Should a parent class reference child classes?

Question

Good morning,

I have inherited some inherited code at work, and it uses a rather unusual design pattern. The only link I could find on the forums in a similar fashion was here . The situation is that the original designer has a generic parent class (not abstract) that has a static factory method that directly references child classes.

Here is an example of this coding style found in several places in legacy code:

public static LoggerFactory getLoggerFactory(LogType type) { switch (type) { case LOG4J: return Log4JLoggerFactory.getInstance(); case LOGBACK: return LogBackLoggerFactory.getInstance(); default: throw new RuntimeException("No logger factory defined for type " + type); } } 

Where Log4JLoggerFactory and LogBackLoggerFactory extend LoggerFactory.

It seems to me that this is really alien to me, but before I significantly changed the code, is there any purpose or benefit for this design template (is there even a formal name for it)?

Any thoughts or advice are appreciated. Thanks!

EDIT: after reading Yishai's answer, I thought for a link to a Wikipedia article on a strategy template . Thank you all for your answers!

Answer 1

This is a very standard template in Java and a general way to implement a strategy template. You see it in the standard API all the time (calendar vs GregorianCalendar, NumberFormat vs DecimalFormat, etc.).

Given that with Injection Dependency all the rage, such a template can really be replaced by a dedicated Factory class with a dedicated Factory interface, but in the absence of a larger constructive reason, I believe that the example you give is a very reasonable design choice.

Answer 2

This is good practice and is called the Factory Method .

The advantage is that you return some specific implementation, but hide it through a common interface or base class. Thus, the client is not concerned about implementation details, but works with the most basic class.

Answer 3

Maybe they configured this way to use Log4J in one environment and Logback in another? I know that sometimes developers prefer the tool for local development, but when the time comes for deployment, you need to use everything that the company approves / approves of.

Answer 4

Whether this is good or bad practice depends on the situation. For example, when the “parent” knows how to create all the children, this can be good practice. When the parent does not know this, this solution will only cause problems.

Testability is another problem: if a parent has many children, it can be difficult to create a parent separately from the children, but again it depends.