Learning Control Structures in Java

1. Control Structures in Java I Controlling Java CS 102-02 Lecture 2-2

2. 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

3. Selection Structures • Two kinds of selection • If…then: Only do it if the condition’s true • If...then...else: Do one thing or the other

4. If I Had a Hammer... • In Java syntax: if ( Expression ) Statement • Expression MUST be a Boolean expression Statement can be a block of statements • There’s no then in them thar hills

5. If I Had an Example.. if (pulse >= 0) System.out.println(“Passed”); if (pulse >= 200) System.out.println(“Slow down!”);

6. 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 {}

7. Compounding a Statement if (temperature <= 30 && windSpeed > 10) { me.bundleUp(); me.complain(“Chicago”, badWeather); me.tryToGraduate(now); me.returnToWarmerClimes(“I love L.A.!”); }

8. The Declaration of Independence • When is a Compound Statement a Block? When it contains a declaration. • A declaration says to Java: “I’m going to use a variable, and it’s going to be of this type.” Graphics graphicsObject; • Not a particular Graphics object yet

9. A Defining Moment • Declarations are good, but there’s more • Definitions associate an initial value with a name Graphics graphicsObject = new Graphics();

10. Why Do We Care? Block = CompoundStatement+Declaration(s) • Declarations in a block are special because they have block scope { Graphics graphicsObject; // some code goes here // graphicsObject still exists } // now it’s gone

11. It’s 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...

12. A Little Medical Diagnosis if (pulse >= 200 || pulse <= 0) System.out.println(“Heart beats gang aft agley”); if (respiration > 75) System.out.println(“You don’t look so good”); else System.out.println(“You look a little flush.”); Pulse Respiration What’s printed? 68 35 You look a little flush. 203 78 Heart beats gang aft agley You don’t look so good 205 40 Heart beats gang aft agley You look a little flush.

13. I’d Hammer in the Morning Else I’d... • In Java, If...Then...Else: if ( Expression ) IfStatement else ElseStatement

14. 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 doesn’t look good”);

15. 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...

16. 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

17. While is a Close Cousin to If • In Java, repeat with while is: while ( Expression ) Statement • A brief example int product = 2; // Def’n or declaration? while (product <= 1000) product = 2 * product; // What’s product here?

18. 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

19. Everything’s a Class • Build a ClassAverage class in Java • Create a ClassAverage object • Invoke its methods

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

21. Data Needs? • What information does the program need to work? • What information will be created in running the program?

22. Programming with Verbs • What actions need to be performed? • Do we need to break them down into methods?

23. 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();

24. What We’re 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(); }

25. 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" ); } }

26. 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" ); } }

27. That’s It Until Next Time • Choosing one branch or another: use if • Watch out for dangling else’s • Looping with condition: can use while • Other ways to loop & branch • Use the most specific