The SerialNumber class validates software serial numbers that are in the format of 5 letters, 4 digits, 5 letters separated by hyphens. The class takes a serial number string as input, splits it into the three groups, and validates each group by length and character type. Getter methods return whether the overall serial number is valid or invalid based on the group validation results.

1. CS4-1
CASESTUDY
4 The SerialNumber Class
In this case study you examine the SerialNumber class, which is used by the Home Soft-
ware Company to validate software serial numbers. A valid software serial number is in
the form LLLLL-DDDD-LLLL, where L indicates an alphabetic letter and D indicates a numeric
digit. For example, WRXTQ-7786-PGVZ is a valid serial number. Notice that a serial number
consists of three groups of characters, delimited by hyphens. Figure CS4-1 shows a UML
diagram for the SerialNumber class.
Figure CS4-1 UML diagram for the SerialNumber class
The fields first, second, and third are used to hold the first, second, and third groups of
characters in a serial number. The valid field is set to true by the constructor to indicate a
valid serial number, or false to indicate an invalid serial number. Table CS4-1 describes
the class’s methods.

2. CS4-2 Case Study 4 The SerialNumber Class
Table CS4-1 SerialNumber class’s methods
Method Description
Constructor The constructor accepts a string argument that contains a serial number.
The string is tokenized and its tokens are stored in the first, second,
and third fields. The validate method is called.
isValid This method returns the value in the valid field.
validate This method calls the isFirstGroupValid, isSecondGroupValid, and
isThirdGroupValid methods to validate the first, second, and third
fields.
isFirstGroupValid This method returns true if the value stored in the first field is valid.
Otherwise, it returns false.
isSecondGroupValid This method returns true if the value stored in the second field is valid.
Otherwise, it returns false.
isThirdGroupValid This method returns true if the value stored in the third field is valid.
Otherwise, it returns false.
The code for the SerialNumber class is shown in Code Listing CS4-1.
Code Listing CS4-1 (SerialNumber.java)
1 import java.util.StringTokenizer;
2
3 /**
4 * The SerialNumber class takes a software serial number in
5 * the form of LLLLL-DDDD-LLLL where each L is a letter
6 * and each D is a digit. The serial number has three groups
7 * of characters, separated by hyphens. The class extracts
8 * the three groups of characters and validates them.
9 */
10
11 public class SerialNumber
12 {
13 private String first; // First group of characters
14 private String second; // Second group of characters
15 private String third; // Third group of characters
16 private boolean valid; // Flag indicating validity
17
18 /**
19 * The following constructor accepts a serial number as
20 * its String argument. The argument is broken into
21 * three groups and each group is validated.
22 */
23

3. Case Study 4 The SerialNumber Class CS4-3
24 public SerialNumber(String sn)
25 {
26 // Create a StringTokenizer and initialize
27 // it with a trimmed copy of sn.
28 StringTokenizer st =
29 new StringTokenizer(sn.trim(), "-");
30
31 // Tokenize and validate.
32 if (st.countTokens() != 3)
33 valid = false;
34 else
35 {
36 first = st.nextToken();
37 second = st.nextToken();
38 third = st.nextToken();
39 validate();
40 }
41 }
42
43 /**
44 * The following method returns the valid field.
45 */
46
47 public boolean isValid()
48 {
49 return valid;
50 }
51
52 /**
53 * The following method sets the valid field to true
54 * if the serial number is valid. Otherwise it sets
55 * valid to false.
56 */
57
58 private void validate()
59 {
60 if (isFirstGroupValid() && isSecondGroupValid() &&
61 isThirdGroupValid())
62 valid = true;
63 else
64 valid = false;
65 }
66
67 /**
68 * The following method validates the first group of
69 * characters. If the group is valid, it returns
70 * true. Otherwise it returns false.
71 */

4. CS4-4 Case Study 4 The SerialNumber Class
72
73 private boolean isFirstGroupValid()
74 {
75 boolean groupGood = true; // Flag
76 int i = 0; // Loop control variable
77
78 // Check the length of the group.
79 if (first.length() != 5)
80 groupGood = false;
81
82 // See if each character is a letter.
83 while (groupGood && i < first.length())
84 {
85 if (!Character.isLetter(first.charAt(i)))
86 groupGood = false;
87 i++;
88 }
89
90 return groupGood;
91 }
92
93 /**
94 * The following method validates the second group
95 * of characters. If the group is valid, it returns
96 * true. Otherwise it returns false.
97 */
98
99 private boolean isSecondGroupValid()
100 {
101 boolean groupGood = true; // Flag
102 int i = 0; // Loop control variable
103
104 // Check the length of the group.
105 if (second.length() != 4)
106 groupGood = false;
107
108 // See if each character is a digit.
109 while (groupGood && i < second.length())
110 {
111 if (!Character.isDigit(second.charAt(i)))
112 groupGood = false;
113 i++;
114 }
115
116 return groupGood;
117 }
118

5. Case Study 4 The SerialNumber Class CS4-5
119 /**
120 * The following method validates the third group of
121 * characters. If the group is valid, it returns
122 * true. Otherwise it returns false.
123 */
124
125 private boolean isThirdGroupValid()
126 {
127 boolean groupGood = true; // Flag
128 int i = 0; // Loop control variable
129
130 // Check the length of the group.
131 if (third.length() != 4)
132 groupGood = false;
133
134 // See if each character is a letter.
135 while (groupGood && i < third.length())
136 {
137 if (!Character.isLetter(third.charAt(i)))
138 groupGood = false;
139 i++;
140 }
141
142 return groupGood;
143 }
144 }
Let’s take a closer look at the constructor, in lines 24 through 41. The following statement, in
lines 28 and 29, creates a StringTokenizer object, using the hyphen character as a delimiter:
StringTokenizer st =
new StringTokenizer(sn.trim(), "-");
Notice that we call the argument’s trim method to remove any leading and/or trailing
whitespace characters. This is important because whitespace characters are not used as
delimiters in this code. If the argument contains leading whitespace characters, they will be
included as part of the first token. Trailing whitespace characters will be included as part
of the last token.
Next, the if statement in lines 32 through 40 executes:
if (st.countTokens() != 3)
valid = false;
else
{
first = st.nextToken();
second = st.nextToken();
third = st.nextToken();
validate();
}

6. CS4-6 Case Study 4 The SerialNumber Class
A valid serial number must have three groups of characters, so the if statement determines
whether the string has three tokens. If not, the valid field is set to false. Otherwise, the
three tokens are extracted and assigned to the first, second, and third fields. Last, the
validate method is called. The validate method calls the isFirstGroupValid,
isSecondGroupValid, and isThirdGroupValid methods to validate the three groups of
characters. In the end, the valid field will be set to true if the serial number is valid, or
false otherwise. The program in Code Listing CS4-2 demonstrates the class.
Code Listing CS4-2 (SerialNumberTester.java)
1 /**
2 * This program demonstrates the SerialNumber class.
3 */
4
5 public class SerialNumberTester
6 {
7 public static void main(String[] args)
8 {
9 String serial1 = "GHTRJ-8975-AQWR"; // Valid
10 String serial2 = "GHT7J-8975-AQWR"; // Invalid
11 String serial3 = "GHTRJ-8J75-AQWR"; // Invalid
12 String serial4 = "GHTRJ-8975-AQ2R"; // Invalid
13
14 // Validate serial1.
15
16 SerialNumber sn = new SerialNumber(serial1);
17 if (sn.isValid())
18 System.out.println(serial1 + " is valid.");
19 else
20 System.out.println(serial1 + " is invalid.");
21
22 // Validate serial2.
23
24 sn = new SerialNumber(serial2);
25 if (sn.isValid())
26 System.out.println(serial2 + " is valid.");
27 else
28 System.out.println(serial2 + " is invalid.");
29
30 // Validate serial3.
31
32 sn = new SerialNumber(serial3);
33 if (sn.isValid())
34 System.out.println(serial3 + " is valid.");
35 else
36 System.out.println(serial3 + " is invalid.");
37

7. Case Study 4 The SerialNumber Class CS4-7
38 // Validate serial4.
39
40 sn = new SerialNumber(serial4);
41 if (sn.isValid())
42 System.out.println(serial4 + " is valid.");
43 else
44 System.out.println(serial4 + " is invalid.");
45 }
46 }
Program Output
GHTRJ-8975-AQWR is valid.
GHT7J-8975-AQWR is invalid.
GHTRJ-8J75-AQWR is invalid.
GHTRJ-8975-AQ2R is invalid.