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The Pitfalls of Inheritance

In Object Oriented Programming, inheritance is a commonly used mechanism to model the relationship between two types. However, modelling such relationship without realizing the impact on the overall application may result in unexpected problems. In this article, I will try to present some internal details of inheritance mechanism based on the Java language and highlight some of the problems that a developers should be aware of when using inheritance of types.

Inheritance Defined…

One of the first questions that we should answer is what do we mean by inheritance. The inheritance is a modelling technique of expressing the relationship between a generalized type and a specialized type. This in essence is a way of expressing IS A relationship. For example, we can say Car IS A Vehicle. In such situations, we will say Car inherits from Vehicle. In the world of Java language, this inheritance relationship is expressed with the following semantics.

class Car extends Vehicle {

In this model, Car is a subclass of the Vehicle super class. Now the next question is what we inherit. By default, the subclass inherits all the interfaces and the implementations from the super class. Note that, the use of the word "interface" in this context does not refer to the interface type specific to the Java language. In OO, the "interface" is a function/method that a particular class exposes to the external world. Coming specific to Java, the inheritance relationship guarantees the following:

• The sub class inherits all the public methods and the implementation in the super class.
• The sub class inherits all the protected methods and their implementation in the super class.
• The sub class also inherits all the public and protected member variables from the super class.
• However, constructors are not part of this inheritance model. We shall also see later the invocation relationship of constructors that exists between a sub class and its immediate super class.

These consequences of inheritance also imply the following things that are vital to how we can use a super class and any sub class of it. In general, when you model and implement an inheritance relationship, you guarantee the following:

• The sub class is capable of accepting all the messages that the super class accepts.
• The sub class can replace the super class anywhere the super class is called for without affecting the final outcome.

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