Objects in Java

1
Objects in Java

• Key concepts and metaphors
• Implicit subclassing
• Static methods
• Constructors
• Instance variables
• Dangers of uninitialised object references

2
Metaphors - inheritance

• A small UK company (Watsonian Squire Ltd) imports
  Royal Enfield motorcycles from India where they
  are still in production
• The basic design has not changed substantially
  since 1949!
• The UK company does two things
• checks the imported bikes and sells them on
  directly
• or modifies the bikes into Clubman and
  Trailie styles by changing some components

3
Metaphors - inheritance

• The company obtains an instance of a basic
  motorcycle superclass object and can use it
  directly
• or it can subclass the object and override a
  feature by providing a new or modified part or
  function
• This would not be possible without the
  cooperation of the Indian factory. The style of
  the motorcycle is not final.

4
What about a simple object?

• To subclass we do this
• public class Spoon extends Cutlery ..
• But what does this do?
• public class TaxCalcLib
• Any class definition with no explicit extends
  clause is inheriting from java.lang.Object
• This provides the bare minimum framework for a
  class to function

5
Structure of a simple class

• import java.net.
• public class HostName
• public static void main(String args)
• InetAddress iaLocal
• try
• iaLocal InetAddress.getLocalHost()
• System.out.println(Name"
  iaLocal.getHostName())
• catch (UnknownHostException e)
• System.err.println("Don't know about local
  host")
• System.exit(0)
• // end of class HostName

6
Static methods

• Note that the example program is part of an
  object called HostName
• Thus the filename must be HostName.java
• But how does the object get instantiated?
• Surprise - it doesnt!
• The example uses a static method (code thats
  part of the class, not the object)

7
Object instantiation

• public class Egg
• private String myColour
• public Egg(String sEggColour) // NB no return
  type!
• myColour sEggColour
• public Egg() // NB no return type!
• myColour Orange
• public static void main(String args)
• Egg e1 new Egg(Green) // create some
  objects
• Egg e2 new Egg() // what colour is this?
• Egg e new Egg6 // no objects created
  here!
• // end of class Egg

8
Constructors

• A constructor is an initialisation method
• It runs when a new object is created
• A class may define many object constructors
• Each constructor differs only in its input
  parameters
• All methods can have multiple flavours
• However, unlike other methods, constructors have
  no return type

9

• public class Egg
• private String myColour // each instance gets
  its own copy
• public Egg(String sEggColour)
• myColour sEggColour
• public Egg()
• myColour "Orange"
• public void reportColour()
• System.out.println(myColour)
• public static void main(String args) // static
  all eggs share this
• Egg e1 new Egg("Green") // green egg
• Egg e2 new Egg() // orange egg
• Egg e new Egg6 // six references to
  eggs
• //
• e1.reportColour() // should be green
• e2.reportColour() // should be orange
• e0.reportColour() // runtime error no eggs
  here yet

10
Comment on the examples

• The idea of using a static method of a class to
  instantiate objects of the same type is common
  practice (e.g. in small programs, for testing)
• It seems counter-intuitive to some people
• A clear separation between the concepts of
• static, class-based code and
• dynamic, object-based code
• helps us gain a better perspective

11
Conclusion

• We have seen
• some of the concepts and language associated with
  object-orientation
• some practical issues of program structure,
  including the use of static methods
• constructors and their use