Java 17 Recipes

7-2. Designing Intelligent Constants
 Problem
You need a type that can represent a fixed set of related constants, and you want to build 
some state and behavior (logic) around your constants in an object-oriented fashion.
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 Solution
Use an enum type and take advantage of type safety because enum types are full-fledged 
Java classes. An enum type can have state and behavior just like any other class, and the 
enum constants, themselves being instances of the enum type, inherit this state and 
behavior. An example best illustrates this. Let’s expand on the example from the previous 
recipe. Imagine that you need to process and validate all the fields from an HTML form 
submitted. Each form field has a unique set of rules for validating its content, based on 
the field type. For each form field, you have the field’s “name” and the value entered in 
that form field. The FieldType enum can be expanded to handle this very easily.
public enum FieldType {
PASSWORD(FieldType.passwordFieldName) {
// A password must contain one or more digits, one or more 
lowercase letters, one or more uppercase letters, and be a
minimum of 6 characters in length.
public boolean validate(String fieldValue) {
return Pattern.matches("((?=.*\\d)(?=.*[a-z]) 
(?=.*[A- Z]).{6,})", fieldValue);
}
},
EMAIL_ADDRESS(FieldType.emailFieldName) {
// An email address begins with a combination of alphanumeric
// Characters, periods, and hyphens, followed by a mandatory 
// Ampersand ('@') character, followed by a combination of
// Alphanumeric characters (hyphens allowed), followed by a
// Ane or more periods (to separate domains and subdomains),
// And ending in 2-4 lphabetic characters representing
// A the domain.
public boolean validate(String fieldValue) {
return Pattern.matches( "^[\\w\\.-]+@([\\w\\-]+\\.)+[A-Z|a-z]
{2,4}$", fieldValue);
}
},
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PHONE_NUMBER(FieldType.phoneFieldName) {
// A phone number must contain a minium of 7 digits.
// Three optional digits representing the area code may appear in
// Front of the main 7 digits. The area code may, optionally,
// Be surrounded by parenthesis. If an area code is included,
// The number may optionally be prefixed by a '1' for
// Long distance numbers. Optional hypens my appear after 
// The country code ('1'), the area code, and the
// first 3 digits of the 7 digit number.
public boolean validate(String fieldValue) {
return Pattern.matches( "^1?[- ]?\\(?(\\d{3})\\)?[- ]?(\\d{3})
[- ]?(\\d{4})$", fieldValue);
}
},
SOCIAL_SECURITY_NUMBER(FieldType.ssnFieldName) {
// A social security number must contain 9 digits with optional
// Hyphens after the third and fifth digits.
public boolean validate(String fieldValue) {
return Pattern.matches("^\\d{3}[- ]?\\d{2}[- ]?\\d{4}$",
fieldValue);
}
}; // End of enum constants definition
// Instance members
//
private String fieldName;
// Define static constants to increase type safety
static final String passwordFieldName = "password";
static final String emailFieldName = "email";
static final String phoneFieldName = "phone";
static final String ssnFieldName = "ssn";
private FieldType(String fieldName) {
this.fieldName = fieldName;
}
public String getFieldName() {
return this.fieldName;
}
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abstract boolean validate(String fieldValue);
// Static class members
//
private static final Map nameToFieldTypeMap =
new HashMap<>();
static {
for (FieldType field : FieldType.values()) {
nameToFieldTypeMap.put(field.getFieldName(), field);
}
}
public static FieldType lookup(String fieldName) {
return nameToFieldTypeMap.get(fieldName.toLowerCase());
}
private static void printValid(FieldType field, String fieldValue, 
boolean valid) {
System.out.println(field.getFieldName() +
"(\"" + fieldValue + "\") valid: " + valid);
}
public static void main(String... args) {
String fieldName = FieldType.passwordFieldName;
String fieldValue = "1Cxy9"; // invalid - must be at least 6 
characters
FieldType field = lookup(fieldName);
printValid(field, fieldValue, field.validate(fieldValue));
fieldName = FieldType.phoneFieldName;
fieldValue = "1-800-555-1234"; // valid
field = lookup(fieldName);
printValid(field, fieldValue, field.validate(fieldValue));
fieldName = FieldType.emailFieldName;
fieldValue = "john@doe"; // invalid - missing .
field = lookup(fieldName);
printValid(field, fieldValue, field.validate(fieldValue));
fieldName = FieldType.ssnFieldName;
fieldValue = "111-11-1111"; // valid
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field = lookup(fieldName);
printValid(field, fieldValue, field.validate(fieldValue));
}
}
Running the preceding code results in the following output.
password("1Cxy9") valid: false
phone("1-800-555-1234") valid: true
email("john@doe") valid: false
ssn("111-11-1111") valid: true