Department of Computer and Information Science,
School of Science, IUPUI
Program Control using Java
- Selection
Dale Roberts, Lecturer
Computer Science, IUPUI
E-mail: droberts@cs.iupui.edu
Dale Roberts
4.4 Control Structures (Cont.)
Selection Statements
if statement
Single-selection statement
if…else statement
Double-selection statement
switch statement
Multiple-selection statement
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4.5 if Single-Selection Statement
if statements
Execute an action if the specified condition is true
Can be represented by a decision symbol (diamond) in a
UML activity diagram
Transition arrows out of a decision symbol have guard conditions
Workflow follows the transition arrow whose guard condition
is true
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Fig. 4.2
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|
if
single-selection statement
UML activity diagram.
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4.6 if…else Double-Selection Statement
if…else statement
Executes one action if the specified condition is true or a
different action if the specified condition is false
Conditional Operator ( ? : )
Java’s only ternary operator (takes three operands)
? : and its three operands form a conditional expression
Entire conditional expression evaluates to the second operand if
the first operand is true
Entire conditional expression evaluates to the third operand if the
first operand is false
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| …else double-selection
statement UML activity diagram.
Fig. 4.3
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if
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|
multiple-selection
statement UML activity
diagram with
statements.
Fig. 5.11
switch
break
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Software Engineering Observation 5.2
Provide a default case in switch
statements. Including a default case
focuses you on the need to process
exceptional conditions.
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// Fig. 5.9: GradeBook.java
// GradeBook class uses switch statement to count A, B, C, D and F grades.
import java.util.Scanner; // program uses class Scanner
public class GradeBook
{
private String courseName; // name of course this GradeBook represents
private int total; // sum of grades
private int gradeCounter; // number of grades entered
private int aCount; // count of A grades
private int bCount; // count of B grades
private int cCount; // count of C grades
private int dCount; // count of D grades
private int fCount; // count of F grades
// constructor initializes courseName;
// int instance variables are initialized to 0 by default
public GradeBook( String name )
{
courseName = name; // initializes courseName
} // end constructor
// method to set the course name
public void setCourseName( String name )
{
courseName = name; // store the course name
} // end method setCourseName
Dale Roberts
Outline
GradeBook.j
ava
(1 of 5)
Lines 8-14
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// method to retrieve the course name
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public String getCourseName()
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{
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return courseName;
} // end method getCourseName
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// display a welcome message to the GradeBook user
public void displayMessage()
{
// getCourseName gets the name of the course
System.out.printf( "Welcome to the grade book for\n%s!\n\n",
getCourseName() );
GradeBook.j
ava
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} // end method displayMessage
Lines 50-54
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public void inputGrades()
{
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Outline
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// input arbitrary number of grades from user
Scanner input = new Scanner( System.in );
Display prompt
int grade; // grade entered by user
System.out.printf( "%s\n%s\n
%s\n
%s\n",
"Enter the integer grades in the range 0-100.",
"Type the end-of-file indicator to terminate input:",
"On UNIX/Linux/Mac OS X type <ctrl> d then press Enter",
"On Windows type <ctrl> z then press Enter" );
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// loop until user enters the end-of-file indicator
while ( input.hasNext() )
{
grade = input.nextInt(); // read grade
total += grade; // add grade to total Loop condition uses method hasNext to
++gradeCounter; // increment numberdetermine
of grades whether there is more data to input
Outline
// call method to increment appropriate counter
incrementLetterGradeCounter( grade );
} // end while
} // end method inputGrades
// add 1 to appropriate counter for specified grade
public void incrementLetterGradeCounter( int numericGrade )
{
(grade / 10 ) is controlling
// determine which grade was entered
switch ( grade / 10 )
expression
{
case 9: // grade was between 90
switch statement determines which
case 10: // and 100
case label to execute, depending on
++aCount; // increment aCount
controlling expression
break; // necessary to exit switch
case 8: // grade was between 80 and 89
++bCount; // increment bCount
break; // exit switch
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GradeBook.j
ava
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Line 57
Line 72
controlling
expression
Lines 72-94
case 7: // grade was between 70 and 79
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Outline
++cCount; // increment cCount
break; // exit switch
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case 6: // grade was between 60 and 69
++dCount; // increment dCount
break; // exit switch
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GradeBook.j
ava
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default: // grade was less than 60
++fCount; // increment fCount
break; // optional; will exit switch anyway
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} // end switch
default case for grade
} // end method incrementLetterGradeCounter
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// display a report based on the grades entered by user
public void displayGradeReport()
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{
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System.out.println( "\nGrade Report:" );
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if ( gradeCounter != 0 )
{
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less than 60
// if user entered at least one grade...
// calculate average of all grades entered
double average = (double) total / gradeCounter;
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Line 91 default
case
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// output summary of results
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System.out.printf( "Total of the %d grades entered is %d\n",
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gradeCounter, total );
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System.out.printf( "Class average is %.2f\n", average );
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System.out.printf( "%s\n%s%d\n%s%d\n%s%d\n%s%d\n%s%d\n",
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"Number of students who received each grade:",
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"A: ", aCount,
// display number of A grades
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"B: ", bCount,
// display number of B grades
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"C: ", cCount,
// display number of C grades
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"D: ", dCount,
// display number of D grades
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"F: ", fCount ); // display number of F grades
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} // end if
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else // no grades were entered, so output appropriate message
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System.out.println( "No grades were entered" );
} // end method displayGradeReport
123 } // end class GradeBook
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Outline
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GradeBook.j
ava
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// Fig. 5.10: GradeBookTest.java
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// Create GradeBook object, input grades and display grade report.
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{
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Outline
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public class GradeBookTest
public static void main( String args[] )
{
// create GradeBook object myGradeBook and
Call GradeBook public
methods to count grades
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// pass course name to constructor
GradeBook myGradeBook = new GradeBook(
"CS101 Introduction to Java Programming" );
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myGradeBook.displayMessage(); // display welcome message
myGradeBook.inputGrades(); // read grades from user
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myGradeBook.displayGradeReport(); // display report based on grades
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} // end main
17 } // end class GradeBookTest
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GradeBookT
est.java
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Outline
Welcome to the grade book for
CS101 Introduction to Java Programming!
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Enter the integer grades in the range 0-100.
Type the end-of-file indicator to terminate input:
On UNIX/Linux/Mac OS X type <ctrl> d then press Enter
On Windows type <ctrl> z then press Enter
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^Z
Grade Report:
Total of the 10 grades entered is 778
Class average is 77.80
Number of students who received each grade:
A: 4
B: 1
C: 2
D: 1
F: 2
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GradeBookT
est.java
(2 of 2)
Program output
Portability Tip 5.1
The keystroke combinations for
entering end-of-file are system
dependent.
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Good Programming Practice 5.8
Although each case and the default case
in a switch can occur in any order, place
the default case last. When the default
case is listed last, the break for that case
is not required. Some programmers
include this break for clarity and symmetry
with other cases.
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switch Multiple-Selection Statement (Cont.)
Expression in each case
Constant integral expression
Combination of integer constants that evaluates to a constant
integer value
Character constant
E.g., ‘A’, ‘7’ or ‘$’
Constant variable
Declared with keyword final
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5.7 break and continue Statements
break/continue
Alter flow of control
break statement
Causes immediate exit from control structure
Used in while, for, do…while or switch statements
continue statement
Skips remaining statements in loop body
Proceeds to next iteration
Used in while, for or do…while statements
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Common Programming Error 5.7
Forgetting a break statement when one
is needed in a switch is a logic error.
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// Fig. 5.12: BreakTest.java
// break statement exiting a for statement.
public class BreakTest
{
public static void main( String args[] )
Loop 10 times
{
int count; // control variable also used after
Exitloop
forterminates
statement
(break)
when count equals 5
for ( count = 1; count <= 10; count++ ) // loop 10 times
{
if ( count == 5 ) // if count is 5,
break;
// terminate loop
Outline
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BreakTest.ja
va
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System.out.printf( "%d ", count );
} // end for
System.out.printf( "\nBroke out of loop at count = %d\n", count );
} // end main
} // end class BreakTest
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Broke out of loop at count = 5
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Program
output
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// Fig. 5.13: ContinueTest.java
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// continue statement terminating an iteration of a for statement.
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public class ContinueTest
{
public static void main( String args[] )
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Loop 10 times
{
and proceed to
for ( int count = 1; count <= 10; count++ ) //Skip
loopline
10 12
times
{
line 7 when count equals 5
if ( count == 5 ) // if count is 5,
continue;
// skip remaining code in loop
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Outline
System.out.printf( "%d ", count );
} // end for
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System.out.println( "\nUsed continue to skip printing 5" );
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} // end main
17 } // end class ContinueTest
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Used continue to skip printing 5
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ContinueTe
st.java
Line 7
Lines 9-10
Program
output
Software Engineering Observation 5.3
Some programmers feel that break and
continue violate structured
programming. Since the same effects
are achievable with structured
programming techniques, these
programmers do not use break or
continue.
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Software Engineering Observation 5.4
There is a tension between achieving
quality software engineering and
achieving the best-performing software.
Often, one of these goals is achieved at
the expense of the other. For all but the
most performance-intensive situations,
apply the following rule of thumb: First,
make your code simple and correct; then
make it fast and small, but only if
necessary.
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Acknowledgements
Deitel, Java How to Program
Dale Roberts