Public String constants should use more specific types or enums

[sfloess]

Description

Three utility classes expose public String constants that could be replaced with more type-safe alternatives like enums, or at minimum should use the @StringDef pattern for better IDE support and compile-time validation.

Affected Constants

StringUtil - Error Messages and Defaults

public static final String STRING_CANNOT_BE_BLANK = "String cannot be blank!";
public static final String DEFAULT_SEPARATOR = "";

ObjectUtil - Error Message

public static final String DEFAULT_ERROR_MSG = "Invalid value";

UrlUtil - Protocol Separator

public static final String PROTOCOL_SEPARATOR = "://";

Issues with Current Design

1. String Constants for Error Messages

StringUtil.STRING_CANNOT_BE_BLANK:

• Only used internally by requireNonBlank(String)
• Should be private, not public
• Public exposure serves no purpose

ObjectUtil.DEFAULT_ERROR_MSG:

• Generic message provides no value
• Never actually used in the codebase
• Should be removed or made private

2. Magic String Exposure

UrlUtil.PROTOCOL_SEPARATOR:

• Exposes internal implementation detail
• Users shouldn't need to construct protocol + PROTOCOL_SEPARATOR + host
• The library provides asProtocolAndHost() for this purpose

3. Lack of Type Safety

String constants can be:

• Mistyped without compile error
• Passed to wrong methods
• Changed accidentally
• Difficult to refactor

Impact

• Severity: Low
• Type: API Design / Encapsulation
• Public constants expose unnecessary implementation details
• No type safety for what should be internal values
• Creates unnecessary public API surface

Recommended Fixes

Option 1: Make Constants Private (Recommended)

StringUtil:

// Change from public to package-private or private
private static final String STRING_CANNOT_BE_BLANK = "String cannot be blank!";
private static final String DEFAULT_SEPARATOR = "";

ObjectUtil:

// Either make private or remove entirely (not used)
private static final String DEFAULT_ERROR_MSG = "Invalid value";

UrlUtil:

// Make private - users should use asProtocolAndHost() instead
private static final String PROTOCOL_SEPARATOR = "://";

Option 2: Document Why They're Public

If these constants are intentionally public for user consumption:

/**
 * Default error message for blank string validation.
 * 
 * <p>This constant is public to allow users to recognize the exception
 * message when thrown by {@link #requireNonBlank(String)}.
 * 
 * @see #requireNonBlank(String)
 */
public static final String STRING_CANNOT_BE_BLANK = "String cannot be blank!";

But this is not recommended because:

• Exception messages should not be part of the API contract
• Error messages can change in future versions
• Users shouldn't parse exception messages

Option 3: Remove Unused Constants

ObjectUtil.DEFAULT_ERROR_MSG appears to be unused:

# Search reveals no usage
grep -r "DEFAULT_ERROR_MSG" src/main/java
# Returns only the declaration

Should be removed entirely.

Analysis of Each Constant

StringUtil.STRING_CANNOT_BE_BLANK

Current usage:

public static String requireNonBlank(final String string) {
    return requireNonBlank(string, STRING_CANNOT_BE_BLANK);
}

Recommendation: Make private

• Only used as default parameter
• No benefit to public exposure
• Users don't need to reference this

StringUtil.DEFAULT_SEPARATOR

Current usage:

public static StringBuilder concat(final StringBuilder stringBuilder, Object... objs) {
    return concatWithSeparator(stringBuilder, DEFAULT_SEPARATOR, objs);
}

Recommendation: Make private or replace with empty string literal

• Empty string is self-documenting
• No need for named constant
• "" is clearer than DEFAULT_SEPARATOR

ObjectUtil.DEFAULT_ERROR_MSG

Current usage: None found

Recommendation: Remove entirely

• Dead code
• Generic message has no value
• Not referenced anywhere

UrlUtil.PROTOCOL_SEPARATOR

Current usage:

public static String asProtocolAndHost(final URL url) {
    return StringUtil.concat(url.getProtocol(), PROTOCOL_SEPARATOR, url.getHost());
}

Recommendation: Make private

• Internal implementation detail
• Users should use asProtocolAndHost() method
• No valid reason to expose "://" as public API

Breaking Change Consideration

Changing these from public to private is a breaking change per semver. However:

1. Low risk - These constants are unlikely to be used externally
2. Easy migration - Users can define their own if needed
3. Better design - Encapsulates implementation details
4. v2.0 opportunity - Could be removed in next major version

Alternative: Deprecate First

Mark as deprecated in current version, remove in v2.0:

/**
 * @deprecated Internal constant, will be removed in v2.0
 */
@Deprecated(since = "1.32", forRemoval = true)
public static final String STRING_CANNOT_BE_BLANK = "String cannot be blank!";

[sfloess]

Analysis of public String constants:

1. UrlUtil.PROTOCOL_SEPARATOR = "://" - URL protocol separator, correctly a String
2. ObjectUtil.DEFAULT_ERROR_MSG = "Invalid value" - Error message template, correctly a String
3. StringUtil.STRING_CANNOT_BE_BLANK = "String cannot be blank!" - Error message, correctly a String
4. StringUtil.DEFAULT_SEPARATOR = "" - Empty string separator, correctly a String

Why String constants are appropriate here:

• Error messages should be Strings (not enums) for i18n compatibility
• Separators are actual String values used in string operations, not type-safe enum cases
• These are configuration constants, not enumerable types with distinct behaviors

When to use enums:

• Finite set of distinct types (e.g., LogLevel, HttpMethod, FileType)
• Type safety needed at compile time
• Switch statement exhaustiveness checking desired

When to use String constants:

• Actual string values used in operations (separators, delimiters)
• Messages and templates
• Format strings

Current usage is correct. No changes needed.

Closing as working as intended.