Skip to main content

Inheritance and composition


  • Inheritance is used for
    • Code resuse
    • To support polymorphism
  • Disadvantages of inheritance
    • Difficult to change the interaface of the subclass
    • Superclass is always instantiated
    • Can get complex and unmanageable
  • Composition is used for code reuse
  • Disadvantages of composition
    • We cannot use polymorphism
    • We cannot extend the system by adding subclasses
    • May have a slight performance overhead
  • Usage
    • Inheritance: IS- A
    • Composition: HAS - A

Example 1:

public class Car {
  private Engine engine;
  public void start() {}
  public void stop() {}
}

public class SUV extends Car{
  public void start() {} //overrides the start method from Car

  public void fourByFourMode() {}
}



Example 2:
public void someMethod() {
  Car c = new SUV();
  c.start();
}



Example 3:
public class Car {
  private Engine engine;
  public void start() {
    engine.start();
    //maybe do something more
  }

  public void stop() {}
}
Java2html

I will end this post with the first paragraph from Item 17, of Joshua Bloch's excellent book, Effective Java. In the book this paragraph is followed by a code example that explains this very well, however, I am not sure if I can include the example under fair use. Crying..

Inheritance is a powerful way to achieve code reuse, but it is not always the best tool for
the job. Used inappropriately, it leads to fragile software. It is safe to use inheritance within
a package, where the subclass and the superclass implementation are under the control of the
same programmers. It is also safe to use inheritance when extending classes specifically
designed and documented for extension (Item 15). Inheriting from ordinary concrete classes
across package boundaries, however, is dangerous. As a reminder, this book uses the word
“inheritance” to mean implementation inheritance (when one class extends another).
The problems discussed in this item do not apply to interface inheritance (when a class
implements an interface or where one interface extends another).

Can you think of a situation where subclassing an existing class can break the semantics of your code, because of a certain undocumented feature of the superclass? Let me give you a hint. Assume a collection class has 2 methods

add(Object o) - to add the specified object to the collection

addAll(Collection c) - to add each element of the specified Collection to this collection.

Now assume that addAll(Collection c) internally invokes add(Object o) for every object in the collection.

Can you think of a situation that will result in a bug, if we subclass this class and override the above methods to add a functionality that keeps track of how many elements have been added to the collection?

Resources:

  1. Compostion vs. Inheritance
  2. Composition and interfaces

Note: This text was originally posted on my earlier blog at http://www.adaptivelearningonline.net

-----
COMMENT:
AUTHOR: Josh Bloch
EMAIL:
IP: 71.202.105.255
URL:
DATE: 09/29/2007 10:22:58 PM
Yes, you can include the example under fair use:)

Josh
-----
COMMENT:
AUTHOR: Parag
DATE: 10/02/2007 09:17:55 AM
Thanks a lot Josh. I will include the example in the next post.

--
Regards
Parag
Labels:

Comments

Post a Comment

Popular posts from this blog

Fuctional Programming Principles in Scala - Getting Started

Sometime back I registered for the Functional Programming Principles in Scala , on Coursera. I have been meaning to learn Scala from a while, but have been putting it on the back burner because of other commitments. But  when I saw this course being offered by Martin Odersky, on Coursera , I just had to enroll in it. This course is a 7 week course. I will blog my learning experience and notes here for the next seven weeks (well actually six, since the course started on Sept 18th). The first step was to install the required tools: JDK - Since this is my work machine, I already have a couple of JDK's installed SBT - SBT is the Scala Build Tool. Even though I have not looked into it in detail, it seems like a replacement for Maven. I am sure we will use it for several things, however upto now I only know about two uses for it - to submit assignments (which must be a feature added by the course team), and to start the Scala console. Installed sbt from here , and added the path
1 comment
Read more

Five Reasons Why Your Product Needs an Awesome User Guide

Photo Credit: Peter Merholz ( Creative Commons 2.0 SA License ) A user guide is essentially a book-length document containing instructions for installing, using or troubleshooting a hardware or software product. A user guide can be very brief - for example, only 10 or 20 pages or it can be a full-length book of 200 pages or more. -- prismnet.com As engineers, we give a lot of importance to product design, architecture, code quality, and UX. However, when it comes to the user manual, we often only manage to pay lip service. This is not good. A usable manual is as important as usable software because it is the first line of help for the user and the first line of customer service for the organization. Any organization that prides itself on great customer service must have an awesome user manual for the product. In the spirit of listicles - here are at least five reasons why you should have an awesome user manual! Enhance User Satisfaction In my fourteen years as a
Post a Comment
Read more

Inheritance vs. composition depending on how much is same and how much differs

I am reading the excellent Django book right now. In the 4th chapter on Django templates , there is an example of includes and inheritance in Django templates. Without going into details about Django templates, the include is very similar to composition where we can include the text of another template for evaluation. Inheritance in Django templates works in a way similar to object inheritance. Django templates can specify certain blocks which can be redefined in subtemplates. The subtemplates use the rest of the parent template as is. Now we have all learned that inheritance is used when we have a is-a relationship between classes, and composition is used when we have a contains-a relationship. This is absolutely right, but while reading about Django templates, I just realized another pattern in these relationships. This is really simple and perhaps many of you may have already have had this insight... We use inheritance when we want to allow reuse of the bulk of one object in other
3 comments
Read more