Saturday, September 29, 2007

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

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