Flow of Control Recitation – 09/(11,12)/2008

CS 180Department of Computer Science, Purdue University

• Now posted on the class webpage.

• Due Wed, Sept. 17 at 10 pm.

• Start early. All questions on the class newsgroup.

• Evening consulting hours.

LWSN B146. MTW 7-10 pm.

Project 2

• If and else parts in braces are preferred for even a single statement.if (count < 3){

total = 0;}is better thanif (count < 3)

total = 0;

Statements in Braces

Statements in BracesBenefits to do this:• Decreases the chance of introducing a bug when

adding a new statement.• Makes the program a little more “self-

documenting” (more readable and easier to maintain).

== Is NOT Always Equal • == should not be used to determine if two

objects have the same value, including two strings

• Even though (s1 == s2) might appear to work, do not use this.

• Use (s1.equals(s2)) instead to compare two strings.

Multibranch if-else Statements• if-else if-else is different from a series of

if statements.

if (score >= 60){

result = ‘P’;

}else if (score > 0){

result = ‘F’;

}else{

result = ‘Z’;

}

if (score >= 60){

result = ‘P’;

}if (score > 0){

result = ‘F’;

}else{

result = ‘Z’;

}

Short-circuit Evaluation• Can prevent problems.if ((number != 0) && (sum/number > 5))

• Can produce problems!if ((x.evaluate()) && (y.reassign()))

• How to run y.reassign() every time?• Use & and | to replace && and || . if ((x.evaluate()) & (y.reassign()))

• But notice & and && are different!

Input & Output of Boolean ValuesExample:

boolean booleanVar = false;System.out.println(booleanVar);System.out.println("Enter a boolean value:");Scanner keyboard = new Scanner(System.in);booleanVar = keyboard.nextBoolean();System.out.println("You entered " + booleanVar);

Input & Output of Boolean ValuesDialog:falseEnter a boolean value: trueYou entered true

The switch StatementSyntax:

switch (Controlling_Expression){

case Case_Label:Statement(s);break;

case Case_Label:…default:…

}

The switch Statement

View sample program Listing 3.4 class MultipleBirths

Sample screen output

The switch Statement• The action for each case typically ends with the

word break.• The optional break statement prevents the

consideration of other cases.• The controlling expression can be anything that

evaluates to an integral type.

The switch Statement

Example without break:switch (grade){

case ‘P’:System.out.print(“You have”);

case ‘F’:System.out.print(“…No, you have not”);

default:}System.out.println(“passed.”);

Enumerations• Consider a need to restrict contents of a variable

to certain values.• An enumeration lists the values a variable can

have.• Example

enum MovieRating {E, A, B}MovieRating rating;rating = MovieRating.A;

Enumerations• Now possible to use in a switch statement

Enumerations• An even better choice of descriptive identifiers for

the constants.

enum MovieRating {EXCELLENT, AVERAGE, BAD}rating = MovieRating.AVERAGE;

case EXCELLENT: ...

Specifying a Drawing Color• When drawing a shape inside an applet’s paint

method, think of the drawing being done with a pen that can change colors.

• The method setColor changes the color of the "pen."canvas.setColor(Color.YELLOW);

• Drawings done later appear on top of drawings done earlier.

Specifying a Drawing ColorView sample program, Listing 3.5

class YellowFace

Sample screen output

Specifying a Drawing ColorPredefined Colors for the setColor Method:

Dialog Box for a Yes/No Question• Used to present the user with a yes/no question.• The window contains

The question text Two buttons labeled yes and no.

Dialog Box for a Yes/No Question

Example:int answer = JOptionPane.showConfirmDialog(null, "End program?", "Click Yes or No:",

JOptionPane.YES_NO_OPTION);if (answer == JOptionPane.YES_OPTION) System.exit(0);else if (answer == JOptionPane.NO_OPTION) System.out.println("One more time");

Dialog Box for a Yes/No Question

A Yes-or No-Dialog Box: