algorithmic if. An algorithmic
ifparticipate in an algorithm that is inherently procedural, where branching is required. No much can be done for these ones. Though they tend to increase the cyclomatic complexity, they are not the most evil. If the algorithm is inherently complex, so be it. - Polymorphic if. A class of object deserve some slightly different treatment each time. This is the polymorphic
if. Following object oriented principles, the treatment should be pushed in the corresponding class, and voila! There are however million of reasons why we don’t want the corresponding logic to be in the class of the object to treat, defeating object oriented basics. In such case, the problem can be alleviated with visitor, decorator, or other composition techniques. - Strategic if. A single class deals with N different situations. The logic is still essentially the same, but there are slight different in each situation. This situation can be refactored with an interface, and several implementations. The implementations can inherit from each other, or use delegation to maximize reuse.
- Dynamic if. Strategic if assumes that the situation doesn’t change for the lifetime of the object. If the behavior of the object needs to change dynamically, the situation becomes even more complicated. Chances are that attributes will be used to enable/disable certain behavior at run-time. Such if can be refactored with patterns such as decorators.
- Null if. Test for nullity is so common that it deserves a special category, even though it could be seen as a special case of another category. Null if can happen to test the termination of an algorithm, the non-existence of data, sanity check, etc. Various techniques exist to eradict such
ifdepending on the situation: Null Object pattern, add as many method signature as required, introducing polymorphism, usage of assertions, etc.
A step-by-step Anti-If refactoring
Here is a step-by-step refactoring of a strategic if I came accross . I plan to send it to the anti-If compaign and took then the time to document it. The code comes from the txfs project.
Let’s start with the original code:
public void writeFile (String destFileName, InputStream data, boolean overwrite)
throws TxfsException
{
FileOutputStream fos = null;
BufferedOutputStream bos = null;
boolean isNew = false;
File f = new File (infos.getPath (), destFileName);
if ( !overwrite && f.exists () )
{
throw new TxfsException ("Error writing in file (file already exist):" +
destFileName);
}
try
{
if ( !f.exists () )
{
isNew = true;
}
DirectoryUtil.mkDirs (f.getParentFile ());
try
{
Copier.copy (data, f);
}
finally
{
if ( isNew && isInTransaction () )
{
addCreatedFile (destFileName);
}
IOUtils.closeInputStream (data);
}
}
catch ( IOException e )
{
throw new TxfsException (“Error writing in file:” + destFileName, e);
}
}
Not very straightforward, isn’t it? The logic is however quite simple: if the overwrite flag is set, file can be written even if it already exists, otherwise an exception must be thrown. In addition to that, if a transaction is active, the file must be added to the list of created file, so that they can be removed in the transaction is rolled back later. The file must be added even if an exception occurs, for instance if the file was partially copied.
What happens is that we have two concerns: (1) the overwrite rule and (2) the transaction rule.
Let’s try to refactor that with inheritance. A base class implements the logic when there is no transaction. And a subclass refines it to support transactions.
public void writeFile (String destFileName, InputStream data, boolean overwrite)
throws TxfsException
{
FileOutputStream fos = null;
BufferedOutputStream bos = null;
boolean isNew = false;
File f = new File (infos.getPath (), destFileName);
if ( !overwrite && f.exists () )
{
throw new TxfsException ("Error writing in file (file already exist):" +
destFileName);
}
try
{
// if ( !f.exists () )
// {
// isNew = true;
// }
DirectoryUtil.mkDirs (f.getParentFile ());
try
{
Copier.copy (data, f);
}
finally
{
// if ( isNew && isInTransaction () )
// {
// addCreatedFile (destFileName);
// }
IOUtils.closeInputStream (data);
}
}
catch ( IOException e )
{
throw new TxfsException (“Error writing in file:” + destFileName, e);
}
}
The transaction concern is removed from the base method. The overridden method looks then like:
public void writeFile (String destFileName, InputStream data, boolean overwrite)
throws TxfsException
{
try
{
super.writeFile( destFileName, data, overwrite );
}
finally
{
addCreatedFile (destFileName);
}
}
But we have then two problems: (1) we don’t know if the file is new, and it’s always added to the list of created file. (2) if the base method throw an exception because the file already exists and the flag is false, we still add it to the list of created file when we shouldn’t.
We could change the base method to have a return code (e.g. FileCreated and NoFileCreated). But return code are not generally a good solution and are quite ugly.
No, what we must do, is remove some responsibility to the method. We then split it into two methods createFile and writeFile. One expects the file to not exsits, the other the file to exists.
void writeFile (String dst, InputStream data ) throws TxfsException
void createFile (String dst, InputStream data ) throws TxfsException
(The method writeFile which takes the additional overwrite flag can be composed out of the two previous one )
So our simplified writeFile method looks like:
public void createFile(String destFileName, InputStream data)
throws TxfsException
{
try
{
super.createFile(destFileName, data);
}
finally
{
// we add the file in any case
addCreatedFile(destFileName);
}
}
Alas, there is still the problem that if super.writeFile fails because the file already exists, we add it to the list.
And here we start to realize that our exception handling scheme was a bit weak. The general TxfsException is rather useless: we need to refine the exception handling to convey sufficient information to support meaningful treatment.
void writeFile (String dst, InputStream data )
throws TxfsException, TxfsFileDoesNotExsistException
void createFile (String dst, InputStream data )
throws TxfsException, TxfsFileExistsAlreadyException
Here is then the final code:
public void createFile(String destFileName, InputStream data)
throws TxfsFileAlreadyExistsException, TxfsException
{
// I can not use a finally block because
// if failure happens because file already existed
// I must not add it the list
try {
super.createFile(destFileName, data);
// we add the file if creation succeeds
addCreatedFile(destFileName);
}
catch (TxfsFileAlreadyExistsException e)
{
// we don't add the file if it already exists
throw e;
}
catch (Throwable e)
{
// we add the file in any other case
addCreatedFile(destFileName);
}
}
}
Conclusion
Anti-If refactoring is not so easy. There are various kind of ifs, some of which are ok, and some of which are bad. Removing ifs can imply changing the design significantly, including the inheritance hierarchy and the exception hierarchy.
A tool to analyze and propose refactoring suggestion would be cool, but for the time being, NoIF refactoring is probably in the hands of developers which can ensure the semantics equivalence of the code before and after the refactoring.
Links
http://www.technology-ebay.de/the-teams/mobile-de/blog/an-exercise-in-unconditional-programming.html
://ewernli 