Wednesday, 28 June 2017

Tell, don't ask

More than twelve years ago Tim Joyce passed on some programming wisdom:

With programs tell don't ask, vice versa for people.

This was a bit abstract for me at the time but last night it came back to me as what is wrong with the code I am currently working on. We store our application configuration in a table in the system's target database and whenever some configuration is needed it is looked up in the database. There was no problem with this approach when the code was written because JUnit had not been invented and testing was not the main part of our discipline. However to write a test we would need a database present, which is an obstacle to fast, distinct, unit tests and has been a blocker to writing tests.

Noncompliant Code Example

public class Example { 
  private String path;
  public void logPath() {
    try {
      path = CachedSystemParameter.getInstance().
                 getParameterValue("PATH");
    } catch (SystemParameterException e) {
      logger.error("[BUSINESS] Error while retrieving system parameter PATH", e);
    }
    logger.info("Path: " + path);
  }
}

Compliant Code Example

By adding sftpPath to the class constructor we can test the business logic without the need for a database fixture.
public class Example { 

  private String path;

  public Example() { 
    this(CachedSystemParameter.getInstance().
                 getParameterValue("PATH"));
  }

  public Example(String path) { 
    this.path = path;
  } 

  public void logPath() {
    logger.info("Path: " + path);
  }
}

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