BIT 115: Introduction To Programming LECTURE 3. Instructor: Craig Duckett

Transcription

1 BIT 115: Introduction To Programming LECTURE 3 Instructor: Craig Duckett cduckett@cascadia.edu

2 Lecture 3 Announcements By now everyone should be up and running with Java, jgrasp, and the Becker Robots on their home or personal computers. Any Problems? Has everyone had a chance to work with the Java programs and the Becker Robots? Reading Assignment for Today Appendix F.1 Extending a Class Chapter 2.1, 2.2 Extending Robot Class Chapter 2.4 Coding Style BIT 115: Introduction To Programming 2

3 TIRED OF CIRCLING THE PARKING LOTS? In case you are not aware, Cascadia has a new option for students and staff who drive to campus. Campus Commuter Services has begun operating the Truly Express shuttle service between campus and an overflow parking lot 1.2 miles away at the Seattle Times building. This should help to alleviate the parking lot traffic jams. The Truly Express makes four stops: 1) Across from the Activities and Recreation Center 2) The parking lot at the Beardslee Professional Building 3) At Beardslee Crossing near the metro bus stop 4) At the Seattle Times parking th Ave NE The Truly Express runs from 7:00 am until 10:50 pm every Monday through Thursday. The schedule is posted on the website. You can also download the live shuttle app to track the Truly Express. The campus parking spots are clearly labeled and a pay station has been installed. If you are parking on a regular basis, you will save money by purchasing a parking permit. You can find information about the shuttle, parking, bus routes, ORCA U-Pass, carpooling, and other commuter services online. Shuttle schedule: Live shuttle tracker app: Parking information:

4 In-Class Exercises (ICEs) You do not have to submit ICEs, since I grade these in-class. If you finish all your ICEs early and I sign you off on them, you are done and you can leave the class early. If you are working on the ICEs until the end of class, but did not finish them, you will still get full credit for working on them, but it is HIGHLY RECOMMENDED that you finish the ICEs on your own outside of class since they have been designed to help you with the concepts that will be used in the Assignments. If you want, you can show me that you have completed the ICEs during the next ICE-time in the next Lecture. Starting with Lecture 4, I will start posting Solutions to most (but not all) of the ICEs. 4

5 H O M E W O R K Assignment 1 is Due LECTURE 5 Zipped & Uploaded to StudentTracker by MIDNIGHT SECTION 1: Monday, January 22 nd It s posted on the website under Assignments menu It will be due by midnight If unsure how to upload to StudentTracker, then bring your work to class, in electronic form, and we will go over how to hand in the homework: Student Tracker How to Use Student Tracker If you re stuck, seek help Talk to the Instructor or a classmate me FYI: You should have enough information after today s lecture to successfully complete Assignment 1 5

6 Assignment Dates (By Due Date) Assignment 1 (LECTURE 5) Section 1: Monday, January 22 nd The Fickle Finger of Fate Assignment 2 (LECTURE 8) Section 1: Wednesday, January 31 st Assignment 1 Revision (LECTURE 10) Section 1: Wednesday, February 7 th Assignment 2 Revision (LECTURE 12) Section 1: Wednesday, February 14 th Assignment 3 (LECTURE 13) Section 1: Wednesday, February 21 st Assignment 3 Revision (LECTURE 16) Section 1: Monday, March 5 th Assignment 4 (LECTURE 18) NO REVISION AVAILABLE! Section 1: Monday, March 12 th 6

7 And Now. The First Quiz! You each get a Quiz hand-out: Put your name on it When you think your drawing is complete, raise your hand 5 minute limit Going forward, always leave your completed quiz beside your throughout the Lecture (and the Quarter) because your Instructor will use these to learn your names 7

8 Lecture 3 Buckle up! This could really Extending a Class : Creating a new type of Robot Style and Java Coding Conventions 8

9 Appendix F.1, Chapter 2.1, 2.2 Extending a Class Extension (B extends A) Extending the Robot Class Superclass and Subclass Constructor Adding a Method (Service) turnaround(); turnright(); The This Keyword (Implicit Parameter) Putting It All Together BIT 115: Introduction To Programming 9

10 Constructors and Extending a Class

11 Object Class Left Right In Java and other programming languages code is rendered from right to left. City Bothell = new City(); City Object City Class No Arguments City Bothell = new City(8, 10); City Object City Class With Arguments Robot Robbie = new Robot(Bothell, 3, 0, Direction.EAST, 0); Robot Object Robot Class With Arguments 11

12 Constructors 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.) BIT 115: Introduction To Programming 12

13 Constructors 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 act, to allow the object to do something. 13

14 BIT 115: Introduction To Programming 14

15 Robot Class Constructor The Robot class constructor has already been built for use and is part of the becker.jar file. We can look at this constructor by examining the Becker Library at There we see there are two types of constructors that can be used, one that has four elements and one that has five elements. Let's look at both these constructors to see what the difference is, and what the individual elements represent. 15

16 Robot Class Constructor The four part constructor represents the creation of a Robot object without a backpack to hold Things. The five part constructor represents the creation of a Robot object with a backpack to hold Things. Both these constructors also represent that type of data that will be stored in memory, and sets up memory locations to hold those values (either four or five) that are passed inside the Robot class's parentheses. These parentheses in programming are called parameters, and the values being passed are called arguments. 16

17 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 In other words, you re going to make a new class from the Robot class so you can add new methods to it like turnright() BIT 115: Introduction To Programming 17

18 Extending a Class 18

19 Extending a Class: Where Class B extends Class A In Plain Old English: Where Class B inherits all the attributes and actions of Class A and then adds it s own functionality by creating new methods 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 Class A class then we need to extend the Class A class (which is itself an extension of Object) and create a new class from it called Class B. Class B has all the actions and attributes of Class A, plus any new ones we give it. 19

20 Extending the Robot Class Now the Robot class has a specific set of actions and attributes. If you create a named Object from the Robot class (like Karel), then Karel can use the move() method and the turnleft() method, but Karel cannot use a turnright() method because that method is not part of the Robot class. If you want to use a turnright() method than you will have to extend the Robot class and make a new class from it so you can create your own methods. In this example, we're going to extend the Robot class and create a new class from it called MrRoboto. public class MrRoboto extends Robot An Object made from the MrRoboto class can do everything an Object made from the Robot class can do, plus any additional methods we create for it, like turnright(). 20

21 Instantiation (Instance) vs. Extension (Extends) Instantiation 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 Let's look at this in the abstract first, and then we'll look at some code to see how it works. 21

22 Think of it as adding an extension to a house. You still get to use all of the original house, but you also get to use the new section you added to the house 22

23 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). BIT 115: Introduction To Programming 23

24 Superclass and Subclass Robot Superclass move() turnleft() pickthing() putthing() MrRoboto Subclass move3() turnaround() turnright() An Object created from the MrRoboto subclass inherits all the actions and attributes of the Robot class, and can use any new actions and attributes (methods) created for the MrRoboto class. 24

25 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 methods (actions, services) go here BIT 115: Introduction To Programming 25

26 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 methods (actions, 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. BIT 115: Introduction To Programming 26

27 Constructor public class MrRoboto extends Robot { public MrRoboto(City thecity, int street, int avenue, Direction adirection) { super(thecity, street, avenue, adirection); } } MrRoboto Robot BIT 115: Introduction To Programming 27

28 Constructor public class MrRoboto extends Robot { public MrRoboto(City thecity, int street, int avenue, Direction adirection) { super(thecity, street, avenue, adirection); } } MrRoboto Robot super BIT 115: Introduction To Programming 28

29 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 BIT 115: Introduction To Programming 29

30 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 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. BIT 115: Introduction To Programming 30

31 Creating New Methods 31

32 Adding New Methods (Actions) public void turnaround() { this.turnleft(); this.turnleft(); } public void move3() { this.move(); this.move(); this.move(); } public void turnright() { this.turnaround(); this.turnleft(); } 32

33 The This Keyword 33

34 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(); } 34

35 Putting It All Together Two Ways of Doing the Same Thing, however: THE CLASS THAT CONTAINS MAIN HAS TO BE THE SAME NAME AS THE FILE Version 1: One Class MrRoboto.java Version 2: Two Classes MrRobotoMain.java 35

36 Putting It All Together MrRoboto2.java MrRoboto.java All on One File On Two Separate Files MrRobotoTest2.java 36

37 Chapter 2.4 Coding Conventions (Style) These coding conventions are not only good for Java, but for other languages as well, including C, C++, C#, JavaScript, Perl, Python, etc., to name a few. 37

38 Please Note! CityFrame! IMPORTANT! CityFrame An older version of becker.jar file contained a class called CityFrame which when used looked something like this: CityFrame City = CityFrame(someCity); Please ignore any reference to this. It will, however, rear its ugly head in the ICEs and Assignments on purpose as an example of old 'legacy' code that should either be "commented out" with // or deleted altogether since it will not allow the program to run correctly! // CityFrame City = CityFrame(someCity); 38

39 Lecture 3 ICE: Creating a New Type of Robot 39