1. A Methodical Approach to Methods
Java's Work Horses CS
Lecture 4-2 1

2. What’s up?
Automatic variables & scope
Recursion
Applets again

3. Scope & Duration In life: In programming: Scope is where you go
Duration is how long you live
In programming:
Scope is where a variable can be referenced
Duration is how long the variable's value exists

4. Automatic Means There When You Need Them
Automatic variables
Automatic duration: automatically created & destroyed
Don't get intialized
Automatic variables include:
Local variables declared (and defined) inside a method
Parameter values passed to a method

5. Initializing Variables
Class instance variables are initialized:
public class Circle {
private int x_center, y_center, radius;
// Each of the above variables is now
// set to 0, because all instance
// variables are initialized, and
// numeric variables are initialized
// to 0. : }

6. Initializing Auto Variables
Automatic variables are not initialized:
public class Circle { :
public double Area() {
double r_squared;
// Next line's no good, because we
// haven't initialized r_squared
// yet.
return Math.PI * r_squared; }

7. Auto in Action Suppose the circle class includes the following method:
public double Area() {
double r_squared;
r_squared = radius * radius;
return Math.PI * r_squared; }
In the course of a program, we call the area method on a Circle object:
Circle testCircle = new Circle(10, 10, 200); :
// r_squared (defined in Area()) doesn't exist
testCircle.Area();
// r_squared did exist, but it's gone now

8. The Rules of Scope Class scope Visible everywhere within a class
class Test {
Test() {
k = 2; // Notice that k hasn't appeared }
int j = 1;
int i = j;
int k; // k is defined here, but its scope
// is the entire class

9. Block Scope Block scope
Variables are visible everywhere within curly braces: { and }.
Includes:
Methods
for loops
Other blocks

10. Hiding Variables
What happens if a variable is declared in the scope of another variable with the same name?
class Test {
static int x = 1;
public static void main(String[] args) {
int x = 0; :
// What's the value of x here? }

11. Telling Them Apart When you run this code: class Test {
static int x = 1;
public static void main(String[] args) {
int x = 0;
System.out.print("x=" + x);
System.out.println(", Test.x=" + Test.x); }
You get:
x=0, Test.x=1

12. Running Again with Recursion
Doing the same thing over and over
Looping
Recursion: functions that call themselves
Recursion is like induction
Example: Factorial
5! = 5 * 4 * 3 * 2 * 1
is the same as
5! = 5 * 4!

13. The Factorial Example How do we calculate the factorial of n?
Multiply n times the factorial of n-1:
n * (n-1)!
n * (n-1) * ((n-1) -1)!
n * (n-1) * ((n-1) - 1) * (((n-1) -1) - 1)!
n * (n-1) *•••((((•••((((n-1) -1) -1) ••• -1)
Now we have to translate this into Java
Remember that n! = n * (n-1)!
Special case when n == 1

14. Factorial Recursion // Recursive definition of method factorial
public long factorial( long number ) {
if ( number <= 1 ) // base case
return 1;
else
// n * (n-1)!
return number * factorial( number - 1 ); }

15. Bottoming Out // Recursive definition of method factorial
public long factorial( long number ) {
if ( number <= 1 ) // base case
// 1! = 1 (By definition, 0! = 1)
return 1;
else
// n * (n-1)!
return number * factorial( number - 1 ); }

16. Visual Café Example The Call window shows the call stack
Call stack is a list of "active" methods
As one method calls another, each method is placed on the stack
For the factorial example, what do you expect to see in the call stack?

17. Why Choose Recursion? Recursion is very similar to iteration
Recursive methods can be rewritten with iteration
Recursion is a different way of looking at the problem
Factorial recursion based on:
n! = n * (n-1)!
Factorial iteration based on:
n! = n * (n-1) * (n-2) * ... * 1

18. Factorial Again // Iterative definition
public long factorialIter ( long number ) {
long factorial = 1;
if (number == 0) {
return 1;
} else {
for(long temp=number; temp != 1; temp--){
factorial *= temp; }
return factorial;

19. Applets Revisited Applets aren't standalone applications
Depend on somebody else to provide numerous functions
Applet class has 21 methods, but 5 of them control interaction between an applet and its container
Browser
Applet

20. The Fab Five Applet Methods I
public void init()
Gets things started
Good place to set up the initial screen appearance
Only gets called once
public void paint(Graphics g)
Draws stuff on the screen
called every time the applet needs to be redrawn

21. The Fab Five Applet Methods II
Three methods used with threads
public void start()
Called after init()
Called again every time the browser returns to the HTML page
public void stop()
Called every time the browser leaves the page
public void destroy()