I’ve recently come to a heuristic called the Law of Demeter (LoD) or Principle of Least Knowledge that says a module should not know about the innards of the objects it manipulates. More precisely, the method of a particular class should call:
- other methods of the same class
- objects created by the method itself
- objects passed as arguments to the method
- objects held in an instance variable of the same class
And that”s all!
So the following code appears to violate the Law:
String output = obj.getContext().getOptions().getDir().getPath();
It’s easy to forget something around such a structure. This one can solve the issue:
String output = obj.getPathFromContext();
I had some difficult time understanding all the roles regarding EJB3. Here is an easy-to-remember description of these roles that I found here.
Imagine a factory producing computers:
Bean Provider:
Chip manufacturer. On the chip, there will be a label with “Warranty void if removed” the chip has the logic and the label sets a Role. If you are not authorized to repair it, warranty voids. (i.e. you cannot access the internal chip if you are not in the role of “Warranty Repair Person”.)
Application Assembler
Mainboard assembler. It takes various chips and puts them on the mainboard. If any additional resistor or cable are required, it will put everything togheter to have something that is some kind of working unit but requires additional assembling.
EJB Server Provider
The EJB Server provider is the Computer Case manufacturer providing a case with a power supply. Is a container for the mainboard
Deployer
As every computer case is different and power voltage vary country by country, the Deployer makes sure to adapt the mainboard to the working environment. In this case adjusts the Voltage on the power supply, and connects the cables. At the same time he will define who are the person allowed to repair it (i.e. provide a list of authorized repair centers)
Persistence Provider
the persistence provider could be the network card company that provides the driver to connect to a network.
System Administrator
Is the person in charge to install the operating system and do necessary configuration changes to the OS to connect to the server, and will make sure to monitor that everything is working fine.
More about Enterprise Java Beans 3.0 and Sun Certified Business Component Developer (SCBCD) certification here.
The book by Robert C. Martin “Clean Code” is a really very useful material for all programmers. You can disagree with him sometimes (as I do), but it still provides some useful notes that helped me to place my programmer experience and knowledge where they belong. Here I’ll provide some key notes that I did while reading the book and that I’d like to memorize.
What you as a programmer SHOULD DO:
1. Give everything meaningful, searchable and pronounceable names.
2. Use names that other programmers can understand. Remember that the code is for programmers, not customers.
3. Functions should be small and smaller than that.
4. Functions should do one thing. They should do it well. They should do it only.
5. The ideal number of arguments for a function is zero. Max 3.
6. Prefer exceptions to returning error codes. Create informative error messages.
7. Prefer meaningful names to comments.
8. Write JavaDoc only for public API’s.
9. Format your code. While working in a team use a single formatting style.
10. Test your code with clean tests. Remember that test code is just as important as production code.
11. Test just one concept per test.
12. Test should follow the FIRST rule:
Fast - tests should be fast.
Independent - tests should not depend on each other
Repeatable - tests should be repeatable in any environment
Self-validating – the test should have a boolean output
Timely - write unit tests before the production code.
What you as a programmer should AVOID:
1. Avoid encoding
2. Avoid duplicate code. Watch out not only duplicate functions but also duplicate functionality.
3. Avoid noisy, redundant, uninformative comments. Remember that one of the more common motivations for writing comments is bad code.
