Inheritance in Java using Robots

1
Inheritance in Java using Robots

• ACSE 2003 Presentation
• Michael Devoy, St. David C.S.S., Waterloo
• mdevoy_at_look.ca

2
Objectives

• Use Robots to
• understand inheritance in Java
• use instance variables
• overload methods (including the constructor)
• override inherited methods
• use arrays

3
Robot Inheritance

• Use inheritance to create a new type of robot
  with additional abilities (smarter robots)
• the child class inherits all the attributes and
  behaviours of the parent class, except the
  constructor
• Java uses the keyword extends to indicate
  inheritance
• benefits
• reduce the amount of code we need to write
• make our program easier to understand
• facilitate the re-use of code
• aid in debugging our program

4
The Problem - the long relay

• Similar to yesterdays problem, but longer, would
  be tedious to code

5
Pattern to define a Robot child class

• import becker.robots.
• public class ltltClassNamegtgt extends Robot
• // define the new Robots constructor method
• public ltltClassNamegtgt (City city, int ave, int
  str, int dir,
• int numThings)
• super(city, ave, str, dir, numThings)
• ltlt other new methodsgtgt

ClassName is the name of the new Robot class
Super refers to the parent, in this situation the
parents constructor is called
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Pattern for new Method
Access may be public (available to any class in
your program) or private (available only to
objects of this class)

• public ltltreturnTypegtgt ltltmethodNamegtgt(ltltparameterLi
  stgtgt)
• ltlt list of statements to executegtgt

For Example public int countThings() int
count 0 while (this.isBesideThing())
this.pickThing() count for
(int i0 i lt count i) this.putThing()
return count
returnType may be any Java primitive variable
type (int, boolean, etc) or class, which is
returned by the method, or void
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Overloading Methods

• Overloading a method means to create a new method
  with the same name as an existing method, but
  with a different list of parameters.
• Example
• move() is a method in the Robot class
• define a new method move(int steps) that has a
  parameter that allows us to dictate how many
  steps to move
• we say the move method is now overloaded
• note we can invoke either of the move methods we
  wish, because Java distinguishes between them by
  the parameter list

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Hands on Inheritance

• Start Ready to Program
• Open RobotRelay2.java
• this is the application class with the main
  method
• Open SmarterRobot.java
• this is the child class
• it already contains a constructor and a helper
  (private) method named turnRight()
• Create the methods stepUp(), stepDown() and
  move(int steps)
• Run the RobotRelay2 program
• Overload the constructor, to set the number of
  Things in the SmarterRobots backpack to 0
• Use this constructor in RobotRelay2.java

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Pattern for Instance Variables

• An instance variable is a variable defined in a
  class which is global to all methods in the class
• Each object (instance) created from the class has
  its own copy of the instance variables
• Instance variables should be declared private

private ltltvariableTypegtgt ltltvariableNamegtgt
ltltinitialValuegtgt

• variableType may be any primitive type of
  variable or a class
• initialValue is optional
• instance variables are often initialized in the
  constructor method
• instance variables values are controlled by
  calls to public methods, ensuring encapsulation

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Overriding Methods

• Overriding methods replace the definition of an
  existing method, inherited from a parent class
• This is accomplished by writing a method
  definition with an identical method signature to
  the method signature in the parent class
• method signature is the method name and parameter
  list
• you can call the parents method, if needed, by
  use of the super keyword

For Example public void move()
stepsTaken // stepsTaken is a private
instance variable super.move()
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Hands on Instance Variables

• Start Ready to Program
• Open WanderingRobot.java
• this is your application class with a main method
• Create a new class which extends RobotSE
• create an instance variable named stepsTaken, as
  an int
• create a constructor (refer back to SmarterRobot)
• override the move method to add 1 to stepsTaken
• add an accessor method called getStepsTaken()
  which returns the value stored in stepsTaken
• create a method named resetStepsTaken() which
  sets the value of stepsTaken to 0
• Run the program

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Arrays

• Java supports arrays of any number of dimensions
• declare a reference to an array so Java knows its
  type, name and dimensions

int age String name Robot karel
double distances

• Instantiate the array, so Java allocates RAM for
  the number of entries

age new int12 name new String5 karel
new Robot 2 distances new double5 7//
note this is an //
array of arrays
//and need not be square
Continued ?
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Arrays

• Initialize each element of the array
• In the case of primitive variable types, assign
  values
• In the case of objects, invoke the constructor
• note array elements are numbered starting at zero

for (int x0 x lt age.length x) agex 0
karel0 new Robot (waterloo, 1, 5,
Directions.EAST, 0) karel1 new Robot
(waterloo, 2, 5, Directions.WEST, 0)
My Notes on arrays
Suns Notes on Arrays
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Hands on Arrays

• Start Ready to Program
• Open WanderingRobot.java
• this is your application class with a main method
• create an array of 5 SmarterRobots
• instantiate each SmarterRobot at a different
  location
• modify the program so each different SmarterRobot
  makes a move in turn
• Run the program

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Resources

• Becker Robots (Documentation Downloads)
• http//www.math.uwaterloo.ca/7Ebwbecker/robo
  ts/
• Introductory Course using Robots for Students
• http//csis.pace.edu/bergin/KarelJava2ed/Kar
  elJavaEdition.html
• Sun Java Class Documentation
• http//java.sun.com/j2se/1.4.2/docs/api/index
  .html
• Sun Java Tutorials
• http//developer.java.sun.com/developer/onlin
  eTraining/
• My ICS4M course
• http//webhome.idirect.com/mdevoy/ics4mi