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

Comments

Popular posts from this blog

Running your own one person company

Recently there was a post on PuneTech on mom's re-entering the IT work force after a break. Two of the biggest concerns mentioned were : Coping with vast advances (changes) in the IT landscape Balancing work and family responsibilities Since I have been running a one person company for a good amount of time, I suggested that as an option. In this post I will discuss various aspects of running a one person company. Advantages: You have full control of your time. You can choose to spend as much or as little time as you would like. There is also a good chance that you will be able to decide when you want to spend that time. You get to work on something that you enjoy doing. Tremendous work satisfaction. You have the option of working from home. Disadvantages: It can take a little while for the work to get set, so you may not be able to see revenues for some time. It takes a huge amount of discipline to work without a boss, and without deadlines. You will not get the benefits (insuran

Some thoughts on redesigning education

Some time back I read a blog post on redesigning education. It asked some very good questions. Stuff which I had been thinking of myself. I left my thoughts on the blog, but I would also like to start a conversation around these ideas with those who read this blog as well. I would like to know what other people think of the issue of redesigning (college) education. I have often thought about how college education can be improved. To answer this question, we first have to ask a very basic question. What is the purpose of education? To me, we need education for 3 things: To learn more about the world around us To lead positive constructive lives To earn a good living / fulfill our ambitions I think education has to a large extent evolved to fulfill #3 (with a bias towards earning a comfortable living). The semester system, along with multiple choice tests, and grading, has made our education system into an assembly line. Students are pushed into the assembly line, given classes, admini

Testing Groovy domain classes

If you are trying to test Grails domain class constraints by putting your unit test cases in the 'test/unit' directory, then your tests will fail because the domain objects will not have the 'valdate' method. This can be resolved in two ways: Place the test cases inside test/integration (which will slow things down) Use the method 'mockForConstraintsTests(Trail)' to create mock method in your domain class and continue writing your test cases in 'test/unit' What follows is some example code around this finding. I am working on a Groovy on Grails project for a website to help programmers keep up and refresh their skills. I started with some domain classes and then moved on to write some unit tests. When we create a Grails project using grails create-app , it creates several directories, one of which is a directory called 'test' for holding unit tests. This directory contains two directories, 'unit', and 'integration' for uni