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How does the EnumSet and EnumMap provide more efficient alternatives to HashSet<Enum> and HashMap<Enum, V> specifically for enum keys? How would you implement an enum that also implements an interface, and what's the benefit of having different enum constants provide different behavior through a shared interface contract? What are the ordinal() and name() methods on an enum constant, and why is relying on ordinal() for persistence or serialization considered dangerous? How do you correctly use an enum as a switch statement/expression's target, and what changed with the introduction of arrow-style switch expressions (Java 14+) regarding exhaustiveness and fall-through? How would you use an enum to represent a fixed set of configuration keys combined with a shared lookup method, avoiding scattered String literal keys throughout a codebase? How would you implement the Strategy pattern using an enum with constant-specific method bodies, and how does this compare to a traditional interface-based Strategy implementation? How does Java implement enums under the hood at the bytecode level (extending java.lang.Enum), and what makes them inherently thread-safe and serialization-safe as singletons? How would you use an EnumMap combined with a switch expression to implement a state machine, and what compile-time safety benefits does an exhaustive switch over an enum provide? How would you implement a custom enum that also functions as a lookup table (mapping an external code to a constant), such as parsing an HTTP status code string into an enum constant, handling unknown values gracefully? How would you implement a custom compareTo()-based ordering for an enum that differs from its natural declaration-order ordering, given Comparable's default implementation uses ordinal()?
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Java › Enums

What is the valueOf() method on an enum, and what happens when you pass a name that doesn't match any declared constant?

Intermediate

Answer

The implicitly-generated static valueOf(String name) method looks up and returns the enum constant whose declared identifier exactly matches the given String (case-sensitive), throwing IllegalArgumentException if no constant with that exact name exists -- useful for parsing an enum from external string input (like a configuration value or API parameter), though because it throws rather than returning a sentinel/null, callers must be prepared to catch IllegalArgumentException or validate the input beforehand if invalid values are a realistic possibility.
enum Status { ACTIVE, INACTIVE, PENDING }

Status s = Status.valueOf("ACTIVE");     // works, returns Status.ACTIVE

try {
    Status invalid = Status.valueOf("active");  // throws IllegalArgumentException -- case-sensitive!
} catch (IllegalArgumentException e) {
    System.out.println("Invalid status value");
}
Real-world example An API endpoint parsing a status query parameter into a Status enum via valueOf() wraps the call in a try-catch to return a clean 400 Bad Request error with a helpful message when a client submits an invalid or mistyped status value, rather than letting an unhandled IllegalArgumentException propagate as a generic 500 error.

Common follow-ups: Why is valueOf() case-sensitive, and what would you do to support case-insensitive lookup instead?;How does this built-in valueOf() differ from the custom fromCode()-style lookup pattern for external representations that don't match Java identifier naming?

Exceptions;RESTful Web APIs & Controllers
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