Events

1
Events

• An event is an object that represents some
  activity to which we may want to respond
• Java makes it easy to write an "event driven
  program" when we want our program to perform
  some action when the following occurs
• the mouse is moved or dragged
• a mouse button is clicked
• a graphical button is clicked
• a keyboard key is pressed
• a timer expires

2
Events

• when we import java.awt.event. we get to use
  the Java event library
• Certain objects, such as applets, keys and
  graphical buttons, generate (fire) an event when
  they occur
• Other objects, called listeners, respond to
  events
• We can write our own listener objects to do
  whatever we want when an event occurs

3

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class SimplestButtonExampleEver
• extends Applet implements ActionListener
• Button aButton new Button("Click Me!")
• public void init()
• add(aButton)
• aButton.addActionListener(this)
• public void paint(Graphics g)
• g.drawString("You rolled a "
• (int)(Math.random()61), 20, 20)
• public void actionPerformed(ActionEvent ae)

4
this means the class I'm inside of

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class SimplestButtonExampleEver
• extends Applet implements ActionListener
• Button aButton new Button("Click Me!")
• public void init()
• add(aButton)
• aButton.addActionListener(this)
• public void paint(Graphics g)
• g.drawString("You rolled a "
• (int)(Math.random()61), 20, 20)
• public void actionPerformed(ActionEvent ae)

5
A Simple Button Applet
6
setLayout(null)setBounds()

• Java will automatically size and place the button
  unless you tell it not to
• public void init()
• //Turn Layout manager off
• setLayout(null)
• //Position Button(X, Y, Width, Height)
• Button1.setBounds(10,20,50,20)

7
Multiple buttons

• If you have more than one button, you can find
  out which one was pressed with e.getSource()
• Button first, second
• //lots of java not shown
• public void actionPerformed(ActionEvent e)
• if (e.getSource() first)
• //do something
• else if (e.getSource() second)
• //do something else

8
Practice Quiz Question What number will be
displayed after buttons b1, b2 and b3 are pressed
in that order?

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class ButtonQuizAP extends Applet
  implements ActionListener
• Button b1, b2, b3
• int nValue
• public void init()
• nValue 4
• b1 new Button("One")
• b2 new Button("Two")
• b3 new Button("Three")
• add(b1)
• add(b2)
• add(b3)
• b1.addActionListener(this)
• b2.addActionListener(this)

9
Nested loops

• "Nested" means "one inside the other"
• Let's say we wanted to make a 10x10 times table
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20
• 3 6 9 12 15 18 21 24 27 30
• 4 8 12 16 20 24 28 32 36 40
• 5 10 15 20 25 30 35 40 45 50
• 6 12 18 24 30 36 42 48 54 60
• 7 14 21 28 35 42 49 56 63 70
• 8 16 24 32 40 48 56 64 72 80
• 9 18 27 36 45 54 63 72 81 90
• 10 20 30 40 50 60 70 80 90 100

10
Nested loops

• Let's start with the first row
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20
• 3 6 9 12 15 18 21 24 27 30
• 4 8 12 16 20 24 28 32 36 40
• 5 10 15 20 25 30 35 40 45 50
• 6 12 18 24 30 36 42 48 54 60
• 7 14 21 28 35 42 49 56 63 70
• 8 16 24 32 40 48 56 64 72 80
• 9 18 27 36 45 54 63 72 81 90
• 10 20 30 40 50 60 70 80 90 100
• It would be easy enough to write a loop that
  makes the numbers 1 through 10

11
Nested loops

• Let's start with the first row
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20 x2
• 3 6 9 12 15 18 21 24 27 30 x3
• 4 8 12 16 20 24 28 32 36 40 x4
• 5 10 15 20 25 30 35 40 45 50 x5
• 6 12 18 24 30 36 42 48 54 60 x6
• 7 14 21 28 35 42 49 56 63 70 x7
• 8 16 24 32 40 48 56 64 72 80 x8
• 9 18 27 36 45 54 63 72 81 90 x9
• 10 20 30 40 50 60 70 80 90 100 x10
• We'd put that loop inside another loop, that
  "multiplies" it

12
Nested loops

• for(int nJ 1 nJ lt 10 nJ)
• System.out.print(nJ '\t')
• //inside loop counts columns 1 to 10
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20 x2
• 3 6 9 12 15 18 21 24 27 30 x3
• 4 8 12 16 20 24 28 32 36 40 x4
• 5 10 15 20 25 30 35 40 45 50 x5
• 6 12 18 24 30 36 42 48 54 60 x6
• 7 14 21 28 35 42 49 56 63 70 x7
• 8 16 24 32 40 48 56 64 72 80 x8
• 9 18 27 36 45 54 63 72 81 90 x9
• 10 20 30 40 50 60 70 80 90 100 x10

13
Nested loops

• for(int nI 1 nI lt 10 nI)
• for(int nJ 1 nJ lt 10 nJ)
• System.out.print( ?? '\t')
• ??
• //outside loop counts rows 1 to 10
• //(inside loop column)outside row
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20 x2
• 3 6 9 12 15 18 21 24 27 30 x3
• 4 8 12 16 20 24 28 32 36 40 x4
• 5 10 15 20 25 30 35 40 45 50 x5
• 6 12 18 24 30 36 42 48 54 60 x6
• 7 14 21 28 35 42 49 56 63 70 x7
• 8 16 24 32 40 48 56 64 72 80 x8
• 9 18 27 36 45 54 63 72 81 90 x9
• 10 20 30 40 50 60 70 80 90 100 x10

14
In Class

• Write an application that generates a 10x10 times
  table using nested loops and displays it using
  System.out.print()
• Hints
• Use "\t" to space numbers
• Use an empty System.out.println() to end the line

15
In Class

• Next, Modify your application so that it produces
  the following output
• (1,1) (1,2) (1,3) (1,4) (1,5) (1,6) (1,7) (1,8) (1
  ,9) (1,10)
• (2,1) (2,2) (2,3) (2,4) (2,5) (2,6) (2,7) (2,8) (2
  ,9) (2,10)
• (3,1) (3,2) (3,3) (3,4) (3,5) (3,6) (3,7) (3,8) (3
  ,9) (3,10)
• (4,1) (4,2) (4,3) (4,4) (4,5) (4,6) (4,7) (4,8) (4
  ,9) (4,10)
• (5,1) (5,2) (5,3) (5,4) (5,5) (5,6) (5,7) (5,8) (5
  ,9) (5,10)
• (6,1) (6,2) (6,3) (6,4) (6,5) (6,6) (6,7) (6,8) (6
  ,9) (6,10)
• (7,1) (7,2) (7,3) (7,4) (7,5) (7,6) (7,7) (7,8) (7
  ,9) (7,10)
• (8,1) (8,2) (8,3) (8,4) (8,5) (8,6) (8,7) (8,8) (8
  ,9) (8,10)
• (9,1) (9,2) (9,3) (9,4) (9,5) (9,6) (9,7) (9,8) (9
  ,9) (9,10)
• (10,1) (10,2) (10,3) (10,4) (10,5) (10,6) (10,7) (
  10,8) (10,9) (10,10)
• Which loop gives the row? Which loop gives the
  column? Change your variable names so that they
  match.

16
What is the output?

• for(char cChar 'a' cChar lt 'c' cChar)
• for(int nI 1 nI lt 4 nI)
• System.out.print(cChar ", " nI '\t')
• System.out.println()

17
2 dimensional arrays

• Can be thought of as a grid with rows and
  columns

column 0 1 2
-3
-1
5
row 0
7
12
13
row 1
18
2 dimensional arrays

• int naNums -3, -1, 5,
• 7, 12, 13
• /In this form, Java automatically sizes the
  array to 2 rows and 3 columns
• Note rows first, then columns /
• System.out.println(naNums12)
• //displays 13
• System.out.println(naNums21)
• //Out of bounds exception!

column 0 1 2
-3
-1
5
row 0
7
12
13
row 1
19
2 dimensional arrays

• int naNums
• /In Java, arrays are objects. If we declare and
  initialize the array seperately, we need to use
  "new" /
• naNums new int23

20
2 dimensional arrays

• int naNums
• naNums new int23
• Typically, nested loops are used to initialize a
  2d array. The outside loop controls the row, and
  the inside the column
• for(int nRow 0 nRow lt 2 nRow)
• for(int nCol 0 nCol lt 3 nCol)
• naNumsnRownCol nCol(3nRow)

column 0 1 2
0
1
2
row 0
3
4
5
row 1
21
A one dimensional array of buttons

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class Buttons extends Applet implements
  ActionListener
• Button buttonArray
• int nRows 3
• int nColumns 4
• int nTotalButtons nRows nColumns
• public void init()
• //declare a grid layout with no space
  between buttons
• setLayout(new GridLayout(nRows, nColumns,
  0, 0))
• //declare a new array of buttons
• buttonArray new ButtonnTotalButtons
• //initialize each of the buttons in the
  array

22
A one dimensional array of buttons

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class Buttons extends Applet
• implements ActionListener
• Button buttonArray
• int nRows 3
• int nColumns 4
• int nTotalButtons nRows nColumns
• public void init()
• //declare a grid layout with no space
  between buttons
• setLayout(new GridLayout(nRows, nColumns,
  0, 0))
• //declare a new array of buttons
• buttonArray new ButtonnTotalButtons

23
A one dimensional array of buttons

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class Buttons extends Applet
• implements ActionListener
• Button buttonArray
• int nRows 3
• int nColumns 4
• int nTotalButtons nRows nColumns
• public void init()
• //grid layout with no space between buttons
• setLayout(new GridLayout(nRows,nColumns,0,0))
• //declare a new array of buttons
• buttonArray new ButtonnTotalButtons
• //initialize each of the buttons in the array
• //with an empty label

24
A one dimensional array of buttons

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class Buttons extends Applet
• implements ActionListener
• Button buttonArray
• int nRows 3
• int nColumns 4
• int nTotalButtons nRows nColumns
• public void init()
• //grid layout with no space between buttons
• setLayout(new GridLayout(nRows,nColumns,0,0))
• //declare a new array of buttons
• buttonArray new ButtonnTotalButtons
• //initialize each of the buttons in the array
• //with an empty label

25
A one dimensional array of buttons

• public void actionPerformed(ActionEvent e)
• //find out which button is the source
• int nButtonNumber 0
• for(int i 0 i lt buttonArray.length
  i)
• if(e.getSource() buttonArrayi)
• nButtonNumber i
• //display button Number at bottom of
  screen
• showStatus("Button number "
  nButtonNumber)
• //change background of buttons
• if(buttonArraynButtonNumber.getBackgrou
  nd()
• !
  Color.red)
• buttonArraynButtonNumber.setBackground
  (
• 
  Color.red)
• else
• buttonArraynButtonNumber.setBackground(
• 
  Color.blue)

26
How would we modify the example for minesweeper?

• When a button is pressed, we need to know two
  numbers the row and column
• We also need to know if the button is hiding a
  mine

27
extending the Button class

• class Cell extends Button
• private boolean bIsBomb
• private int myRow, myCol
• . . .

28
extending the Button class

• class Cell extends Button
• private boolean bIsBomb
• private int myRow, myCol
• public Cell(??)
• ??

29
extending the Button class

• class Cell extends Button
• private boolean bIsBomb
• private int myRow, myCol
• public Cell(??)
• ??
• public int getRow()return myRow

30
In actionPerformed

• To find out the row (and column) of the button
  that was pressed
• public void actionPerformed(ActionEvent e)
• int nRow ((Cell)e.getSource()).getRow()

31
In actionPerformed

• To find out the row (and column) of the button
  that was pressed
• //Get the object that generated the event
• e.getSource()
• //Cast it as a Cell
• (Cell)e.getSource()
• //Then ask it what row it came from
• ((Cell)e.getSource()).getRow()

32
How to get started on Minesweeper

• Start a new Applet Project. Copy the sample
  button applet code into your project
• Change nRows and nCols to constants public
  final static int nROWS 10
• Modify the array to be a 2 dimensional array.
  Button buttonArray will become
  Button buttonArray and so on
• You can delete the code in actionPerformed. Were
  going to use a different technique.
• Write a new class Cell that extends Button
• Change the applet from an array of Buttons to an
  array of Cells.

33
How to get started on Minesweeper

• AB students should use a Set to make sure that
  the correct number of bombs is placed
• Don't forget import java.util.

34
What will be displayed?

• public class Recursion extends Applet
• public void init()
• mystery()
• public void mystery()
• System.out.println(Math.random())

35
What will be displayed now?

• public class Recursion extends Applet
• public void init()
• mystery()
• public void mystery()
• System.out.println(Math.random())
• mystery()

36
The program will repeatedly display random
numbersproblem is it won't stop!
37
Recursion A method that calls itself

• public void mystery()
• System.out.println(
• Math.random())
• mystery()
• Recursion is another way of making a loop
• Recursion is hard to controlit's very easy to
  create an infinite loop that never stops

38
In recursion, the "stopping point" is called the
base case

• public void mystery(int nTimes)
• if(nTimes 0)
• System.out.println("Stop!")
• else
• System.out.println(
• Math.random())
• mystery(nTimes-1)
• The base case stops the recursive calls

39

• public class Recursion extends Applet
• public void init()
• mystery(10)
• public void mystery(int nTimes)
• if(nTimes 0)
• System.out.println("Stop!")
• else
• System.out.println(
• Math.random())
• mystery(nTimes-1)

40
Recursion

• There are two basic ways to make things happen
  "over and over again" in programming
• Loops
• Recursion
• In theory, anything you can do with loops, you
  can do with recursion ( vice versa)

41
A loop that "counts" from 1 to 10

• public class Recursion extends Applet
• public void init()
• for(int nI 1 nI lt 10 nI)
• System.out.println(nI)

42
A Recursive Function that "counts" from 1 to 10

• public class Recursion extends Applet
• public void init()
• count(1)
• public void count(int nTimes)
• if(nTimes 10)
• System.out.println(nTimes " Stop!")
• else
• System.out.println(nTimes)
• count(nTimes1)

43
Just like a loop, it has a starting point, a
stopping point, and a way to get from one to the
other

• public class Recursion extends Applet
• public void init()
• count(1)
• public void count(int nTimes)
• if(nTimes 10)
• System.out.println(nTimes " Stop!")
• else
• System.out.println(nTimes)
• count(nTimes1)

44
Find the output

• import java.awt.
• import java.applet.
• public class Recursion extends Applet
• public void init()
• mystery(2)
• public void mystery(int nNum)
• if(nNum gt 5)
• System.out.println("Base Case")
• else
• System.out.print(nNum ", ")
• mystery(nNum 1)

45
Recursive Methods that return values

• A Recursive Method is a method that calls itself
• public int Mystery(int nNum)
• if(nNum 0)
• return 0
• else
• return 2 Mystery(nNum - 1)

46
Recursive Methods

• A Recursive Method is a method that calls itself
• public int Mystery(int nNum)
• if(nNum 0)
• return 0
• else
• return 2 Mystery(nNum - 1)

Recursion
47
Recursive Methods

• What would be returned by the call Mystery(0)?
• public int Mystery(int nNum)
• if(nNum 0)
• return 0
• else
• return 2 Mystery(nNum - 1)

0
0
48
Recursive Methods

• What would be returned by the call Mystery(1)?
• public int Mystery(int nNum)
• if(nNum 0)
• return 0
• else
• return 2 Mystery(nNum - 1)

1
1
49
Recursive Methods

• Mystery(1) 2 Mystery(0) 20
• public int Mystery(int nNum)
• if(nNum 0)
• return 0
• else
• return 2 Mystery(nNum - 1)

1
1
50
Recursive Methods

• What would be returned by the call Mystery(2)?
• public int Mystery(int nNum)
• if(nNum 0)
• return 0
• else
• return 2 Mystery(nNum - 1)

2
2
51
Recursive Methods

• Mystery(2) 2 Mystery(1)
• 2 2 Mystery(0) 2 2 0
• public int Mystery(int nNum)
• if(nNum 0)
• return 0
• else
• return 2 Mystery(nNum - 1)

2
2
52
Recursive Methods

• What would be returned by the call Mystery(10)?

53
Recursive Methods

• What would be returned by the call Mystery(10)?
• Mystery(10) 2 Mystery(9)
• 22Mystery(8)
• 222Mystery(7)
• 2222Mystery(6)
• 22222Mystery(5)
• 222222Mystery(4)
• 2222222Mystery(3)
• 22222222Mystery(2)
• 222222222Mystery(1)
• 2222222222Mystery(0)20

54
Recursive Methods

• Let's remove some code from Mystery
• Now what would be returned by the call
  Mystery(10)?
• public int Mystery(int nNum)
• if(nNum 0) //delete 3 lines
• return 0
• else
• return 2 Mystery(nNum - 1)

55
Recursive Methods

• Mystery(10) 2 Mystery(9)
• 22Mystery(8)
• 222Mystery(7)
• 2222Mystery(6)
• 22222Mystery(5)
• 222222Mystery(4)
• 2222222Mystery(3)
• 22222222Mystery(2)
• 222222222Mystery(1)
• 2222222222Mystery(0)
• 22222222222Mystery(-1)
• 222222222222Mystery(-2)
• and so on, and so on
• Infinite Recursion! Stack Overflow!
• CRASH!

56
The Base Case

• Every recursive method must have a base case
• The base case is where the recursion stops
• public int Mystery(int nNum)
• if(nNum 0) //base case
• return 0
• else
• return 2 Mystery(nNum - 1)

57
Preventing Stack Overflow

• What would happen with the following call?
• Mystery(-1)
• How could the method be improved?
• public int Mystery(int nNum)
• if(nNum 1)
• return 1
• else
• return 3 Mystery(nNum - 1)

58
Practice Quiz Question

• Find the return values of the calls to the
  Mystery method shown below
• Mystery(1)
• Mystery(2)
• Mystery(5)
• public int Mystery(int nNum)
• if(nNum 1)
• return 1
• else
• return 3 Mystery(nNum - 1)

59
Javabat Bunny Ears

• http//www.javabat.com/prob?idRecur1.bunnyEars
• We have a number of bunnies and each bunny has
  two big floppy ears. We want to compute the total
  number of ears across all the bunnies recursively
  (without loops or multiplication).

60
Problem Create a Fibonacci function

• The first two Fibonacci numbers are 1 1
• The next Fibonacci numbers can be found by adding
  the previous two
• 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89. . .

61
Problem Create a Fibonacci function

• We could describe the Fibonacci numbers like this

62

• public int fibonacci(int nNum)
• ???

63

• public int fibonacci(int nNum)
• if(nNum 0 nNum 1)
• ???

64

• public int fibonacci(int nNum)
• if(nNum 0 nNum 1)
• return 1
• else
• ???

65

• public int fibonacci(int nNum)
• if(nNum 0 nNum 1)
• return 1
• else
• return
• fibonacci(nNum-2)
• fibonacci(nNum-1)

66
Problem Create a non-recursive Fibonacci function

• public int fibonacci(int nNum)
• ???

67
Problem Create a non-recursive Fibonacci function

• public int fibonacci(int nNum)
• int nPos 0
• int nFibonacci1 1
• int nFibonacci2 1
• while(nPos lt nNum)
• int nTemp nFibonacci2
• nFibonacci2
• nFibonacci2 nFibonacci1
• nFibonacci1 nTemp
• nPos
• return nFibonacci1

68
Practice Quiz Question

• Write a simple applet that
• finds the factorial of an integer
• 3! 3 2 1
• Write a recursive factorial method
• Start with an Applet like
• import java.awt.
• import java.applet.
• public class FactorialPractice extends Applet
• public int factorial(int nNum)
• ??? //your 4 lines of code here
• public void paint(Graphics g)
• System.out.println(factorial(3))
• System.out.println(factorial(1))
• System.out.println(factorial(10))
• System.out.println(factorial(0))
• System.out.println(factorial(-3))

69
Head vs. Tail Recursion

• The position of the recursive call in a recursive
  method can have a BIG effect on the way the
  method works
• If the recursive call is at the end of the
  method, it's called tail recursion
• If the recursive call is at the beginning of the
  method, it's called head recursion

70
Tail Recursion

• public void tailRecursion(int nNum)
• if(nNum gt 10)
• System.out.println("base case")
• else
• System.out.print(nNum ", ")
• tailRecursion(nNum 1)

71
Head Recursion

• public void headRecursion(int nNum)
• if(nNum gt 10)
• System.out.println("base case")
• else
• headRecursion(nNum 1)
• System.out.print(nNum ", ")

72
Tail Recursion

• What would the following calls display?
• tailRecursion(10)
• tailRecursion(9)
• tailRecursion(1)
• public void tailRecursion(int nNum)
• if(nNum gt 10)
• System.out.println("base case")
• else
• System.out.print(nNum ", ")
• tailRecursion(nNum 1)

73
Tail Recursion

• What would the following calls display?
• tailRecursion(10)
• tailRecursion(9)
• tailRecursion(1)
• base case
• 9, base case
• 1, 2, 3, 4, 5, 6, 7, 8, 9, base case

74
Head Recursion

• What would the following calls display?
• headRecursion(10)
• headRecursion(9)
• headRecursion(1)
• public void headRecursion(int nNum)
• if(nNum gt 10)
• System.out.println("base case")
• else
• headRecursion(nNum 1)
• System.out.print(nNum ", ")

75
Head Recursion

• What would the following calls display?
• headRecursion(10)
• headRecursion(9)
• headRecursion(1)
• base case
• base case
• 9,
• base case
• 9, 8, 7, 6, 5, 4, 3, 2, 1,

76
Head vs. Tail Recursion

• Tail recursion works like you would expect, it
  goes "forward"
• Head recursion "stacks up" and then "unwinds", it
  goes backwards!

77
Javabat Bunny Ears 2

• http//www.javabat.com/prob?idRecur1.bunnyEars2
• Hint 3 possibilites base case, odd bunny, even
  bunny

78
Javabat Triangle

• http//www.javabat.com/prob?idRecur1.triangle
• 1,3,6,10,. . . .

79
Javabat sumDigits

• http//www.javabat.com/prob?idRecur1.sumDigits
• 123,456 can be split into two parts
• 123,456 10 6 (the ones digit)
• 123,456 / 10 12,345 (the rest of the digits)
• The sum of all the digits the one's digit plus
  the sum of the rest of the digits
• What's the base case?

80
Recursion in 2d arrays

• Problem Write a "Remove Blob" method. A Blob
  is a collection of one or more dark adjacent
  cells.

• There are 4 Blobs on the right
• Clicking on any dark cell should remove the
  entire Blob

81
A BlobCell class

• class BlobCell extends Button
• private int myRow, myCol
• public BlobCell(int nR, int nC)
• myRow nR
• myCol nC
• public int getRow()return myRow
• public int getCol()return myCol

82
The "Applet" class

• public class RemoveBlobDemo extends Applet
• implements ActionListener
• BlobCell buttonArray
• final static int nROWS 9
• final static int nCOLUMNS 9
• // init() not shown
• public void actionPerformed(ActionEvent e)
• int nRow
• ((BlobCell)e.getSource()).getRow()
• int nCol
• ((BlobCell)e.getSource()).getCol()
• removeBlob(nRow,nCol)

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The removeBlob method

• public class RemoveBlobDemo extends Applet
• implements ActionListener
• . . .
• public void removeBlob(int nRow,int nCol)
• if(buttonArraynRownCol.getBackground()
• .equals(Color.black))
• buttonArraynRownCol.
• setBackground(Color.white)
• . . . .

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The removeBlob method

• public class RemoveBlobDemo extends Applet
• implements ActionListener
• . . .
• public void removeBlob(int nRow,int nCol)
• if(buttonArraynRownCol.getBackground()
• .equals(Color.black))
• buttonArraynRownCol.
• setBackground(Color.white)
• //Check 8 neighbors
• if(nRow gt 0 nCol gt 0 )
• removeBlob(nRow-1,nCol-1)

85
Practice Quiz Questions

• Which of the following calls to this method
  create infinite recursion?
• public void mystery(int nNum1, int nNum2)
• if(nNum1 ! nNum2)
• mystery(nNum1 1, nNum2 - 1)
• mystery(2,3)
• mystery(3,3)
• mystery(2,4)
• none of the above

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substring() one argument

• returns the rest of the String beginning at that
  index
• Examples
• "unhappy".substring(2) returns "happy"
• "Harbison".substring(3) returns "bison"
• "emptiness".substring(9) returns "" (empty
  String)

87
substring() two arguments

• returns part of the String beginning at that
  first index, and ending one position before the
  second index
• Examples
• "hamburger".substring(4, 8) returns "urge"
• "smiles".substring(1, 5) returns "mile"

88
JavaBat practice problemRecur1 gt countX

• Given a string, compute recursively (no loops)
  the number of lowercase 'x' chars in the string.
• countX("xxhixx") ? 4
• countX("xhixhix") ? 3
• countX("hi") ? 0

89
Practice Quiz Questions

• Complete the following method so that clicking on
  a black button changes it's background to white
  AND the background of all the adjacent black
  buttons to the right

public void removeRowRight(int nRow,int nCol)
if(buttonArraynRownCol.getBackground(
).equals(Color.black))
buttonArraynRownCol.setBackground(Color.white)
if(__________________________)
_________________________________________

90

• A class of 30 students rated their cs teacher on
  a scale of 1 to 10. The response array is a 30
  element integer array of the student reponses.
  An 11 element array will count the number of
  occurrences of each response. For example
  freq6 will count the number of students who
  responded 6. The quantity freq0 will not be
  used. Here is a program that counts the
  students' responses and outputs the results
• public class StudentEvaluations
• public static void main(String args)
• int responses 6,6,7,8,10,1,5,4,6,7,5,4,
  3,4
• ,4,9,8,6,7,10,6,7,8,8,9,6,7,8,9
  ,2
• int freq new int11
• for(int i0iltresponses.lengthi)
• freqresponsesI
• Suppose the last entry in the initializer list
  for the responses array was incorrectly typed as
  12 instead of 2. What would be the result of
  running the program?
• A rating of 12 would be listed with a frequency
  of 1
• A rating of 1 would be liste with a frequency of
  12
• An ArrayIndexOutOfBoundsException would be thrown
• A StringIndexOutOfBoundsException would be thrown
• A NullPointerException would be thrown