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

Flow of Control Recitation – 09/(11,12)/2008 CS 180 Department of Computer Science, Purdue University

Project 2 • 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.

If and else parts in braces are preferred for even a single statement. if (count < 3) { total = 0; } is better than if (count < 3) total = 0; Statements in Braces

Statements in Braces Benefits 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 runy.reassign()every time? Use& and|to replace&& and || . if ((x.evaluate()) & (y.reassign())) But notice& and && aredifferent!

Input & Output of Boolean Values Example: 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 Values Dialog: false Enter a boolean value: true You entered true

The switch Statement Syntax: switch (Controlling_Expression) { case Case_Label: Statement(s); break; case Case_Label: … default: … }

Theswitch Statement View sample program Listing 3.4class MultipleBirths Sample screen output

Theswitch Statement The action for each case typically ends with the word break. The optionalbreakstatement prevents the consideration of other cases. The controlling expression can be anything that evaluates to an integral type.

The switch Statement Example withoutbreak: 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. Exampleenum 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 Color View sample program, Listing 3.5class YellowFace Sample screen output

Specifying a Drawing Color Predefined 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 labeledyesand 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:

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