Control Structures in Java I

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Control Structures in Java I
Controlling Java

• CS 102-02
• Lecture 2-2

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Control Structures Control Flow

• Program inertia
• Java programs start with the first statement
• Jump to the next, and the next
• until...
• Control structures change the program flow

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Selection Structures

• Two kinds of selection
• Ifthen Only do it if the conditions true
• If...then...else Do one thing or the other

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If I Had a Hammer...

• In Java syntax
• if ( Expression ) Statement
• Expression MUST be a Boolean expression
• Statement can be a block of statements
• Theres no then in them thar hills

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If I Had an Example..

• if (pulse 0) System.out.println(Passed)
• if (pulse 200)
• System.out.println(Slow down!)

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Compound Statements

• If Java expects a statement, you can also use a
  compound statement
• if ( Expression ) Statement
• Compound statement is a group of statements,
  enclosed in

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Compounding a Statement
if (temperature 10)
me.bundleUp() me.complain(Chicago,
badWeather) me.tryToGraduate(now)
me.returnToWarmerClimes(I love L.A.!)
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The Declaration of Independence

• When is a Compound Statement a Block? When it
  contains a declaration.
• A declaration says to Java Im going to use a
  variable, and its going to be of this type.
• Graphics graphicsObject
• Not a particular Graphics object yet

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A Defining Moment

• Declarations are good, but theres more
• Definitions associate an initial value with a
  name
• Graphics graphicsObject new Graphics()

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Why Do We Care?

• Block CompoundStatementDeclaration(s)
• Declarations in a block are special because they
  have block scope
• Graphics graphicsObject
• // some code goes here
• // graphicsObject still exists
• // now its gone

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Its Minty Fresh

• Scope is where a variable is visible
• More details on scope when we get to methods
• And now, back to our regularly scheduled
  lecture...

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A Little Medical Diagnosis
if (pulse 200 pulse n(Heart beats gang aft agley) if (respiration
75) System.out.println(You dont look so
good) else System.out.println(You look a
little flush.) Pulse Respiration Whats
printed? 68 35 You look a little flush.
203 78 Heart beats gang aft agley You dont
look so good 205 40 Heart beats gang aft
agley You look a little flush.
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Id Hammer in the Morning Else Id...

• In Java, If...Then...Else
• if ( Expression )
• IfStatement
• else
• ElseStatement

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Pile on the If Statements

• if (grade 90)
• System.out.println(Wow, an A!)
• else if (grade 80)
• System.out.println(Not bad, a B!)
• else if (grade 70)
• System.out.println(Hanging in with a C.)
• else if (grade 60)
• System.out.println(Oh my, a D.)
• else
• System.out.println(It doesnt look good)

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While You Were Sleeping

• While you were sleeping, I
• Told your family we were engaged
• Hit on your brother
• Fell in love with him
• See what happens when you doze off for a few
  days...

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Variety Might be the Spice of Life...

• Repetition is the meat and potatoes
• Keep going until
• Fixed number of times (count up, count down)
• A condition becomes true
• A condition becomes false
• An exception/error occurs

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While is a Close Cousin to If

• In Java, repeat with while is
• while ( Expression ) Statement
• A brief example
• int product 2 // Defn or declaration?
• while (product
• product 2 product
• // Whats product here?

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Determining a Class Average

• The problem from the book
• Develop a class-averaging program that will
  process an arbitrary number of letter grades each
  time the program is run

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Everythings a Class

• Build a ClassAverage class in Java
• Create a ClassAverage object
• Invoke its methods

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Java Applications

• Java Its not just for applets anymore
• Browsers need not apply
• Applications are programs which can run on their
  own.
• User interface console or graphical

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Data Needs?

• What information does the program need to work?
• What information will be created in running the
  program?

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Programming with Verbs

• What actions need to be performed?
• Do we need to break them down into methods?

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The Data We Need
import java.io. public class Average
public static void main( String args ) throws
IOException double average // number
with decimal point int counter, grade,
total // initialization phase
total 0 counter 0 //
processing phase System.out.print( "Enter
letter grade, Z to end " ) grade
System.in.read()
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What Were Doing With It
while ( grade ! 'Z' ) if ( grade
'A' ) total total 4
else if ( grade 'B' ) total
total 3 else if ( grade 'C' )
total total 2 else if (
grade 'D' ) total total 1
System.in.skip( 2 ) counter
counter 1 System.out.print( "Enter
letter grade, Z to end " ) grade
System.in.read()
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The End Result
// Termination phase if ( counter ! 0 )
average (double) total / counter
System.out.println( "Class average is "
average ) else
System.out.println( "No grades were entered" )

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The Average Program
// Fig. 2.9 Average.java // Class average
application with // sentinel-controlled
repetition. import java.io. public class
Average public static void main( String
args ) throws IOException double
average // number with decimal point int
counter, grade, total //
initialization phase total 0
counter 0 // processing phase
System.out.print( "Enter letter grade, Z to end
" ) grade System.in.read()
while ( grade ! 'Z' ) if ( grade
'A' ) total total 4
else if ( grade 'B' ) total
total 3 else if ( grade 'C' )
total total 2 else if (
grade 'D' ) total total 1
System.in.skip( 2 ) counter
counter 1 System.out.print( "Enter
letter grade, Z to end " ) grade
System.in.read() //
termination phase if ( counter ! 0 )
average (double) total / counter
System.out.println( "Class average is " average
) else System.out.println(
"No grades were entered" )
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Thats It Until Next Time

• Choosing one branch or another use if
• Watch out for dangling elses
• Looping with condition can use while
• Other ways to loop branch
• Use the most specific