April 8, 1998CS102-02Lecture 2-2 Control Structures in Java I CS 102-02 Lecture 2-2 Controlling Java

April 8, 1998 CS102-02 Lecture 2-2

Control Structures in Java I

CS 102-02

Lecture 2-2

Controlling Java


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


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


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


If I Had an Example..

if (pulse >= 0) System.out.println(“Passed”);

if (pulse >= 200)

System.out.println(“Slow down!”);


Compound Statements

• If Java expects a statement, you can also use a compound statementif ( Expression ) Statement

• Compound statement is a group of statements, enclosed in {}


Compounding a Statement

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


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


A Defining Moment

• Declarations are good, but there’s more

• Definitions associate an initial value with a name

Graphics graphicsObject = new Graphics();


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


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...


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 agleyYou don’t look so good

205 40 Heart beats gang aft agleyYou look a little flush.


I’d Hammer in the Morning Else I’d...

• In Java, If...Then...Else:

if ( Expression )

IfStatement

else

ElseStatement


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!”);


System.out.println(“Hanging in with a C.”);


System.out.println(“Oh my, a D.”);

else

System.out.println(“It doesn’t look good”);


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...


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


While is a Close Cousin to If

• In Java, repeat with while is:while ( Expression ) Statement

• A brief exampleint product = 2; // Def’n or declaration?

while (product <= 1000)

product = 2 * product;

// What’s product here?


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


Everything’s a Class

• Build a ClassAverage class in Java

• Create a ClassAverage object

• Invoke its methods


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


Data Needs?

• What information does the program need to work?

• What information will be created in running the program?


Programming with Verbs

• What actions need to be performed?

• Do we need to break them down into methods?


The Data We Needimport 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();


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(); }


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" ); }}


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" ); }}


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

Documents

April 8, 1998CS102-02Lecture 2-2 Control Structures in Java I CS 102-02 Lecture 2-2 Controlling Java