Teaching Java —with OO first
David Gries
Computer Science Cornell University
Ithaca, NY 14850
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Principle 1. Reveal the programming process, in order to ease
and promote the learning of programming.
Principle 2. Teach skills, and not just knowledge, in order to
promote the learning of programming.
Michael Caspersen discusses these in his PhD thesis.
Approach based on cognitive science, educational
psychology, cognitive skill acquisition, research in
programming methodology.
Introduces stepwise improvement.
Caspersen, M.E. Educating Novices in the Skills of
Programming. PhD Dissertation, Computer Science,
Aarhus, Denmark, June 2007.
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Principle 1. Reveal the programming process, in order to ease
and promote the learning of programming.
Principle 2. Teach skills, and not just knowledge, in order to
promote the learning of programming.
Most texts teach programs, not programming. They focus
largely on knowledge, not skill.
CS people in general put little emphasis on the programming
process.
Every lecture should deal in some way with the skill of
program development.
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Principle 3. Present concepts at the appropriate level of
abstraction.
• Kim Bruce. Using abstractions to make concepts concrete.
SIGCSE Education Award lecture, 2005.
• Peter Denning et al. Computing as a discipline. CACM 1989.
Three major paradigms, or cultural styles, used in CS:
• theory
• abstraction (modeling)
• design
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Principle 3. Present concepts at the appropriate level of
abstraction.
The computer is not the appropriate level of abstraction to
describe variables, assignment, and other programming
language concepts!
"A variable is name for a memory location used to hold a
value of some particular data type."
"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."
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Principle 3. Present concepts at the appropriate level of
abstraction.
The computer is not the appropriate level of abstraction to
describe variables, assignment, and other programming
language concepts!
Computer-centric view:
• Can create unnecessary and confusing detail —especially for
a novice, who doesn't yet know about much about computers
and computing.
• Gives the impression that only the computer can execute a
program.
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Principle 3. Present concepts at the appropriate level of
abstraction.
The computer is not the appropriate level of abstraction to
describe variables, assignment, and other programming
language concepts!
"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."
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Algol 60 language definition —CACM, Jan 1963.
"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 … variable …. The process will be understood to
take place in three steps:
4.2.3.1. Subscript expressions occurring in the left part
variable are evaluated in sequence from left to right.
4.2.3.2. The expression of the statement is evaluated.
4.2.3.3. The value of the expression is assigned to the left
part variable, with any subscript expressions having values
as evaluated in step 4.2.3.1."
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Principle 4. Order the material so as to minimize the
introduction of terms or topics without explanation
—as much as possible, define a term when you introduce it.
When teaching Java, principle 3
mandates teaching OO first.
/** Print "Hello World" */
public class FirstClass {
public static void main(String[] pars) {
System.out.println("Hello World);
}
Almost every line of a Java program
}
deals with a class or an object!!
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Principle 5. Use unambiguous, clear, precise, terminology.
Don't use pointer and reference. Novices do not know what
they mean.
Use inheritance properly.
Java version: private components are not inherited.
Better version: all components are inherited from the superclass,
because they all appear in each object of the subclass.
Use parameter – argument, not formal parameter – actual
parameter.
These sentences from texts completely confuse the issue:
“The semantics of an assignment for primitive types and for
objects is different.”
“When an object is passed to a method, we are actually
passing a reference to that object.”
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Facilitating the teaching of OO first
• An IDE that does not require a Java application —e.g. DrJava
or BlueJ.
• An appropriate notational machine —model of execution.
• Constructive alignment: Course outcomes, teaching/learning
activities, and assessment methods are all aligned.
• Biweekly quizzes: Students are told exactly what will be on
the quiz and are expected to get 100/100 on them.
• Closed labs.
• One-on-one sessions.
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Notational machine
Two aspects:
• Structural/organizational —ØØ
• Procedural —algorithms, conditionals, loops, arrays, etc.
When teaching Java, principle 3 mandates teaching OO first.
We provide a model for classes and objects that
(1) Can be taught after the students know only about (a) types
and expressions, (b) variables, (c) the declaration <type>
<variable>; and (d) the assignment statement.
(2) Does not mention the computer.
(3) Is in terms of a concept with which students are already
familiar.
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A class is a file drawer. It contains
manila folders, all having the same
kind of information
C1
Whoever creates the folder
gets to decide on the name
that goes on the tab. Must
be unique!
name
B. Clinton
address
New York
owes
$250.00
Patient
Names that go on tabs of manila folder form a new type.
Importance cannot be overestimated.
Allows us to eliminate terminology like pointers and
references.
manila folder: an object or instance of the class
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A class is a file drawer. Its manila
folder contain the same fields and
instructions (methods)
C1
Whoever creates the folder
gets to decide on the name
that goes on the tab. Must
be unique!
name
B. Clinton
address
New York
owes
$250.00
Patient
getName() { … }
Deposit(double) { … }
Instructions in folder for people to carry out: methods
Function: returns a value
Procedure: does something, does not return a value
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pat
C1
variable contains
the name of the
folder
C1
name
B. Clinton
address
New York
owes
$250.00
Patient
getName() { … }
Deposit(double) { … }
pat.getName()
function call. Its value is “B. Clinton”
pat.deposit(250.0); procedure call. Subtract 250.0 from field
owes.
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C2
m
?
C2
variable contains
the name of the
folder
C2
new Patient()
name
?
address
?
owes
0.0
Patient
getName() { … }
Deposit(double) { … }
An expression: create a new folder (put it in
file-drawer Patient) and give as the value of
the expression the name of the folder.
C2
m= new Patient(); A statement: evaluate new Patient()
and store its value (the name of the new
folder) in variable m.
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j
a0
variable contains
the name of the
folder
a0
show() hide()
setTitle(String)
getHeight()
setSize(int, int)
getX() getY()
isResizable()
javax.swing.JFrame
getTitle()
getWidth()
setLocation(int, int)
setResizable(boolean)
A manila folder (object) of class javax.swing.JFrame()
Maintains a window on your monitor, with methods
(instructions) for manipulating it
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j
a0
variable contains
the name of the
folder
a0
show() hide()
setTitle(String)
getHeight()
setSize(int, int)
getX() getY()
isResizable()
javax.swing.JFrame
getTitle()
getWidth()
setLocation(int, int)
setResizable(boolean)
SquareJFrame
area() {…}
setHeightToWidth() { …}
Drawing a folder (object) for a subclass. Students learn how
to draw such folders. It is important for later understanding
that they know that the complete method resides in the folder.
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j
a1
a0
a1
a0
…
k
a1
JFrame
SquareJFrame
area() …
…
JFrame
SquareJFrame
area() …
Assignment is seen to follow the normal rules for an
assignment: To execute
j= k;
evaluate expression k (gives the value a1) and store its value
in j.
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Model helps, right from the beginning
• Students understand what an object is, right from the
beginning. The explanation is not in terms of a computer.
• The pictorial nature of the model is important. The name on
the tab is most important. Our students learn to draw objects
for subclasses, and they use it later to learn various concepts
(inheritance, overriding, the inside-out rule for determining
what declaration a method call or variable refers to.
• Pointer and reference are not used.
UML diagrams are inadequate because they do not have the
equivalent of the name on the tab of an object. Although it
would be easy to extend UML diagrams for objects to
include it.
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i
Inside-out rule: To determine to
which variable declaration a
variable reference refers, search in
the current scope, the enclosing
scope, its enclosing scope, etc.
until it is found. (Similar rule for
method declarations and calls.)
a1
v
2
C
4
SC
w p) {
m(int
int lv;
while ( … ) {
int n; … sv … n
}
}
sv
Inside-out rule used in just about every
programming language and in predicate logic:
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m() { …}
SC's file
drawer
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Closed labs, after lectures 1, 3, 5, …, give practice with concepts.
Lecture 1. Types, expressions, variables, assignments
Lecture 2. Objects
Lecture 3. Class def. Subclass of JFrame with 2 methods
Lecture 4. Fields, getter/setter methods. Junit testing
Lecture 5. Class hierarchy. Static variables
Lecture 6. Methods; method bodies; local variables; if-statement
Lecture 7. Super-this; inside-out rule. Stepwise refinement
Lecture 8. Overriding; constructors in subclasses
Stepwise refinement
Lecture 9 and 10. Recursion
Lecture 11. Casting about; instanceof
Lecture 12. Loops
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David and Paul Gries. A Multimedia Introduction to
Programming Using Java. Springer Verlag, NY 2005.
Comes with a CS that has 250-odd 2-5 minute lectures
with synched animation. Here, we can really concentrate
on program development.
Webpage for last Spring's course.
http://www.cs.cornell.edu/courses/cs100j/2007sp/
You can get slides of lectures and labs, assignments, etc.
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