A Methodical Approach to Methods Java's Work Horses CS 102-02 Lecture 4-2 1

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

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

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

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. : }

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; }

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

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

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

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? }

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

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!

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

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 ); }

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 ); }

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?

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

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;

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

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

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()