1. Variable Scope & Lifetime
CS 161
February 11, 2000

2. Overview
A few more words about Program #3
JavaDoc
Variable Scope

3. Program #3 Some Changes Some Clarification Some Removals
Draw half bar may be special case of drawing full bar
Some Clarification
Methods are required
Documentation for methods are required
Some Removals
Do not turn in JavaDoc generated files

4. Software Development Software Engineering Java to the Rescue
Documentation
Documentation is hard to keep accurate & up to date
The source code is the best source of documentation
Java to the Rescue
Well, a program included with the Java Development Kit (JDK) JavaDoc
Reads Java source code and produces documentation

5. JavaDoc
JavaDoc is a command line program which extracts comments from Java source code and formats them into HTML pages
Works on public
packages, classes, interfaces, methods & variables
Also protected methods & variables
Extracts comments which begin with /**
And Special Keywords

6. JavaDoc General Tags @since text @deprecated text @see link
Where text is a description of the version which introduced this feature
@deprecated text
Where text is a description of methods which you plan on making obsolete in a future release.
You should suggest an alternative
@see link
Where link is text or an HTML reference tag
<a href="otherpage.html">label</a>

7. Class & Interface Tags @Author Name @Version text
Where Name is your (or the program author's) name.
You may want to include an address or a company home page
@Version text
Where text is a description of this version of the source code

8. Method Tag @param @param variable description
Where variable is one of the formal arguments in the following method
The description is a description of what the variable is used for, any restrictions or limitations expected or enforced (e.g. range). It’s not a bad idea to include type information
The descriptions provided as examples in the textbook are anemic
All of the param tags for a given method need to be grouped together in the same comment without intervening tags.

9. Method Tag @return @return description @throws class description
The description you provide should indicate what the return value is, what it can be used for, special values, units, and so on.
@throws class description
If your class throws an exception (which must be handled inside of a try-catch block), you would describe it with this tag.

10. An Example Program Program Fac.java /**
A Java Application to demonstrate factorials
@author Cay Horstman
@version Computing Concepts w/ Java 2 Essentials 2nd Ed
@see <a href="
For CS 161, Winter 2000</a> */
public class Fac {

11. Factorial Program, Continued
/**
The main method is where execution starts.
It instantiates a ConsoleReader object to get
interactive input from the user. It calls the
recursive <B>factorial</B> method.
@param args An array of strings consisting of the
words on the command line. <I>These
are ignored</I> */
public static void main(String[] args) {
ConsoleReader console = new ConsoleReader(System.in);
System.out.println("Please enter a number:");
int n = console.readInt();
System.out.println(n + "! = ” + factorial(n)); }

12. Factorial Program, Continued
Program Fac.java (continued)
/**
Recursively computes the factorial of an integer.
@param n an integer >= 0
@return n! <I>(if n is out of bounds, a 1 is returned)</> */
public static int factorial(int n) {
if (n <= 0)
return 1;
else
int result = n * factorial(n - 1);
return result; }

13. Program Fac.java (continued)
/**
Computes the factorial of an integer.
@param n an integer >= 0
@return n! */
public static int factorial(int n) {
if (n == 0)
return 1;
else
int result = n * factorial(n - 1);
return result; }

14. Running JavaDoc Z:\CS161\Factorial\docs>javadoc Factorial.java
Loading source file Factorial.java...
Constructing Javadoc information...
Building tree for all the packages and classes...
Building index for all the packages and classes...
Generating overview-tree.html...
Generating index-all.html...
Generating deprecated-list.html...
Building index for all classes...
Generating allclasses-frame.html...
Generating index.html...
Generating packages.html...
Generating Factorial.html...
Generating serialized-form.html...
Generating package-list...
Generating help-doc.html...
Generating stylesheet.css...
Z:\CS161\Factorial\docs>

18. Java Variables
Variables in Java can be ether primitives or objects (based on classes)
We can divide variables into 4 groups
Instance (Class) variables
Static Variables
Local (method) variables
Parameter Variables

19. Variable Lifetime The lifetime of a variable describes the time
A variable is created
Through the time that it is destroyed
And that storage is available for re-use
When an object is constructed (instantiated)
All instance variables are created
They are initialized according to their declaration statements
The position in the Java source code is relatively unimportant
It does start at the top and go to the bottom

20. The Duration of a Variable's Lifetime
When the object which created the object is no longer useable
The method that created it ends
All reference to that object are gone
The variable is disposed
And the storage space is available for other variables

21. Static Variables
When a class with static variables is referenced for the first time
All static variables are created
Any initialization is performed
When the object which loaded the static class is no longer needed
(which is hard to tell)
The static variables of that class are destroyed

22. Parameter Variables
Parameter variables are the variables based on the formal arguments to a method
These are always copies of the value of primitive values declared elsewhere
Or values from expressions
These remain 'in scope' throughout the lifetime of that method call
They are recreated with each method call

23. Local Variables These are variables declared within a method
Or other code block
e.g. for ( int j = 0; … ) { … }
It exists only within that code block

24. Initialization All instance variables are automatically initialized
0 for numbers
false for boolean
null for objects
unless we specify otherwise
Parameter variables are initialized with copies of their actual arguments

25. Key ideas Lifetime (duration) Scope Shadow Alias
When the variable (or object) exists in your program
Scope
Where the variable can be used
Shadow
When the scope of one variable takes precedence over another of the same name
Alias
The same object is referenced by two different names

26. Overview / To Do Program #3 JavaDoc Variables Monday Wednesday Friday
Kinds
instance, static, parameter & local
Lifetime/Duration
Scope
Shadowing
Aliases
Monday
Testing & Debugging
Wednesday
OOP & Class Design
Program #3 Due
Friday
Inheritance
Quiz #4
WS #4 & HW #5