Inheritance

When developing software applications the use of inheritance can greatly reduce development times by allowing the reuse of code with little or no modification by building on classes that are already defined and available to the developer.  This is achieved by forming a new class, known as the derived class, which takes over the attributes of a pre existing class which is known as the base class.   The question is how does this actually assist the developers to build a project more efficiently?

Consider a class called Person that contains a person’s name, address, date of birth, sex, and phone number.  Now consider a class called Employee that will contain employee number, salary, holiday entitlement, employee name, address, date of birth, sex and phone number.  The employee class contains the same details as the person class with the addition of the employee number, salary and holiday entitlement.   Rather than creating the employee class from scratch the person class can be used as base to build on.  The employee class will inherit the pre defined attributes from the person class and the developer can add the employee number, salary and holiday entitlement to it. 

My Findings
The main issue I have come across when taking over pre existing projects is that people lose sight of which class the methods and properties should be located.  On simple class definitions like above it is easy to see why the person class has the attributes it does and why the employee class inherits the person class and adds the extra attributes.  However on larger more complex project this can become more difficult especially when the requirements of a project are constantly changing.  When I have picked up project like this making the changes to the classes to rectify these miss placed attributes can be time consuming activity but I have found that it greatly reduces development time in the future due to there being less confusion and a greater understanding of how the classes work together.

What I would like to know is if this lack of sight is a common development issue or have I been unfortunate with the projects that I have inherited from other development teams?  Your experiences of this kind of issue would interest me greatly.