A Methodical Approach to Methods

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A Methodical Approach to Methods
Java's Work Horses

• CS 102-02
• Lecture 4-2

2
Whats up?

• Automatic variables scope
• Recursion
• Applets again

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Scope Duration

• In life
• 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

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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

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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.

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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

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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
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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

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Block Scope

• Block scope
• Variables are visible everywhere within curly
  braces and .
• Includes
• Methods
• for loops
• Other blocks

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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?

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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
• x0, Test.x1

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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!

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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

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Factorial Recursion
// Recursive definition of method
factorial public long factorial( long number )
if ( number lt 1 ) // base case return
1 else // n (n-1)!
return number factorial( number - 1 )
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Bottoming Out
// Recursive definition of method
factorial public long factorial( long number )
if ( number lt 1 ) // base case // 1!
1 (By definition, 0! 1) return 1 else
// n (n-1)! return number
factorial( number - 1 )
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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?

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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

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Factorial Again
// Iterative definition public long factorialIter
( long number ) long factorial 1 if
(number 0) return 1 else
for(long tempnumber temp ! 1 temp--)
factorial temp return factorial
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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

Applet
Browser
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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

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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()