Case Study: Class GradeBook Using an Array to Store Grades

Case Study Class GradeBook Using an Array to Store Grades

This section further evolves class GradeBook, introduced in Chapter 3 and expanded in Chapters 45. Recall that this class represents a grade book used by a professor to store and analyze a set of student grades. Previous versions of the class process a set of grades entered by the user, but do not maintain the individual grade values in instance variables of the class. Thus, repeat calculations require the user to reenter the same grades. One way to solve this problem would be to store each grade entered in an individual instance of the class. For example, we could create instance variables grade1, grade2, ..., grade10 in class GradeBook to store 10 student grades. However, the code to total the grades and determine the class average would be cumbersome, and the class would not be able to process any more than 10 grades at a time. In this section, we solve this problem by storing grades in an array.

Storing Student Grades in an Array in Class GradeBook

The version of class GradeBook (Fig. 7.14) presented here uses an array of integers to store the grades of several students on a single exam. This eliminates the need to repeatedly input the same set of grades. Array grades is declared as an instance variable in line 7therefore, each GradeBook object maintains its own set of grades. The class's constructor (lines 1014) has two parametersthe name of the course and an array of grades. When an application (e.g., class GradeBookTest in Fig. 7.15) creates a GradeBook object, the application passes an existing int array to the constructor, which assigns the array's reference to instance variable grades (line 13). The size of the array grades is determined by the class that passes the array to the constructor. Thus, a GradeBook object can process a variable number of grades. The grade values in the passed array could have been input from a user or read from a file on disk (as discussed in Chapter 14). In our test application, we simply initialize an array with a set of grade values (Fig. 7.15, line 10). Once the grades are stored in instance variable grades of class GradeBook, all the class's methods can access the elements of grades as needed to perform various calculations.

Figure 7.14. GradeBook class using an array to store test grades.

(This item is displayed on pages 307 - 310 in the print version)

 1 // Fig. 7.14: GradeBook.java
 2 // Grade book using an array to store test grades.
 3
 4 public class GradeBook
 5 {
 6 private String courseName; // name of course this GradeBook represents
 7 private int grades[]; // array of student grades
 8 
 9 // two-argument constructor initializes courseName and grades array
10 public GradeBook( String name, int gradesArray[] )
11 {
12 courseName = name; // initialize courseName
13 grades = gradesArray; // store grades
14 } // end two-argument GradeBook constructor
15 
16 // method to set the course name
17 public void setCourseName( String name )
18 {
19 courseName = name; // store the course name
20 } // end method setCourseName
21 
22 // method to retrieve the course name
23 public String getCourseName()
24 {
25 return courseName;
26 } // end method getCourseName
27 
28 // display a welcome message to the GradeBook user
29 public void displayMessage()
30 {
31 // getCourseName gets the name of the course
32 System.out.printf( "Welcome to the grade book for
%s!

",
33 getCourseName() );
34 } // end method displayMessage
35 
36 // perform various operations on the data
37 public void processGrades()
38 {
39 // output grades array
40 outputGrades();
41 
42 // call method getAverage to calculate the average grade
43 System.out.printf( "
Class average is %.2f
", getAverage() );
44 
45 // call methods getMinimum and getMaximum
46 System.out.printf( "Lowest grade is %d
Highest grade is %d

",
47 getMinimum(), getMaximum() );
48 
49 // call outputBarChart to print grade distribution chart
50 outputBarChart();
51 } // end method processGrades
52 
53 // find minimum grade
54 public int getMinimum()
55 {
56 int lowGrade = grades[ 0 ]; // assume grades[ 0 ] is smallest
57 
58 // loop through grades array 
59 for ( int grade : grades ) 
60 { 
61  // if grade lower than lowGrade, assign it to lowGrade
62  if ( grade < lowGrade ) 
63  lowGrade = grade; // new lowest grade 
64 } // end for 
65 
66 return lowGrade; // return lowest grade
67 } // end method getMinimum
68 
69 // find maximum grade
70 public int getMaximum()
71 {
72 int highGrade = grades[ 0 ]; // assume grades[ 0 ] is largest
73 
74 // loop through grades array
75 for ( int grade : grades )
76 {
77 // if grade greater than highGrade, assign it to highGrade
78 if ( grade > highGrade )
79 highGrade = grade; // new highest grade
80 } // end for
81 
82 return highGrade; // return highest grade
83 } // end method getMaximum
84 
85 // determine average grade for test
86 public double getAverage()
87 {
88 int total = 0; // initialize total
89 
90 // sum grades for one student
91 for ( int grade : grades ) 
92  total += grade; 
93 
94 // return average of grades
95 return (double) total / grades.length;
96 } // end method getAverage
97 
98 // output bar chart displaying grade distribution
99 public void outputBarChart()
100 {
101 System.out.println( "Grade distribution:" );
102 
103 // stores frequency of grades in each range of 10 grades
104 int frequency[] = new int[ 11 ];
105 
106 // for each grade, increment the appropriate frequency
107 for ( int grade : grades ) 
108  ++frequency[ grade / 10 ]; 
109 
110 // for each grade frequency, print bar in chart
111 for ( int count = 0; count < frequency.length; count++ )
112 {
113 // output bar label ( "00-09: ", ..., "90-99: ", "100: " )
114 if ( count == 10 )
115 System.out.printf( "%5d: ", 100 );
116 else
117 System.out.printf( "%02d-%02d: ",
118 count * 10, count * 10 + 9 );
119 
120 // print bar of asterisks
121 for ( int stars = 0; stars < frequency[ count ]; stars++ )
122 System.out.print( "*" );
123 
124 System.out.println(); // start a new line of output
125 } // end outer for
126 } // end method outputBarChart
127 
128 // output the contents of the grades array
129 public void outputGrades()
130 {
131 System.out.println( "The grades are:
" );
132 
133 // output each student's grade 
134 for ( int student = 0; student < grades.length; student++ )
135  System.out.printf( "Student %2d: %3d
", 
136  student + 1, grades[ student ] ); 
137 } // end method outputGrades
138 } // end class GradeBook

Figure 7.15. GradeBookTest creates a GradeBook object using an array of grades, then invokes method processGrades to analyze them.

(This item is displayed on pages 311 - 312 in the print version)

 1 // Fig. 7.15: GradeBookTest.java
 2 // Creates GradeBook object using an array of grades.
 3
 4 public class GradeBookTest
 5 {
 6 // main method begins program execution
 7 public static void main( String args[] )
 8 {
 9 // array of student grades 
10 int gradesArray[] = { 87, 68, 94, 100, 83, 78, 85, 91, 76, 87 };
11
12 GradeBook myGradeBook = new GradeBook(
13 "CS101 Introduction to Java Programming", gradesArray );
14 myGradeBook.displayMessage();
15 myGradeBook.processGrades();
16 } // end main
17 } // end class GradeBookTest

 

Welcome to the grade book for
CS101 Introduction to Java Programming!

The grades are:

Student 1: 87
Student 2: 68
Student 3: 94
Student 4: 100
Student 5: 83
Student 6: 78
Student 7: 85
Student 8: 91
Student 9: 76
Student 10: 87

Class average is 84.90
Lowest grade is 68
Highest grade is 100

Grade distribution:
00-09:
10-19:
20-29:
30-39:
40-49:
50-59:
60-69: *
70-79: **
80-89: ****
90-99: **
 100: *

 

Method processGrades (lines 3751) contains a series of method calls that result in the output of a report summarizing the grades. Line 40 calls method outputGrades to print the contents of the array grades. Lines 134136 in method outputGrades use a for statement to output each student's grade. A counter-controlled for must be used in this case, because lines 135136 use counter variable student's value to output each grade next to a particular student number (see Fig. 7.15). Although array indices start at 0, a professor would typically number students starting at 1. Thus, lines 135136 output student + 1 as the student number to produce grade labels "Student 1: ", "Student 2: ", and so on.

Method processGrades next calls method getAverage (line 43) to obtain the average of the grades in the array. Method getAverage (lines 8696) uses an enhanced for statement to total the values in array grades before calculating the average. The parameter in the enhanced for's header (e.g., int grade) indicates that for each iteration, the int variable grade takes on a value in the array grades. Note that the averaging calculation in line 95 uses grades. length to determine the number of grades being averaged.

Lines 4647 in method processGrades calls methods getMinimum and getMaximum to determine the lowest and highest grades of any student on the exam, respectively. Each of these methods uses an enhanced for statement to loop through array grades. Lines 5964 in method getMinimum loop through the array, and lines 6263 compare each grade to lowGrade. If a grade is less than lowGrade, lowGrade is set to that grade. When line 66 executes, lowGrade contains the lowest grade in the array. Method getMaximum (lines 7083) works the same way as method getMinimum.

Finally, line 50 in method processGrades calls method outputBarChart to print a distribution chart of the grade data using a technique similar to that in Fig. 7.6. In that example, we manually calculated the number of grades in each category (i.e., 09, 1019, ..., 9099 and 100) by simply looking at a set of grades. In this example, lines 107108 use a technique similar to that in Fig. 7.7 and Fig. 7.8 to calculate the frequency of grades in each category. Line 104 declares and creates array frequency of 11 ints to store the frequency of grades in each grade category. For each grade in array grades, lines 107108 increment the appropriate element of the frequency array. To determine which element to increment, line 108 divides the current grade by 10 using integer division. For example, if grade is 85, line 108 increments frequency[ 8 ] to update the count of grades in the range 8089. Lines 111125 next print the bar chart (see Fig. 7.15) based on the values in array frequency. Like lines 2324 of Fig. 7.6, lines 121122 of Fig. 7.14 use a value in array frequency to determine the number of asterisks to display in each bar.

Class GradeBookTest That Demonstrates Class GradeBook

The application of Fig. 7.15 creates an object of class GradeBook (Fig. 7.14) using the int array gradesArray (declared and initialized in line 10). Lines 1213 pass a course name and gradesArray to the GradeBook constructor. Line 14 displays a welcome message, and line 15 invokes the GradeBook object's processGrades method. The output reveals the summary of the 10 grades in myGradeBook.

Software Engineering Observation 7.1

A test harness (or test application) is responsible for creating an object of the class being tested and providing it with data. This data could come from any of several sources. Test data can be placed directly into an array with an array initializer, it can come from the user at the keyboard, it can come from a file (as you will see in Chapter 14), or it can come from a network (as you will see in Chapter 24). After passing this data to the class's constructor to instantiate the object, the test harness should call upon the object to test its methods and manipulate its data. Gathering data in the test harness like this allows the class to manipulate data from several sources.