Alice in Action with Java Chapter 10 Flow Control in Java.

Alice in Action with Java

Chapter 10Flow Control in Java

Alice in Action with Java 2

Flow Control In Java

• Purpose of using selective and repetitive execution– Implement methods that produce complex behaviors

• Selective flow control statements: if and switch• Repetitive flow control statements: while, for, do


Introductory Example: pH

• Background for the use of pH values – pH: measures the acidity of a solution– pH scale of values: 0 – 14 – Labels for ranges in pH scale: acidic, neutral, alkaline

• Main elements in a user story built around pH values– pH values of water samples have been recorded– Given pH values of samples, provide correct labels

• Divide the problem into two subproblems– Build a class to model pH values and operations– Write a program that uses the class to label samples


Subproblem 1: A PH Class

• The PH class needs a private instance variable – double type variable is named myValue– myValue will be initialized by a constructor

• PH constructor– Takes an argument that sets myValue– If pH argument is invalid, an error message displays– Otherwise, pH argument is assigned to myValue

• The label()method– Returns error message if value is outside range– Otherwise, returns one of three labels


Subproblem 1: A PH Class (continued)


Subproblem 2: Processing Multiple pH Values

• Objective: display labels for sequence of pH values

• Pseudocode for PhLabeler program– Set the boolean variable named “done” to false– While NOT done:

• Prompt the user to enter a pH value (-1 to quit)

• Read ph from the keyboard

• If ph < the minimum pH value: set done to true;

• Else: display phLabel(ph)

• Observation: if statement nested in a while loop


Subproblem 2: Processing Multiple pH Values (continued)


Selective Execution

• Directing flow based on the value of a condition

• Two statements that provide selective execution– if statement: general-purpose selection structure– switch statement: multi-branch selection structure


Java’s if Statement

• General-purpose selection structure

• Selects from one or more groups of statements

• The else portion of the if statement is optional

• One-branch if: envisioned as branching flow

• Two-branch if: flow follows one of two branches


Java’s if Statement (continued)



• Pattern for Java’s if statement: if(Condition)Statement1 [else Statement2]

– Condition: any boolean expression

– Statementi: set of Java statements within { }

– Brackets are optional if only one statement is in path

• Multi-branch if statements – Flow can move along multiple paths– Nest additional if statements in the else clause


Java’s switch Statement

• Objective: create a PetLicense class

• Instance variables used in PetLicense class– char type named myCode stores license code– double type named myFee stores license fee

• Constructor for a PetLicense object – Takes a single char type argument– Uses multi-branch if to select appropriate fee – If data is valid, instance variables are initialized– If data is invalid, an error message is displayed

• switch: concise alternative to the multi-branch if


Java’s switch Statement (continued)

• Pattern for Java’s switch statement switch(IntegerCompatibleExpression){ CaseList1 StatementList1 ... CaseListN StatementListN default: StatementListN+1 }

– The condition is an integer-compatible expression– Each case corresponds to a literal value– The use of default statement optional– Use of break statement below a case is recommended

• PetLicense class meets criteria for use of switch



• One switch statement can be nested within another

• A nested switch statement is used in TShirt class• TShirt constructor

– Takes on String argument named size– myPrice for standard sizes set in cases ‘S’, ‘M’, ‘L’– myPrice for ‘XS’and ‘XL’ are set in case ’X’– Case ’X’ includes a nested switch statement

• Other methods in TShirt class– A method to return mySize and one to return myPrice– toString()returns String value of data members


Repetitive Execution

• Program execution flows sequentially by default• if and switch perform statements selectively

• Repetitive execution: control flows in loops

• Three repetition statements: while, for, and do


Java’s while Statement

• Used for processing a series of values

• Input loops: read and process a series of values

• Sentinel-controlled input loops– Utilize a sentinel (invalid value) to falsify a condition

• Problem: extract the initials in a name

• Members implemented in the Initials class– Instance variables called myName and myInitials– Constructor to initialize the instance variables – Methods to return myName and myInitials


Java’s while Statement (continued)

• StringTokenizer class:used to split Strings

• Overview of the Initials class constructor– String argument (name) is passed to constructor– Instance variables are initialized – StringTokenizer object called names is initialized– names and while loop are used to extract initials

• General pattern for Java’s while statement while(Condition)Statement – Statement comprises one or more statements– Curly braces ({ }) required with multiple statements


Java’s for Statement

• Repetition structure for solving counting problems

• Counting input loop– Simpler design than the sentinel-controlled input loop– Provides repetition when number of inputs is fixed

• Illustration: computing the city’s air pollution index– A for loop counts from 1 to NUM_READINGS– Each iteration gets a reading and adds it to sum – After loop terminates, index is computed and output

• Java’s for loop is very flexible– Example: the Java for loop can count down


Java’s for Statement (continued)



• Overview of NinetyNineBottlesSong program– for loop in constructor counts down from 99 to 0– Each iteration of the loop adds a verse to myLyrics– main()method constructs and displays the song

• Plotting a function of the form y = f(x)– for loop iterates through x values and computes f(x)

• SineWaves class for plotting a sine function – SineWaves should be compiled with Plotter class– Two methods use for loops to plot the sine waves

• Range of values and increment amounts differ



• TextGraphics class illustrates nested for loops

• Understanding drawBox()in TextGraphics class– Method takes two arguments for height and width– Outer for loop counts the rows (builds the height)– Inner for loop prints asterisk symbol through width

• General pattern for Java’s for loop for (InitialExpr; Condition; ChangeExpr)

Statement– Curly braces ({ }) required with multiple statements– Scope of loop control variable goes to end of loop only


Java’s do Statement

• Pattern for Java’s do statement: do

Statement while(Condition);

– Loop provides one-trip behavior with posttest condition– You must type a semicolon after the condition

• Overview of the GuessingGame class – Generates pseudo-random number between 1 and 100– Retrieves input from user until the number is guessed – Critical logic: multi-branch if nested in a do loop


Java’s do Statement (continued)



• Query-controlled input loop– A loop that terminates when user enters specific value

• Overview of the GameController class – Utilizes GuessingGame object – Repeats game using query-controlled do loop

• Default constructor– Constructor provided by Java– Used when a class does not include instance variables

• GameController utilizes a default constructor


Choosing the Right Loop

• Solving a problem with fixed counting – Recommendation: use the for loop

• Solving a problem without fixed counting– If one-trip behavior is needed, use a do loop– If zero-trip behavior is needed, use a while loop

• Using guidelines to choose loop for Initials– Cannot formulate problem as a counting type

• Number of words in a name not known in advance

– Zero-trip behavior is needed, as name might be null– Result of analysis: while loop is selected


Choosing the Right Loop (continued)


Example: The Chaos Game

• Overview of the ChaosGame class– Uses the algorithm for generating a Sierpinski Triangle– drawVertices()plots vertices of outer triangle– drawPoints()plots the vertices of the inner triangles

• switch statement is nested in a for loop

• New point generated and plotted with each iteration

– More points to interpolate leads to better resolution

• Observations – Structure emerges from seemingly random behavior– Computers quickly help users visualize patterns in data


Example: The Chaos Game (continued)

Alice in Action with Java Chapter 10 Flow Control in Java.

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