Download Customizing a Class and Testing - Lecture Slides | CS 1110 and more Quizzes Computer Science in PDF only on Docsity!
1
CS1110 11 Sept 2007. Customizing a class & testing
Quote for the day:
There is no reason anyone would want a computer in their home.
--Ken Olson, founder of Digital Equipment Corp. (DEC), 1977.
The company was a huge player in computer hardware and software in CS
academia in the 1970’s. The PDP machines were well known. The VAX had
unix on it, and C, and Lisp. It was the main computer in most CS
departments of any stature. DEC was bought by COMPAQ in the late 1990’s.
Quiz 2 on Tuesday (16 September):
How do you evaluate a new expression (see slide 8)?
What is the purpose of a constructor (see slide 7)?
- Fields (variables in a folder); getter & setter methods.
Secs 1.4.1 (p. 45) & 3.1 (pp. 105–110 only)
- Constructors. Sec. 3.1.3 (p. 111–112)
- Testing methods. Appendix I.2.4 (p. 486)
2
One-on-One Sessions
Next two weeks, hold a 1/2-hour one-on-one session on a
computer with each student in CS1110.
Purpose : See how well you understand what we have done,
let you ask questions, give you help. Graded 0-1: you get 1 if
you took part in a session. Not counted in course grade.
Purpose: to help you.
Instructors : Gries, TAs, consultants.
How to sign up : Visit the CMS for the course. Click on the
assignment One-on-one. You will see a list of times and
instructors. Choose one. First-come-first-served.
Not registered in the CMS? Email Maria Witlox immediately
and ask her to register you: [email protected]
3
YOU CAN EXECUTE AN ASSIGNMENT
STATEMENT
Quiz webpage said to look at top of page 28. It says:
To execute the assignment, evaluate the expression
and store the value in the variable.
Quiz did not ask for a description of how the computer does it,
or what you type in DrJava, it asked for you to execute the
assignment statement.
j 32
j= j + 2;
j = j + 2
j = 32 + 2
j = 34
4
Field: a variable that is in each folder of a class.
a
Chapter title (^) …
number (^) …
previous … public class Chapter {
private String title; // Title of the chapter
private int number; // Number of the chapter
private Chapter previous; // previous chapter (null if none)
We generally make fields
private instead of public , so
that they cannot be referenced
from methods that are outside
the class.
Declarations
of fields
5
Getter and setter methods
a
Chapter title …
number …
previous …
/** An instance describes a chapter
of a book */
public class Chapter {
// Title of the chapter
private String title;
/** = title of the chapter */
public String getTitle() {
return title;
}
/** Set chapter title to t */
public void setTitle(String t) {
title= t;
}
Getter methods (functions) get
or retrieve values from a folder.
Setter methods (procedures) set
or change fields of a folder
getTitle() setTitle(String t)
6
Initialize fields when a folder is first created
new Chapter()
creates a object but doesn’t allow us
To say what values should be in it.
We would like to be able to say:
new Chapter(“I am born”, 1, null )
to set the title to “I am born”, the
chapter number to 1, and the previous
chapter to null.
For this, we use a new kind of
method, the constructor.
a
Chapter title (^) …
number (^) …
previous …
getTitle() setTitle(String t)
7
Purpose of a constructor:
To initialize (some) fields of a newly created object
/** An instance describes a chapter of a book */
public class Chapter {
private String title; // Title of chapter
private int number; // No. of chapter
private Chapter previous; // previous
// chapter (null if none)
/** Constructor: an instance with title t,
chapter number i, and previous
chapter p (null if none) */
public Chapter(String t, int i,
Chapter p) {
title= t;
number= i;
previous= p;
}
}
a
Chapter title (^) …
number (^) …
previous …
getTitle() setTitle(String t)
Chapter(String t,
int i, Chapter c)
The name of a constructor is the name of the class.
Do not put a type or void here 8
New description of evaluation of a new-expression
new Chapter(“I am born”, 1, null )
- Create a new folder of class
Chapter, with fields initialized to
default values (e.g. 0 for int )
–of course, put the folder in the
file drawer.
- Execute the constructor call
Chapter(“I am born”, 1, null)
- Use the name of the new object
as the value of the new
-expression.
a
Chapter title (^) …
number (^) …
previous …
getTitle() setTitle(String t)
Chapter(String t,
int i, Chapter c)
Memorize this new definition! Today! Now!
9
Testing —using JUnit
Bug : Error in a program.
Testing : Process of analyzing, running program, looking for bugs.
Test case : A set of input values, together with the expected output.
Debugging : Process of finding a bug and removing it.
Get in the habit of writing test cases for a method from the
specification of the method even before you write the method.
A feature called Junit in DrJava helps us develop test cases
and use them. You have to use this feature in assignment A1.
10
- c1= new Chapter(“one”, 1, null );
Title should be: “one”; chap. no.: 1; previous: null.
- c2= new Chapter(“two”, 2, c);
Title should be: “two”; chap. no.: 2; previous: c1.
To create a testing framework: select menu File item new Junit
test case…. At prompt, put in class name ChapterTester. This
creates a new class with that name. Save it in same directory as
class Chapter.
The class imports junit.framework.TestCase , which provides
some methods for testing.
Need a way to run these test cases, to see whether the fields
are set correctly. We could use the interactions pane, but then
repeating the test is time-consuming.
Here are two test cases
11
/** A JUnit test case class.
public class ChapterTester extends TestCase {
/** A test method.
(Replace "X" with a name describing the test. Write as
many "testSomething" methods in this class as you wish,
and each one will be called when testing.) */
public void testX() {
}
One method you can use in testX is
assertEquals(x,y)
which tests whether expected value x equals y
12
A testMethod to test constructor and getter methods
/** Test first constructor and getter methods getTitle,
getNumber, and getPrevious */
public void testConstructor() {
Chapter c1= new Chapter("one", 1, null);
assertEquals("one”, c1.getTitle(), );
assertEquals(1, c1.getNumber());
assertEquals(null, c1.getPrevious());
Chapter c2= new Chapter("two", 2, c1);
assertEquals("two”, c2.getTitle());
assertEquals(2, c2.getNumber());
assertEquals(c1, c2.getPrevious());
}
Every time you click button Test in
DrJava, this method (and all other
testX methods) will be called.
first
test
case
second
test
case
assertEquals(x,y):
test whether x equals y ;
print an error message
and stop the method if
they are not equal.
x: expected value,
y: actual value.
A few other methods that
can be used are listed on
page 488.
