Teaching programming at Introductory Level

• Why has introductory computer science courses become so advanced in terms of conceptual materials presented [Roberts04a]?

• Keeping a generation that has grown up with XBox, PSP, Mobile Phone Games… interested in our courses requires a different approach.

• How to get them interested in computer science?

How institutions attract students to CS(ACM report)

• By allowing them to create similar applications in the first year (at a high level of abstraction)– Graphics – Events and concurrent programming– Distributed objects etc.

• Convince students at the end of first year– Computing is Cool! Programming is Fun!– I can do real life applications (Adam Boas)

• Leaving in-depth matters to be covered later– Caching, Transactions, Building Components, Optimization …– Sydney uni spends much of first year programming and design

• Institutions forced to adopt appropriate paradigms• Java has proved to be suitable – but not without

problems

Role of ACM Java Task Force• Java is widely taught as the first language. Most have moved

from “whether Java” to “if Java then how” ?

• Despite increasing usage “satisfaction is mixed”. Problems common to all modern languages

• Complexity associated with huge API

• Instability – changing API, tools

• ACM Task Force formed to review the language, APIs, and tools from the perspective of introductory computing education

• ACM Task Force to develop a stable collection of pedagogical resources including a set of new packages. Feedback was invited.

What are the problems?• Modern O-O languages caters to many demands trying

to meet user needs • As a result the number of programming details a new

user has to must master has increased greatly (compare with Pascal)

• Next program shows the number of constructs that must be learnt even before they can get a simple program to work using Java (earlier version).– Chaining – try catch block– Converting String to double– Use of special class for formatting output

import java.io.*;import java.text.*;public class Add1{ static public void main(String args[]) { InputStreamReader stdin = new InputStreamReader(System.in); BufferedReader console = new BufferedReader(stdin); double a = 0,b = 0, c; String s1,s2; try {

System.out.print("Enter value for a :"); s1 = console.readLine(); a = Double.parseDouble(s1);

System.out.print("Enter value for b :"); s2 = console.readLine(); b = Double.parseDouble(s2); } catch(IOException ioex) { System.out.println("Input error"); System.exit(1); } c = a + b; DecimalFormat dF = new DecimalFormat("0.00"); System.out.println(a + " + " + b + " = " + dF.format(c)); System.exit(0); }}

Try { } catch() { } Needed because readLine() may throw an exception

Though it will not happens when reading from keyboard)

Needed to format output

BRISR program

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Equivalent program in C#include <stdio.h>int main(int argc, char*argv[]) { int a,b,c; printf(“Enter values for a and b"); scanf("%d ",&a); scanf("%d",&b);

c = a+b; printf(“%d %d = %d\n",a,b,c);}

How can J2SE 5 help?import java.util.Scanner;public class Add2{ static public void main(String args[]) { Scanner console = new Scanner(System.in); double a,b, c; System.out.print("Enter value for a and b :"); a = console.nextDouble(); b = console.nextDouble(); c = a + b; System.out.printf("%f + %f = %.2f",a,b,c); }}

Use of Scanner class requires no chaining

Scanner method reads numbers directly

(patterns can be used)

try {...} catch() {…} avoided as no checked exception thrown by Scanner methods

New printf() method makes formatted output simple

What are some new J2SE 5 features?

• Scanner class • Formatted output• Using/Creating Generic classes• Autoboxing• Enhanced for loop• Type safe enum• Variable arguments• Static import

Using Generic classes• All JCF classes can be used as generic classes

• For example, an ArrayList instance can be created to store only Account objects by passing the type (Account) to the constructor and the reference as in:

ArrayList<Account> accList = new ArrayList<Account>();

• Similarly to map a customer name (String) to Account objects we can use:HashMap<String,Account> hashMap

= new HashMap<String,Account>();

Example using ArrayListimport java.util.*;class Account { }class Customer { }

public class TestGenericArray{ public static void main(String[] args) { ArrayList<Account> accList = new ArrayList<Account>(); accList.add( new Account() ); accList.add( new Account() );

ArrayList<Customer> custList = new ArrayList<Customer>(); custList.add( new Customer() ); custList.add( new Customer() );// custList.add( new Account() );

System.out.println("Entries in accList =" + accList); System.out.println("Entries in custList =" + custList); }}

Compiler detects type mismatch

Autoboxing• Box/Unbox

– Wrap a primitive into a class to be used as an object and extract primitive value from object manually

• AutoBox/AutoUnbox– Compiler does the work

ArrayList a = new ArrayList(10);int num = 10;a.add(new Integer(num));int x = ((Integer)a.get(0)).intValue(); System.out.println(x);

BoxingUnBoxing

ArrayList<Integer> a = new ArrayList<Integer>(10);int num = 10;a.add(num);int x = a.get(0);System.out.println(x);

With JDK 1.5

Before JDK 1.5

Enhanced for loop

• Iterations are made easy

• Before (iterating through a set)Iterator iterator = set.iterator();

while ( iterator.hasNext())

System.out.print(iterator.next() + " ");

• With JDK 1.5for (String s : set )

System.out.print(s + " ");

Variable Arguments

• Java 1.5 allows vararg specification allowing any number of arguments

• It is of the form– int… args– String… messages

• You can have only one vararg in method specifiction and it must be the last.

class Utility{ /** Returns the largent of any number of int values */ public static int max(int... arg) { if ( arg.length == 0) { System.out.println("Error: Max of zero values"); System.exit(0); } int largest = arg[0]; for (int i=1; i<arg.length; i++) if (arg[i] > largest ) largest = arg[i]; return largest; }}

public class TestUtility{ public static void main(String args[]) { int max1 = Utility.max(10,40,30); int max2 = Utility.max(10,40,30,60); System.out.println("max1 = " + max1 + " max2 = " + max2); }}

Example: Variable Arguments

Type Safe enums

import java.util.*;public class Test3 { enum WorkDay {MONDAY,TUESDAY,WEDNESDAY,THURSDAY,FRIDAY}; public static void main(String args[]) { WorkDay meetingDay = WorkDay.THURSDAY; System.out.println(meetingDay); }}

• Like C enum

• Type Safe

• Can be printed

Static import

By adding a import statement for static method you can avoid having to precede the method with class name.

import static java.lang.Character.toUppercase;

toUpperCase(firstCharacter);

// instead of Character.toUpperCase(firstCharacter);

Remaining problems teaching/learning(according to Java Task Force)

• Scale• Instability (methods deprecated, new models)• Static methods• Exceptions• Conceptual difficulty of Graphics model• Writing event driven code

– due to event model– due to difficulty of concurrent programming

Java Task Force Solutions• Graphics Package

– Uses an extensible hierarchy of grapical objects rooted at GObject

GObjectGCompound

GArc GImage GLabel GOval

GPen

GRect

GTurtle

GPolygon

• Program Package

– Uses an hierarchy of Program classes (ConsoleProgram, DialogProgram, GraphicsProgram)

• Others

ACM approachPros:• Helps avoid overloading student with details• Makes available pedagogical resources• Beta version of JTF classes released

Cons• Are we adding to the scale of complexity?• Will will be chasing moving target (new features in Java

creates new problems)• Can all staff agree on these or other classes (such as

developed by IBM) ?• Availability of resources

Our own Java Task Force needed

Our challenge

Changing profile of students

• Students with high enter scores are usually self-motivated

• Average students are willing to try– If the problem is perceived to be interesting– If it is not too difficult to get started

• Average students need more guidance and feedback

Our Approach thus far

• Use of helper classes / Container classes to hide complexity in the initial stages

– ConsoleReader class– Date class– From this year JCF from first semester

• Regular Assessments (9 excluding exam)– Demos, WebLearn tests, MS-Test,

Assignments (3)

Our Approach thus farUse some Animation, Games, Robot Control, Aircraft Simulation etc to motivate the students using a layered Approach. Use given

Graphical objects(emphasis on algorithms)

Student extend given Graphical objects

(Emphasis OO Progr.)

Students use Swing classesEmphasis GUI

Students use Threads, sockets, MVC,

RMI to design apps

Diagram Editor

Aircraft Landing

BattleShip Multiplayer Game

Layered Approach

How can our JTF help ?• Study the ACM JTF classes and recommend adoption (or

otherwise)• Assignments form very important part of student learning

and Java allows us to provide the required level of abstraction to teach important concepts – but it takes time.

• Develop and share pedagogical resources. • Invite speakers from Industry and get student feedback.

Apr 4 speaker from Sun.• Decide what Java features needs to be taught & the

depth. • Avoid duplication in electives. Currently no Java course in

school (only used as a vehicle)

Discussion

A Parameterized Library class

import java.util.*;

class Library<E> { private ArrayList<E> resources = new ArrayList<E>(); public void add(E x) { resources.add(x); }

public E retrieveLast() { int size = resources.size(); if (size > 0) { return resources.get(size-1); } return null; }}

The generic library class has a single type parameter E, allowing it to store objects of type E.

Using parameterized type E

Allows any object of type E to be added

objects retrieved are of type E

Uses the services of parameterized ArrayList reference Constructor

Converting a class to Generic class import java.util.*;class Library { private ArrayList resources = new ArrayList(); public void add(Media x) { resources.add(x); } public Media retrieveLast() { int size = resources.size(); if (size > 0) { return (Media) resources.get(size-1); } return null; }}

public class TestLibrary{ public static void main(String args[]) { Library myBooks = new Library(); myBooks.add(new Book()); myBooks.add(new Book());// myBooks.add(new Video()); Book lastBook = (Book) myBooks.retrieveLast(); lastBook.print(); }}

Media

Book Video

Cannot detect at compile time resulting in runtime error when retrievingExplicit cast needed

For storing Book objects

Using the Parameterized Library

public class TestLibrary{ public static void main(String args[]) { Library<Book> myBooks = new Library<Book>(); myBooks.add(new Book()); myBooks.add(new Book()); Book lastBook = myBooks.retrieveLast(); lastBook.print();

Library<Video> myVideos = new Library<Video>(); myVideos.add(new Video()); myVideos.add(new Video()); myVideos.add(new Video()); Video lastVideo = myVideos.retrieveLast(); lastVideo.print(); }}

• When using the parameterized (generic) Library class a type must be passes to the type parameter E.

• Note the element extracted from the parameterized Library need not be cast.

Creating a Library to store Book objects

Creating a Library to store Video objects

No casting needed

Things to take note when creating and using a Parameterized classes

• Primitive types cannot be passed as parameters. ArrayList<Integer> numbers = new ArrayList<Integer>(); ArrayList<int> numbers = new ArrayList<int>();

• When a class uses parameterized type T, the type parameter T can be used as a reference but not for constructing.T object = …T[] a = … = new T(); = new T[10];

• The parameter can be used for casting E e2 =(E) new Object();E[] e3 = (E[])new Object[10];

• Generic classes cannot be array base type Library<Video> videoLibs[] = new Library<Video>[10];ArrayList<Video> vidLibs[] = new ArrayList<Video>[10];

References

[Roberts04a]

The dream of a common language: The search for simplicity and stability in computer science education. Proceedings of the Thirty-Fifth SIGCSE Technical Symposium on Computer Science Education, Norfolk, VA, March 2004.