Teaching CS1 with Karel the Robot in Java

Transcription

1 Teaching CS1 with Kare the Robot in Java Byron Weber Becker Department of Computer Science University of Wateroo Wateroo, Ontario, Canada N2L 3G1 Abstract Most current Java textbooks for CS (and thus most current courses) begin either with fundamentas from the procedura paradigm (assignment, iteration, seection) or with a brief introduction to using objects foowed quicky with writing objects. We have found a third way to be most satisfying for both teachers and students: using interesting predefined casses to introduce the fundamentas of object-oriented programming (object instantiation, method cas, inheritance) foowed quicky by the traditiona fundamentas of iteration and seection, aso taught using the same predefined casses. Kare the Robot, deveoped by Richard Pattis [6] and weknown to many computer science educators, has aged gracefuy and is a vita part of our CS 1 curricuum. This paper expains how Kare may be used and the advantages of doing so. 1 Introduction We began the deveopment of our Java-based introductory programming course for CS majors and Math students in At the outset we reaized the obvious: we woud have to make a paradigm shift from procedura programming to object-oriented programming. What surprised us was the second paradigm shift required - the paradigm for teaching an OO anguage (Java) is different from teaching a procedura anguage such as Pasca, C or even C++. The first time we taught the course we used a textbook which was very we-written but paced objects reativey ate - about where a Pasca text woud pace records. We were uneasy with this, feeing that objects reay ought to be introduced and used from day 1, but as we coud not find a satisfactory text which supported this we made an attempt to provide our own exampes parae to the text. Permission to make digita or hard copies of a or part of this work for persona or cassroom use is granted without fee provided that copies are not made or distributed for profit or commercia advantage and that copies bear this notice and the fu citation on the first page. 'To copy otherwise, to repubish, to post on servers or to redistribute to ists, requires prior specific permission and/or a fee. SIGCSE /01 Charotte, NC, USA 2001 ACM ISBN /01/ $5.00 It was a disaster. The fundamenta object-oriented concepts (object instantiation, method cas, inheritance) came too ate in the course. Students didn't have enough time to master the topics. This was worsened by the fact that most of our students come with some (procedura) programming experience from high schoo. When the eary part of the course focused on iteration and seection (without objects, except for the itte we provided ourseves), most students thought they aready knew it a. After reviewing textbooks again, our standard rant went ike this: "Textbooks start with a vague, airy-fairy descriptions of objects (Java is an object-oriented anguage after a), but then go into primitive types. No objects. Chapter 3 is seection using payro as an exampe. No objects. Chapter 4 is iteration - no objects. Suddeny, in chapter 5, students are expected to use and write objects. And they say, 'Remind me what an object is... What are they good for? How do I use them? What are the characteristics of a good object?'." Shorty thereafter we discovered Kare the Robot in Kare++: A Gente Introduction to the Art of Object- Oriented Programming [2], the most recent incarnation of Rich Pattis' idea from the eary 1980's. It was a reveation to us. This was how we wanted to teach Java. There were just two probems. First, the book is reay oriented towards C++ rather than Java. Second, it's ony the introduction to an introductory course, covering the first 4-5 weeks. After convincing the pubisher to aow us to transate Kare++ to Java, adding a second textbook, writing software to support Kare, and rewriting a our ectures, we have a course we are very happy with. The remainder of this paper discusses Kare the Robot and the advantages we see in starting the course by using an interesting object. 2 Kare and Karers Word Kare inhabits a very simpe word. There are avenues running north and south numbered one to infinity and streets running east and west, aso numbered one to infinity. Was may bock avenues or streets. Beepers may be paced on the intersections of the avenues and streets. Severa robots may exist within the same word. Within this word, robots may move forward from one intersection to an adjacent intersection uness the way is bocked by a wa. They may turn eft 90 degrees to face a 50

2 different direction. Robots may pick up a beeper from their current intersection (if one or more are present), or pace a beeper on their current intersection (provided they are carrying at east one beeper). Robots may carry beepers between intersections in their "beeper bag" and may detect whether or not their beeper bag is empty. Robots may aso determine if they are facing north (or any of the other 3 compass positions), if there is a beeper on the current intersection or if there is another robot on the current intersection. The word and the actions of any robots within it are shown visuay on the computer monitor. A sampe is shown in Figure 1. If the goa of the robot is to pick up a of the beepers, then one possibe soution is shown in Figure 2. This exampe may be seen running as an appet at 01/sampes.htm. A second on-ine exampe shows four robots soving the same probem. A third exampe uses threads so that four robots work simutaneousy (instead of taking turns) to sove the probem. ' Fast package HarvestSquare; import cslib.kare.robot; import cslib.kare.word; pubic cass Main extends Object { pubic static void main (String[] args) { //Instantiate a new word, initiaized with was and //beepers as specified in a fie. Word square = new Wor 1 d ("HarvestAroundSquare.txt") ; E Instanti~e a new Robot and add it to the word E the corner of 2nd avenue and 3~ street facing east. HarvesterRobot kare = new HarvesterRobot(); square.addrobot(kare, 2, 3, Word.EAST); H Moveintotheintiaposition, then pickthe beepe~ H and turnoff. kare.move(); kare.pickboundedrectange(); kare.turnoff(); f + eeeet ' I. T /** A new kind of robot which knows how to harvest beepers aid out in a rectange with a wa marking the end of each side. */ cass HarvesterRobot extends Robot { /** Pick the beepers in a rectange where the end of each side is bounded by a wa. */ pubic void pickboundedrectange() { for(int side=0; side<4; side++) { this.pickuntiwa () ; this.turnleft () ; } : i* I ri fi it i i Z. Avenues Sow Figure 1: An initia situation with one robot (an arrowhead), tweve beepers (circes) and ten was (rectanges). 3 Differences from Previous Work Our impementation of Kare differs from Kare++ and the origina Kare in a number of significant ways. The most obvious is that we use Java whie the previous two use their own anguages. Kare uses a Pasca-ike anguage whie Kare++ uses a anguage simiar to C++. Neither anguage, however, supports parameters or non-robot variabes whie /** Pick one beeper from each intersection between the current ocation and the nearest wa in the direction faced by the robot. */ private void pickuntiwa() { whie (this.frontscear()) { this.move(); this.pickbeeper () ; Figure 2: One soution to pick up a the beepers in the word shown in Figure 1. our approach aows the fu power of the Java programming anguage - incuding objects which do not extend Robot, oca and instance variabes of any type and threads. The previous impementations were programs which integrated a deveopment environment with a simuator. 51

3 The robot and word casses were buit right into the combined program. Our approach uses a standard Java deveopment environment. When students write a robot program they simpy import the required casses from a ibrary. This removes the need to switch environments mid-way through the course, but is somewhat more compex at the beginning. Because the previous impementations buit the definition of the robot's word into the simuator, the behaviors of the word coudn't change. In our impementation the word is a fuy extendabe cass. We encourage students to extend it with new behaviors for pacing was and beepers, for instance. 4 Course Outine A typica CS 1 course woud use Kare for the first four to five weeks to introduce objects, inheritance, seection, iteration, and reated concepts. After this introduction, robots woud be eft behind in favor of a wide range of exampes. 4.1 Week 01 : Instantiating and Using Objects In the first week we describe robots and the word they inhabit. After reviewing severa simpe programs, students understand how to instantiate objects, invoke an object's methods and that it is possibe to have severa objects beonging to the same cass, each with its own state. A typica homework probem is to instruct one or more robots to perform a fixed task such as retrieve a beeper from a given intersection. 4.2 Week 02: Extending Existing Casses During the first week students often ask "Can a robot turn right?" (or move forward more than one intersection at a time or...) - which they can't. This is the perfect opportunity to extend the Robot cass with new behaviors. For instance, to create a new cass of robots capabe of turning right, we write import cslib.kare.robot; pubic cass RightRobot extends Robot { pubic void turnright() { this.turnleft (); this.turnleft(); this.turnleft () ; } This basic idea is quicky expanded into step-wise refinement to sove a more compex probem such as picking up a the beepers in a fixed-size rectanguar area. A natura extension, parameterized methods, is deferred to eave sufficient time for discussing inheritance and stepwise refinement thoroughy. 4.3 Week 03: Seection and Iteration In the third week we tacke seection and iteration. The Robot cass incudes severa methods which return Booean vaues, incuding frontiscear (is there a wa immediatey in front of the robot?), nexttoabeeper (is there a beeper on the same intersection as the robot?), facingnorth (is the robot facing North?), and anybeepersinbeeperbag (is the robot carrying one or more beepers?). These aow a number of interesting probems such as the one shown in Figure 1, constructing a histogram out of beepers, or having the robot run a hurdes race where hurdes are represented by was. Vauereturning methods and Booean expressions fit naturay with the discussion of the provided predicates. 4.4 Week 04: Methods with Parameters Methods can be made much more genera, of course, with parameters. Simper probems incude a move method which takes an integer distance parameter or a harvestpot method which coects a the beepers from an area defined by the parameters. A more compex exampe is a contractor robot which has severa subcontractors to hep buid a house. The subcontractors are passed via parameters and give a good opportunity to compare passing primitive types with passing reference types. 4.5 Week 05: Instance Variabes In the ast week spent with robots, we extend the Robot cass with additiona instance variabes. We start with a DeiveryRobot which keeps track of how many moves it makes so that a customer can be charged for its services. After this reativey brief exampe we quicky move to other, non-robot, exampes such as a date or bank account cass. After introducing the fundamentas of object-oriented programming, the remainder of the course focuses on a wide range of topics incuding arrays, graphica user interfaces, and object-oriented design. 4.6 Discussion Taking five weeks to cover these topics seems sow, but it's not. Topics such as procedura decomposition, parameters, iteration and seection are hard for most beginners. They were hard back in the days of Pasca - and that hasn't changed just because we have moved to object-oriented anguages. In the Pasca predecessor to this course we spent two weeks on contro structures, two weeks on subprograms, and another week on records. Many courses may be abe to move faster. In fact, we do. By 1995 we observed that most of our incoming students had some previous programming experience. After making this a prerequisite for our "first" course (and providing a remedia course for those who didn't have it) we shaved a week off the schedue suggested above. Some peope assume robots are over-used and that students soon tire of them. This is not the case. We coud not use them onger, but just over four weeks is not too ong. Students enjoy the visua aspects and the anthropomorphism they make possibe. In the course evauations many students say they thoroughy enjoyed the robots; few mention them negativey. 52

4 We do need to mention often in cass that we are ony using robots as a too to earn about programming. It's not a course about robots - it's a course about programming! We reinforce this with frequent ausions to other possibe casses such as Empoyee or Date or Account, indicating that the concept under discussion appies to those situations, too. 5 Advantages There are a number of advantages to using Kare the Robot to introduce objects. A number have been auded to aready. They are more carefuy enumerated here. 5.1 Object-Oriented Fundamentas First Kare the Robot emphasizes the fundamenta concepts in OO programming (instantiating objects, invoking methods, extending existing casses) from the beginning. It's important that students be abe to practice these concepts as ong as possibe and that they not be eft to the midde or end of the course where they might be skimmed over ighty if time is tight. Of course the foundations of procedura programming (seection, iteration, procedura decomposition) are aso very important. The Robot cass aows them to be naturay introduced with the OO fundamentas so that they, too, can be practiced throughout the course. Many courses reverse these sets of fundamentas - introducing seection and iteration before using objects, sometimes treating objects as fancy Pasca records. There are at east two probems with this approach: a paradigm shift and missed opportunities. Students are often asked to write programs typica of those in chapter 3 of [4]. These are programs which count, sum or generate comments based on user input. None of these programs use objects except for System. out, strings, and (once or twice) an instance of NumberFormat. Later, the students must shift from a "do it a mysef right here" paradigm to a "find/buid some objects to cooperate in getting the job done" paradigm. The missed opportunities of this approach are in the richness of tasks that can be done with existing objects. Why shoud students be summing numbers when they coud be doing more interesting things which are pedagogicay just as vaid? Another common approach in existing textbooks is to make extensive use of System.out and the String cass as exampe objects. These programs use objects but the object creation is hidden by the run-time system (in the case of System. out) or specia anguage support (strings). Creating objects - incuding mutipe instances from the same cass - is an important part of students understanding what objects are and how they work. By using a sufficienty compex and interesting predefined cass such as Robot students can begin using the OO fundamentas from the beginning and avoid a paradigm shift ater on. 5.2 Robots are Visua The fact that robot words have a visua representation provides a number of benefits. Many probems can be specified using a picture of the initia situation and another of the fina situation, pus a few ines of text. We find that students have fewer questions about homework probems specified this way. The animation provides visua feedback on the correctness of an agorithm. If the student's program doesn't resut in the same image as the probem statement, there must be a bug (of course, just because it ooks the same doesn't mean it was done right...). Students can often see where their program goes wrong simpy by watching the animation. Because the human brain is highy optimized to process visua input this is faster and easier than, say, scanning a ist of numbers from a consoe program. Because robots provide output visuay, traditiona input and output can be deayed. This is p~ticuary attractive in Java where I/O is cumbersome, at best. 5.3 Robots are Fun Robots are fun! This is, perhaps, the biggest advantage of the approach. Students enjoy directing robots to do various tasks. Acting out a program in cass is more fun than tracing a isting. Visua output is more fun than textua output. 6 Reated Work Since we deveoped this course textbooks have, in genera, moved objects earier. Most, however, use a scattering of Java casses such as String or Random (which we find inferior to the approach recommended here) or expect students to write their own casses amost as soon as they earn about objects. See [1] for a more thorough treatment. A few textbooks come coser. Sack [9] uses turte graphics rather than a robot but uses a spira approach. Chapter 2 is a difficut chapter, packed with concepts iustrated with turte graphics. Chapters 4, 5, 7 and parts of 10 then come back to cover the concepts in more depth. Morei [5] uses a "CyberPet" as an interesting exampe to iustrate object-oriented fundamentas as we as iteration and seection. The difference is that it is not a pre-defined cass, as are the robots. Instead students deveop it as they go aong. Wu [10] gives a we-crafted rationae for using objects eary. The objects his students use, however, are a simpified interactive I/O ibrary used in the context of other probems. Otterbein Coege uses Kare the Robot as an introduction to programming. They have a sophisticated environment deveoped by Duane Buck which supports a Java-ike syntax (as we as Pasca and Lisp) and has an integrated compier and execution environment. Differences from our approach is that our students use a Java deveopment 53

5 environment, importing the necessary casses. Their students use one environment for Kare and a different one for other programs. In the Otterbein environment ony a singe robot can be used in any given program. The environment is avaiabe at JKareRobot/. Their arger approach is discussed in [3]. Joseph Bergin has made his own transation of Kare++ to Java avaiabe on the web, aong with casses impementing the robot. It may be found at KareJava/Kare++JavaEdition.htrn. Our own work is at Rich Pattis, the originator of Kare the Robot, has extended the idea into artificia ife simuations. The resut is a much more compex set of casses for students to master, but one that aso affords many more possibiities [7]. 7 Future Directions The argest difficuty we have encountered is that Kare++, and thus our transation of that book, is ony an introduction to object-oriented programming. It doesn't cover numerous topics expected in a CS1 course and so we jump to a standard text after severa weeks. Students often compain of this discontinuity. To overcome this I am writing a fu CS1 textbook which starts with Kare but aso incudes a the expected CS1 topics. This text wi aso incude a number of innovations beyond those discussed here. First, we currenty discuss attributes of an object (instance variabes) quite ate. After stressing in week 1 that an object has both behavior and attributes, it seems wrong to focus on behavior for so ong without mentioning attributes. In the text I wi cover object usage, extending objects (behaviors), seection, and then attributes, bringing two core object-oriented concepts into a much coser reationship. Second, there wi be a richer coection of casses. In addition to the current Robot and word (renamed city) wi be a CigyBock - the immediate environment of a robot. Students wi be abe to extend the CityBock cass and robots wi be abe to interact with CityBock objects. Picking up and putting down beepers then becomes just one exampe of possibe interactions. After studying how casses interact to impement beepers, students might go on to impement a robot that rakes eaves into pies or spreads pies of sat onto the streets or city bocks that dispay a different coor after being visited by a robot. Many of these projects make good use of instance variabes. This aso aows simuations inspired by artificia ife [7, 8]. Third, I am introducing a parae track which reinforces concepts using non-robot objects. Currenty too many students don't recognize that they are earning genera concepts via Kare the robot. Incuding other exampes shoud aeviate this. Since students ove graphics and GUI appications, I wi foow [4] and others in providing a "GUI Suppement." In chapter 1 we wi instantiate and invoke methods on a robot object foowed by instantiating and invoking methods on a JFrame object. In chapter 2 students wi extend robots to do new things and aso extend JFramo to do something new - paint a scene. Other chapters wi foow a simiar structure. 8 Concusions We have been extremey peased with using interesting predefined casses (Kare the Robot and its word) to teach object-oriented fundamentas (object instantiation, method cas, inheritance) and traditiona procedura fundamentas (iteration, seection) eary in the course. The resut has been a course that is fun for both students and instructors, and where students understand the fundamenta concepts eary, aowing them to approach advanced topics with confidence. References [1] Becker, Byron Weber. Pedagogiesfor Teaching CS1 in Java. papers/sigcse2001/iavapedagogies/index.htm. [2] Bergin, Joseph, Mark Stehik, Jim Roberts, and Richard Pattis. Kare++: A Gente Introduction to the Art of Object-Oriented Programming. John Wiey & Sons, [3] Buck, Duane and David J. Stucki. Design Eary Considered Ha~fu: Graduated Exposure to Compexity and Structure Based on Leves of Cognitive Deveopment. SIGCSE Buetin 1 (2000), p [4] Lewis, Johia and Wiiam Loftus. Java Software Soutions. Addison-Wesey, [5] Morei, Raph. Java, Java, Java: Object-Oriented Probem Soving. Prentice-Ha, [6] Patfis, Richard E.. Kare the Robot: A Gente Introduction to the Art of Programming, John Wiey & Sons, [7] Pattis, Richard E. Teaching top in C++ Using an Artificia Life Framework. SIGCSE Buetin 1 (1997), p [8] Resnick, Mitche. Turtes, termites, and traffic jams: exporations in massivey parae microwords. The MIT Press, [9] Sack, James M. Programming and Probem Soving with Java. Brooks/Coe, [10]Wu, C. Thomas. An Introduction to Object-Oriented Programming with Java. WCB/McGraw-HiI1,