Principled approach to teaching OO first David Gries Computer Science, Cornell 1

Principled approach to teaching OO first Teach programming, not programs Teach skills, not just knowledge 2 Stepwise refinement (stepwise improvement) top-down programming Clear, precise specification before writing a routine Enforce JUnit testing Class invariants, loop invariants, and their use in the programming process: correctness concerns done informally Tony Hoare Klaus Wirth Edsger W Dijkstra David Gries

3. Present concepts at the appropriate level of abstraction 3 The computer itself is not the right level of abstraction for beginners. Give them a model they can understand without mentioning the computer and its memory. The computer must always know the type of value to be stored in the memory location associated with a variable. An object reference variable actually stores the address where the object is stored in memory. An object has its own unique identity, which distinguishes it from all other objects in the computer’s memory …. An object’s identity is handled behind the scenes by the Java virtual machine and should not be confused with the variables that might refer to that object. (1) gives impression that only computers can execute programs. (2) confuses people who have little idea of memory, virtual machines, and how computers work.

3. Present concepts at the appropriate level of abstraction 4 Algol 60 language definition does not mention the computer. “The purpose of the algorithmic language is to describe computational processes. … A variable is a designation given to a single value. Assignment statements serve for assigning the value of an expression to a variable …. The process will … be understood to take place in three steps as follows: Any subscript expressions occurring in the left part variable are evaluated in sequence from left to right The expression of the statement is evaluated The value of the expression is assigned to the left part variable, with any subscript expressions having values as evaluated in step

3. Present concepts at the appropriate level of abstraction 5 Problem: students don’t know how to execute the assignment statement. Variable: 1. A name associated with a value. 2. A named box with a value inside it. x 5 x= x+2; To execute the assignment: (1) evaluate the expression and (2) store its value in the variable. The computer itself is not the right level of abstraction for beginners. Give them a model that they can understand without mentioning the computer and its memory. To evaluate new C(...) (1) create an object of class C (2) execute constructor call C(...) (3) yield as the value of the expression the “name” of the new object To execute procedure call p(...) (1) draw a frame for the call (2) assign arg values to pars (3) execute method body (4) erase the frame for the call

4. Order material to minimize the introduction of terms or topics without explanation 6 Two aspects to a program Structural-organizational Algorithmic Expression Assignment Conditional statement Loop Subroutine call Subroutine (method) Object Class Interface When using Java, this principle almost requires teaching OO first Requires an IDE that removes the need for a method main or an applet DrJava BlueJ I don’t mention method main or applets until week 11, and then I can do both on the same day.

5. Use unambiguous, clear, precise terminology 7 Formal parameter — actual parameter Pointer-reference A parameter is a variable that is declared within the parentheses of a method header. An argument is an expression that occurs within the parentheses of a method call. y x an object y= x; a1 a2 a1 Expression x evaluates to a1

6. Name the things you want to talk about 8 an object a1 x

Use a standard format for drawing object. Force students to use it 9 x a1 an object a1 JFrame w 4 get Title() setTitle(String) getWidth() show().... An object contains all the instance components (variables and methods) defined in the class and inherited by the class —see later how to include inherited components in the object a6 JFrame w 7 get Title() setTitle(String) getWidth() show()....

Objects of a subclass 10 a1 JFrame get Title() setTitle(String) getWidth() show().... a1 Demo public class Demo { } extends JFrame private int w; w 4 /** = area of window */ public int area() { return getWidth() * getHeight(); } area() { return getWidth()*getHeight(); } bottom-up rule: Search for a component from the bottom up —gives the overriding method toString()

The inside-out rule for determining which component is referenced 11 a7 JFrame get Title() setTitle(String) getWidth() show().... Demo w 7 area() { return getWidth()*getHeight(); } a1 JFrame get Title() setTitle(String) getWidth() show().... Demo w 4 area() { return getWidth()*getHeight(); } Look in the current construct, its surrounding construct, its surrounding construct, etc., until a matching component is found. file drawer for class Demo s 6 m() {... } static fields, methods

Contents of first lectures Expressions, variables, assignment Lab: practice: types int, double, boolean, string; casting; assignment 02. Objects (students see JFrame objects) 03. The class and subclass definitions (simple function/proc decs with return statements and method calls Lab: write simple function/proc declarations in a subclass of JFrame 04. Fields, getter/setter methods, simple constructors 05. Static components, the class hierarchy, JUnit testing Lab: use JUnit test cases to find and fix errors in a given program; practice with static components 06. How a method call is executed. if- and if-else statements. local variables. Two lectures per week, with a closed, mandatory lab between them

Contents of first lectures Inside-out rule; super-this; stepwise refinement Lab: practice: write functions (if- and if-else; no loops!) 08. Constructors in subclasses; stepwise refinement 09. Wrapper classes; stepwise refinement Lab: Learn about class Vector 10. Recursion 11. Recursion Lab: Writing recursive functions 12. Casting among class-types; operator instanceof; function equals. Two lectures per week, with a closed, mandatory lab between them

Assignments Maintain rhinos. Storyline on endangered species. fields, setter/getter methods, boolean expressions, JUnit testing. /** = true if r not null and this rhino is older than r; false otherwise */ public boolean isOlder(Rhino r) { return r != null && (year < r.year || (year == r.year && month < r.month)); } 2. Maintain rhinos. extension of 1 to include a static variable (number of Rhinos created) and more comparison methods. 3. Write several functions using assignment, if, if-else. JUnit testing A. Translate between rgb, cmyk, hsv color models B. TimeZone: an instance maintains a time in one of 12 time zones C. Maintain dates in various forms and translate between them D. Bowling: an instance maintains a bowling frame. The calculation of the score in the frame requires thinking.

Assignments Turtle graphics. Draw shapes, bouncing balls. Loops: repetitive polygons, spirals, radiating lines Recursion: Koch snowflakes, Sierpinski triangles 5. Mozart dice game. Create a random waltz, building the mp3 file from mp3 files for the individual measures, and play it. 6. Manipulate jpg files —reflect, transpose, convert to gray scale, etc. 7. Hide a message in a jpg file.

A principled approach to teaching programming 16 Using Java, OO comes first Allows OO stuff to be used (and understood!!!) throughout the course Interesting, inspiring assignments generate enthusiasm Stress good programming practices —good specifications, class invariants, stepwise refinement, integrated testing using JUnit First assignment requires mastery, everyone gets 100/100 Allows students to learn from their mistakes, without penalty Cornell: I teach 130 to 160 students each semester. At least 50% have not programmed before. Mainly engineering, but from all other disciplines, too