Transcript BIT 143: C++ Programming: Data Structures

Instructor: Craig Duckett
[email protected]
Lecture 2 Announcements
Home Computers and/or Laptops/Notebooks
By now you should have installed Java SE 8 and jGRASP on your computers, set up the
becker.jar file, and started working with some of the supplied .java files from Lecture 1.
• How many have done this successfully?
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How many have tried to do this and have still not gotten jGRASP to compile and run? If
you have your laptops with you, I will stick around for a bit after today's class to see if
we can get you up and running. I'll also stick around for a bit after next Monday's class.
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I've added a TROUBLESHOOTING walk-through page to the BIT115 website if you are
getting the jGRASP wedge2 PATH error. This should be fixed with the latest version of
jGRASP, but just in case…
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IMPORTANT! Always download the .java files from the web site, save them to a local
folder, and run them from there. If you try to run them directly off the site, they go in the
browser cache and get renamed, and then things won't work!
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There are other issues to remember, which we'll cover in today's class.
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NOTE: It is essential that you get jGRASP working on your home machines or laptops if
you are going to be successful in this class (unless you are going to do all your coding on
the PCs here at the college).
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Lecture 2 Announcements
Attention: Windows 8 Users
If you are having trouble getting the software to install, one way to bring
compatibility to install the Java SE JDK and jGrasp, etc, is to download and
install a program called Classic Shell:
http://download.cnet.com/Classic-Shell/3000-2072_4-75553853.html?tag=mncol;1
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It's okay if right now you are feeling
somewhat overwhelmed, nervous,
confused, or just scared spitless!
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Hey! You're learning a new language (Java) and a new way of thinking
about things, so it is perfectly natural if you're feeling a bit
overwhelmed right now, or a bit nervous, or even a bit confused.
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Knowing this, we'll be taking baby steps for the next few lectures. I'll be
holding your hand and walking you through Java and code step-by-step.
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So: Do not be afraid to ask questions!
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And: Do not be afraid to come see me during my office hours (or by
special appointment).
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Also: Once I know the exact days, times, and places, going to the Open
Labs for additional one-on-one help is greatly encouraged!
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StudentTracker
Reminder: You Will Need to Create an Account in
Order to Submit Your Assignments
http://ccc.panitzco.com/mpanitz/StudentTracker
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Some Sites/Links of Interest
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Udacity – Free Online Courses
MIT OpenCourseWare – Free Online Courses
Coursera – Free Online Courses
Standford Engineering Everywhere – Online CS Courses
Academic Earth – Online CS Courses
Khan Academy – Free Online Tutorials
Code Academy – Interactive Code Learning
Code School – Interactive Code Learning
CoderDoJo – Code Club Community
Code.org – Code Club Community
Girls Who Code – Code Club Community
Girl Develop It – Code Club Community
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Lecture 2 Announcements
Reading Assignment for Today
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Chapter 1.5: Types of Errors
Chapter 1.4.5 – Tracing Code
Note: there are ‘BIT 115 Student Open Source Guides’ that students wrote from previous
quarters that may deal with the various topics of your readings and lessons.
This is a great tool, and highly recommended!
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The Becker Robot and Java Libraries
Finding and Fixing Errors
Setting up a City
Tracing a Program
Extending a Class (Introduction; we’ll cover again on Monday)
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Quarter-Long Game Plan:
• Up until the Mid-Term:
– 1st part of the quarter
Overview of topics, conceptual understanding, not as much freeform problem-solving
• After the Mid-Term:
– 2nd part of the quarter
Focus on learning and more problem-solving on your own
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Save A Copy Of All Work
(Highly recommended!)
• In-Class Exercises, Exams, Quizzes, etc.
– If you work with someone, make sure each of you gets a copy
– For In-Class Exercises, you do not have to turn it in
• (I’ll be checking it as I walk around and watch you work)
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If you lose track of it, and I unintentionally record it incorrectly in
StudentTracker, then whatever I’ve got will stand as the grade. Best then to
save your work for proof, if needed, in the event of a discrepancy. If you do
find a grading error in StudentTracker, then please don't hesitate to let me
know!
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Quizzes and Exams
• Starting next lecture, you should assume that the quizzes and
exams will all be 100% pen/pencil-and-paper.
You bring the pen or pencil, I will supply the paper
– Start Practicing Now (My Mantra: Practice! Practice! Practice!)
• Try writing code from memory,
• and/or using Notepad or NotePad++ and then copy-and-pasting into
jGRASP to check your work
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H Assignment 1 is due LECTURE 4
th by midnight
Wednesday,
July
15
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– It’s posted on the website under Assignments
– It will be due by midnight on day of Lecture 4
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• If you are confused, bring your work to class, in electronic form, and
we will go over how to hand in (upload) the homework:
Student Tracker
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(We’ll go over this now with “Rex Winkus”)
How to Use Student Tracker (Instructions)
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stuck, seek help!
R • If–you’re
Talk to the Instructor or a classmate or Open Lab helper
– Email me
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BIT 115: Introduction To Programming
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Assignment 1
• By now, you should all have jGrasp installed at home,
along with the Java JDK, and the becker.jar mapped
properly in jGrasp.
– It’s easy to get stuck at home, and then have to finish
your work at school
– Remember for future assignments: The building is
closed on Sundays
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H A Quick "Peek" at Assignment 1
O Let's Have a Quick Look at Assignment 1
M (so there are no unforeseen surprises down the road!)
1 is due: Monday, July 14 (uploaded to StudentTracker by
E Assignment
midnight)
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What You Need To Turn In to StudentTracker for A1
A1_Part_1.java
.java file only (no .class)
A1_Part_2.doc
MS Word .doc
A1_Part_3.doc
MS Word .doc
A1_Part_4.doc
MS Word .doc
A1_Part_5.java
.java file only (no .class)
(You'll create this from Starting_Template.java file)
Personal Data Sheet (Optional)
MS Word .doc
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Assignment Announcements
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Assignment 1 DUE Lecture 4, Wednesday, July 15th, by midnight
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Assignment 2 DUE Lecture 7, Monday, July 27th, by midnight
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Assignment 1 Revision DUE Lecture 8, Wednesday, July 29th, by midnight
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Assignment 2 Revision DUE Lecture 10, Wednesday, August 5th, by midnight
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Assignment 3 DUE Lecture 11, Monday, August 10th, by midnight
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Assignment 3 Revision DUE Lecture 13, Monday, August 17th, by midnight
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Assignment 4 DUE Lecture 15, Monday, August 24th, by midnight
NO REVISION AVAILABLE
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Extra Credit 01 DUE Lecture 15, Monday, August 24th, by midnight
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And now …. The Quiz!
NO QUIZ TODAY
We will have our very first 'warm-up' Quiz starting next Monday, June 13th
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Today’s Topics
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The Becker Robot Library
The Java Library
Errors: How to find and fix them
Important Terms (like Class, Object, Method, Parameter, etc)
Dissecting the Code: What it means and what it does
How to set up a City
Tracing Code
(Predicting What A Program Will Do BEFORE you Run It)
• Extending a Class (Introduction)
– We will go over this again on Monday, June 13th
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The Becker Robot Library
Direct Link: http://www.learningwithrobots.com/doc/
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The Java Library
Direct Link: http://docs.oracle.com/javase/1.5.0/docs/api/index.html
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Three Types of Errors
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Chapter 1.5: Types of Errors
3 Different General Categories of Errors:
 Compile-Time Errors (Syntax Errors)
 Run-Time Errors (Application Errors)
 Intent (Logical) Errors
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Compile-Time Errors (Syntax Errors)
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Anything that goes wrong when you compile the file is a compile-time error!
Point out that the Output Panel is often small, but it can be resized, and you should get used to
figuring out which line the error pertains to.
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Java is CASE SENSITIVE
- Java is different from java, is different from JaVa
- This means you must type in names, etc, EXACTLY the same
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File name MUST be the same as the class name
You need the import becker.robots.*; at the top of each robot file
Forgetting to Compile BEFORE Running
- You should compile the file EVERY SINGLE TIME you change it
- You should also wait UNTIL the compilation finishes BEFORE trying to run it
Strategy For Fixing Compile-Time Errors:
Follow the debugging strategy (listed in your text) to find and correct the syntax errors in the
FindErrors.java program later this evening when you do your In-Class Exercises.
1. Compile the program to get a list of errors;
2. Fix the most obvious errors, beginning with the first error reported
3. Compile the program again to get a revised list of the remaining errors.
Run-Time Errors (Crashing Errors)
Anything that causes the program to crash while it's running. If you encounter a run-time error
when you are compiling and running your Java programs, then this would be a jGrasp problem, or
a Windows Operating System problem, and NOT a Java Problem. The Java language does a lot to
protect you from this type of error, so you shouldn't see many of these.
Intent Errors (Logical Errors)
The program compiles and runs without crashing, but it doesn't do what you want it to.
Example:
- Robot takes an extra leftTurn, and runs off the screen
- Robot doesn't pick up a Thing when it's supposed to.
- Robot collides with a Wall (the book calls this a run-time error, but it technically isn’t)
As you continue learning to code you'll make fewer "typo" type mistakes, and end up with more
Intent errors then Compile-Time errors.
You need to come up with a personal strategy to figure out where the Intent problems are, and
how to fix them. One Option: Use a Program Debug Table (show where this is on web site)
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Lecture 2 ICE:
Finding And Fixing Errors In Your Program
• In-Class Exercise Directions 2.1
• FindErrors.java
Program Debug Table
We'll Wait and Do All the ICEs
Together at the End of Lecture
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Important Terms to Remember
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Some Important Terms To Remember
A class is a set of functions that can work together to accomplish a task. It can contain or manipulate data,
but it usually
does so according to a pattern rather than a specific implementation. An instance of a class is
Under
considered an object. Until an object is instantiated from a class, the class can’t actually do anything.
Course Wide Information
An object receives all of the characteristics of a class, including all of its default data and any actions that can
be performed by its functions. The object is for use with specific data or to accomplish particular tasks.
A method simply refers to a function that is encased in a class. It usually entails an action.
A parameter is a variable that is passed into a function that instructs it how to act or gives it information to
process. Parameters are also sometimes called arguments.
A field or property is a default set of data stored in a class. A class can have multiple properties and can
be changed dynamically through the methods of the class. These are sometimes called attributes.
Inheritance is one of the keys that make OOP tick. Simply put, classes can inherit methods and fields
from other classes by extending them and each class can be extended by multiple classes. This means that
you can start with a base (or parent) class that contains shared characteristics among several classes. That
base class can then be extended by other classes (children) that are similar but are meant for slightly
different purposes. Any changes in the parent class will automatically cascade to its children.
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Class (The “Idea” of Features and Functions)
A class is a collection of functions that can work together to accomplish a task. It can contain or manipulate
data, but
it usually does so according to a pattern rather than a specific implementation. An instance of a
Under
class is considered an object. Until an object is instantiated from a class, the class can’t actually do anything.
Course Wide Information
Example
The Robot class. The Robot class contains all the features and functions that a robot might
have once it is created (or instantiated) as an object. Until then, all the Robot features and
functions are just the idea how a robot might work, but being an idea the class doesn’t do any
work itself..
http://www.learningwithrobots.com/doc/
Just like we might have an idea how a car should look and operate—body, engine, four tires,
steering wheel, gas and brake pedals, etc—an idea of a car is not the same of an actual
working car. Here the idea of a car is a class, and an actual working car is an object.
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"Brakes"
"Engine"
"Lights"
"Transmission"
"Wheels"
"Chassis"
"Seats"
"Power"
"Car" Class
"Car" Object
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Object
(An Instance of the “Idea” Made Actionable)
An object receives all of the characteristics of a class, including all of its default data and any actions that
Under by its functions. The object is for use with specific data or to accomplish particular tasks.
can be performed
Course Wide Information
Example
When we create an object from the Robot class we do so by creating a named instance of the
Robot class that will possess all the features and functions that were outlined as belonging to
the Robot class as an idea. Once the object is instantiated, what was once the idea of a Robot
now becomes an actual Robot that possesses specific ‘Robot’ properties (its shape, size, color,
speed) and that can perform specific ‘Robot’ actions (like move, turn left, put down a thing,
pick up a thing, etc).
http://faculty.cascadia.edu/cduckett/bit115/Becker/Documentation/index.html
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Method
(An “Action” That Might Be Performed by the Object)
A method simply refers to a function that is encased in a class. It usually entails an action.
Example
After we have created a named object from the Robot class that possesses all the properties
(or attributes) belonging to robots, the robot can now perform some kind of action or actions
(like move, turn left, put down a thing, pick up a thing, etc). These actions are the methods
belonging to the Robot classes, and which an instance of the Robot class can perform. A robot
can move because the Robot class has a method called move() which will allow the robot to
move one space forward each time it is called upon to do so.
Although we might want the robot to mix a martini, the robot can not because the Robot class
does not contain a mixAMartini() method. Later on in this course we will learn how we might
make a new kind of class that could contain a mixAMartini() method, but the Robot class we
are using now does not have just such a method.
http://faculty.cascadia.edu/cduckett/bit115/Becker/Documentation/index.html
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Parameter (“Information” or “Instruction” That Might Be Given)
A parameter is a variable that is passed into a function that instructs it how to act or gives it information
to process.
Parameters are also sometimes called arguments.
Under
Course Wide Information
Example
When we have a robot object call upon a method in order to perform some action, that
method may or may not take a parameter which is an additional piece of information or an
instruction that might extend its functionality.
Currently most of the methods belonging to the Robot class do not take any parameters
(although in a moment we’ll look at a Robot method that does take a parameter). For example,
the move() method when called upon will allow the robot to move one space forward. You’ll
notice that the move() method has two “empty” parentheses which represents that it doesn’t
take a parameter. Had the programmer of the Robot class designed the move() method
differently, then it might have taken a parameter to represent how many places the robot
should move. For example move(1) might make the Robot move one space forward, move(2)
might make the Robot move two spaces forward, move(3) might make the robot move three
spaces forward, and so on. Since the programmer did not design the move() method this way,
using the move() method like this will not work. This was only a theoretical example.
http://faculty.cascadia.edu/cduckett/bit115/Becker/Documentation/index.html
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Field or Property (The “Attributes” of Class and Its Objects)
A field or property is a default set of data stored in a class. A class can have multiple properties and can
Under
be changed
dynamically through the methods of the class. These are sometimes called attributes.
Course Wide Information
Example
When we create a named instance of a robot from the Robot class, this robot object comes
with a set of predefined properties and attributes. For example, by default the new robot
object is so many pixels high and so many pixels wide, its shape is an arrowhead, its color is
red, and its location and placement and direction in the City is defined by the parameters that
were passed to it (which we will discuss in the next Dissecting the Code section).
For example, one of the properties of the Robot class is that a robot will move one space
forward in .5 seconds. This speed, .5 seconds, is a pre-defined property of the Robot class and
any instances of objects made using the Robot class.
Now these pre-defined properties might be changed dynamically while the program is running
if just such a feature was developed programmatically to do so. In the case of the robot’s
default speed of .5 seconds for every move, there is a setSpeed() method which can be used to
change the robot’s initially defined speed. For example setSpeed(4) will move the Robot twice
as fast (the default speed is 2).
http://faculty.cascadia.edu/cduckett/bit115/Becker/Documentation/index.html
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Inheritance (Extending a Class’s Features to a New Class )
Inheritance is one of the keys that make OOP tick. Simply put, classes can inherit methods and fields
from other classes by extending them and each class can be extended by multiple classes. This means that
you canUnder
start with a base (or parent) class that contains shared characteristics among several classes. That
base class can then be extended by other classes (children) that are similar but are meant for slightly
different purposes. Any changes in the parent class will automatically cascade to its children.
Course Wide Information
Example
We will learn about inheritance and how to put it to good use next week. A brief example
what inheritance is can be explained by looking at our Robot class. Right now the Robot class
has a turnLeft() method but it doesn’t have a turnRight() method. If you created a Robot object
and wanted it to turn right then you would have to call upon it to turn left three times!
Now we might create a new type of Robot class (perhaps called MrRoboto) that would extend
all the features and functions of the Robot class (like move(), turnLeft(), etc) and also create its
own new set of methods (like turnAround(), turnRight(), etc). In this example we are using the
original Robot class to create our new MrRoboto class, and as such the MrRoboto class is
inheriting all the original features and functions of the Robot class. This is a great tool of
Object Oriented Programming (or OOP) because this means the programmer doesn’t have to
code all the Robot class features and functions from scratch to use in the new MrRoboto class.
All the programmer has to do is code the new methods (like turnAround() or turnRight()) since
the MrRoboto class inherited all the original methods (like move(), turnLeft(), etc).
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Dissecting the Code
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Dissecting the Code:
What It Means and What It Does (Quiz2.java)
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import becker.robots.*;
public class Quiz2 extends Object
{
public static void main(String[] args)
{
City toronto = new City();
Robot Jo = new Robot(toronto, 0, 3, Direction.NORTH, 0);
new
new
new
new
Thing(toronto, 2, 2);
Wall(toronto, 3, 3, Direction.EAST);
Wall(toronto, 3, 3, Direction.NORTH);
Wall(toronto, 3,3, Direction.SOUTH);
Jo.turnLeft();
}
}
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import becker.robots.*;
This imports from the becker.jar collection all the classes contained in the
robots file so they would be functionally available in Java (JDK) and any code
written using it. These classes include the Robot.class, the Thing.class, the
City.class, the Wall.class, and over a hundred more.
NOTE: Java loads some
classes by default into the
compiler, while others it does
not. Later on we will have to
import
additional
Java
classes in our code to get our
programs to work
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public class Quiz2 extends Object
public
class
Quiz2
extends
Object
'public' so other classes can use it.
a 'class' is a blueprint for making
'objects' that can do something
'Quiz2' is the name of the class. It can be any name.
The program uses this name when it runs to know
what 'objects' and 'methods' to use in the program.
Java, like other object-orientated languages, supports
class inheritance. Inheritance allows one class to
"inherit" the properties of another class. For example,
all Java objects are inherited from the java.lang.Object
class which is a fundamental part of Java and the Java
libraries. Our Quiz2 class is inheriting the properties of
Java's Object class, and so extends its actions and
attributes in the program hierarchy.
http://www.tutorialspoint.com/java/java_object_classes.htm
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Note: main is a method()
public static void main(String[ ] args)
The method is public because it is accessible to the JVM (Java Virtual Machine) to begin
execution of the program, and can be accessed by any other class or method.
The static keyword signifies the fact that this method can be invoked without creating an
instance of that class (an object). Main is called before any objects are made, hence static.
void signifies that this method does not return anything. In other words no new or altered
data is sent out from it for use by the rest of the program. Nothing is returned, so void.
Just like in the C “procedural” language and other languages, this is the main method in a
Java program. When you run a Java program, execution begins in the main method.
String is the data type that could be passed to the method, although no data type needs to
be passed. An example of a data type is an int or float. The [] represents unknown quantity.
args is the name of the parameter. We can pass any number of arguments to this method.
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City toronto = new City();
Names the object

Creates the new City object
We want to make a new instance of the City class (City.class) found in the becker > robots
directory (inside becker.jar) and call this new object Toronto
The City class contains all the attributes and actions necessary to set up a city when a named
city object (like toronto) is created, include shape, size, color, streets, avenues, etc.
By itself, the City class can't do anything. It's just a collections of ideas and concepts. You
need an object made from the City class (like toronto here) to actually do something with a
city.
NOTE:
In Java, '=' is an assignment operator, and points right to left; "==" is an equals sign.
EXAMPLE:
x = 1 assigns 1 to variable space x and x == 1 means x equals 1.
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import becker.robots.*;
public class Quiz2 extends Object
{
public static void main(String[] args)
{
City toronto = new City();
Robot Jo = new Robot(toronto, 0, 3, Direction.NORTH, 0);
new
new
new
new
Thing(toronto, 2, 2);
Wall(toronto, 3, 3, Direction.EAST);
Wall(toronto, 3, 3, Direction.NORTH);
Wall(toronto, 3,3, Direction.SOUTH);
Jo.turnLeft();
}
}
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Robot Jo = new Robot(toronto, 0, 3, Direction.NORTH, 0);
Names the Robot  Creates the new Robot using five parameters (arguments)
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Our instance of the Robot object is named Jo
Jo is placed in a City called Toronto
Jo is starting out on Street 0
Jo is starting out on Avenue 3
Jo is starting out facing North
Jo is starting out with 0 (zero) Things in its backpack
Now, another way to "construct" this is with only four parameters (arguments), by leaving out
the number of Things in the backpack
Robot Jo = new Robot(toronto, 0, 3, Direction.NORTH);
If you are not going to be picking up or putting down Things in your program, then you can
"construct" your Robot without this fifth 'backpack' parameter.
Later on, when we start creating our own types of Robots and methods, some of the ICES will
be set up in such a way that parts of the code will use five parameters and other parts of the
code will use four parameters and this will cause an error. We'll go over this in greater detail
when the time comes.
Look: http://www.learningwithrobots.com/doc/
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new
new
new
new
Thing(toronto, 2, 2);
Wall(toronto, 3, 3, Direction.EAST);
Wall(toronto, 3, 3, Direction.NORTH);
Wall(toronto, 3, 3, Direction.SOUTH);
These will work by default. For stuff that just sits there, we don’t have to actually
give them unique names (e.g., BrickWall) but we can’t talk about them in code,
however a Robot can pick up/put down an unnamed object. Why would you want
to give a wall a unique name?
If you look at the becker library you will discover that Wall actually extends Thing,
that is to say Wall has inherited all the initial properties of Thing
Jo.turnLeft();
turnLeft() is one of the methods of the Robot class, along with move(),
pickThing(), putThing(), frontIsClear(), countThingsInPackback(), and
several more. Because Robot can turn left, Jo can turn left.
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Something to Remember:
In parameter order, street comes first
and avenue comes second.
Streets go West-East (left-right)
Avenues go North-South (up-down)
A neat trick to remember the difference
is to recall that the 'A' and 'V' in Avenue
point up and down.
Lecture 2 ICE: Setting Up a City
• In-Class Exercise Directions 2.2
• Starting_Template.java
We'll Wait and Do All ICEs
Together at the End of Lecture
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Chapter 1.4.5 – Tracing Code
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PREVIEW: Extending a Class
This is important stuff, so we will be going over
it twice – once today as a “preview” and again
during next Monday’s lecture (Lecture 3) with
an accompanying ICE.
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Constructor
Here, when we create a new instance (an object) of the Robot class, a ‘hidden’ default
constructor works in the background to make sure that Kelsey inherits all the attributes and
methods available to Robots, including its placement on a particular Street and Avenue and
Direction in a particular City, and that it can use all of the actions (methods) available to the
Robot class (including move(), pickThing(), turnLeft(), putThing(), frontIsClear(), etc.)
http://www.learningwithrobots.com/doc/
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Constructor
Constructors have one purpose in life. To create an instance of a class. This
can also be called creating an object, as in:
The purpose of a method, by contrast is much more general. The purpose of a
method is to execute Java code, to allow the object to do something.
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Now … what if these is an action that you might want Kelsey to do that isn’t
found in the Robot class?
For instance, instead of invoking the the turnLeft() method three times, you
could just call up a turnRight() ?
The problem is, the Robot class does not have a turnRight() command
(method). The Robot class has been finalized. You cannot add to it.
The good news is, you can create a new method like turnRight() that will do
what you want the robot to do!
But in order to make this happen, you need to extend the Robot class …
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Extending a Class:
Where ClassB extends ClassA
In Plain Ol’ English: Where ClassB “inherits” the attributes and actions of ClassA
… then adds new functionality to them.
When we’re not interested in extending a class because we’re happy with the
methods that come with that class just the way they are, then we declare our class the
‘normal’ default way: public class Example extends Object
Object is the top class of all class hierarchies. When a new
instance of anything is made in Java, then it inherits all the
attributes and actions of the Object class. You can’t get a new
object without Object.
Object
Class
Hierarchy
However, if we want to add new functionality (methods) to the Robot class (like
turnRight) then we need to extend the Robot class (which is itself an extension of
Object)
public class MrRoboto extends Robot
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Extending a Class:
Where ClassB extends ClassA
Instance vs. Extension?
Instance creates a new object from a class, but extension extends a new
class from a class through inheritance, allowing for an improved class that
might offer additional attributes and services (methods) not available in the
original class …
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Extending the Robot Class
public class MrRoboto extends Robot
MrRoboto
extends
inherits
Robot
MrRoboto “inherits” all of
the Robot attributes and
services and then can have
additional attributes and
services of its own (i.e.,
those not shared by Robot).
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Superclass and Subclass
Robot
Superclass
MrRoboto
Subclass
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Constructor
import becker.robots.*;
public class MrRoboto extends Robot
{
public MrRoboto(City theCity, int street, int avenue, Direction aDirection)
{
super(theCity, street, avenue, aDirection);
}
//New service or services go here
}
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Constructor
import becker.robots.*;
public class MrRoboto extends Robot
{
// This declares the parameters used by Robot “inside” of MrRoboto
public MrRoboto(City theCity, int street, int avenue, Direction aDirection)
// This passes on information received by the parameters used by Robot ‘inside” of MrRoboto
{
super(theCity, street, avenue, aDirection); //Instead of Robot here, Java uses the keyword super
}
//New service or services go here
}
Constructors fulfill a special roll. They are responsible for ensuring an object is set up properly when
it is created, and that it can be immediately used once it is created. This construction process is
known as initialization. Two other details about constructors: they must have the same name as the
class and they do not have a return type, not even a void.
NOTE: We will talk briefly about return types in just a few minutes, and go over them in greater detail in an upcoming
lecture.
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Constructor
public class MrRoboto extends Robot
{
public MrRoboto(City theCity, int street, int avenue, Direction aDirection)
{
super(theCity, street, avenue, aDirection);
}
}
MrRoboto
Robot
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Constructor
public class MrRoboto extends Robot
{
public MrRoboto(City theCity, int street, int avenue, Direction aDirection)
{
super(theCity, street, avenue, aDirection);
}
}
MrRoboto
Robot super
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Constructor
public class MrRoboto extends Robot
{
public MrRoboto(City theCity, int street, int avenue, Direction aDirection)
{
super(theCity, street, avenue, aDirection);
}
}
MrRoboto
 imagine a conduit …
super
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Constructor
public class MrRoboto extends Robot
{
public MrRoboto(City theCity, int street, int avenue, Direction aDirection)
{
super(theCity, street, avenue, aDirection);
}
}
bothell, 3, 2, Direction.SOUTH
MrRoboto
super
BIT 115: Introduction To Programming
Since MrRoboto is inheriting the
Robot parameters, the Robot still
needs those parameters in order
for MrRoboto to inherit them.
This is why it appears as if there
are two sets of parameters: one
set to pass through MrRoboto, a
second set for Robot to receive
them, where Robot sends them
back to MrRoboto by extension.
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Adding New Services
public void turnAround()
{ this.turnLeft();
this.turnLeft();
}
public void move3()
{ this.move();
this.move();
this.move();
}
.
public void turnRight()
{ this.turnAround();
this.turnLeft();
}
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The this keyword
The new Java feature in the new services we created is the use of the
this keyword.
The keyword this is useful when you need to refer to an instance of the
class from its method, but without having to refer to it by a specific
name. Why? Because when you create the new method, you don’t
know the name of the particular robot that is going to use it, so ‘this’ is
a kind of placeholder name.
The this keyword helps us to avoid name conflicts, and also creates a
shortcut to having to invent a unique name for each field in the
different methods.
public void turnAround()
{ this.turnLeft();
this.turnLeft();
}
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Putting It All Together
Two Ways of Doing the Same Thing
Version 1: One Class
MrRoboto.java
Version 2: Two Classes
MrRobotoMain.java
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Putting It All Together
MrRoboto2.java
MrRoboto.java
MrRobotoTest2.java
All on One File
On Two Separate Files
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Lecture 2 ICEs
Now: We will do all the Lecture 2 In-Class Exercises in this order:
• In-Class Exercises 2.1
• In-Class Exercises 2.2
• In-Class Exercises 2.3
Work on these ICEs to the End of Class Time
CityFrame – An older version of becker.jar file contained a class called
CityFrame which when used looked something like this:
CityFrame City = CityFrame(seattle);
Please ignore any reference to this. It will, however, rear its ugly head in the ICEs
on purpose as an example of old 'legacy' code that should either be "commented
out" or deleted altogether!
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