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How do you retrieve all constants of an enum type, and how would you iterate over them? How do you add fields, constructors, and methods to a Java enum, and how would you implement an enum representing planets with mass and radius that calculates surface gravity? 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? What is the valueOf() method on an enum, and what happens when you pass a name that doesn't match any declared constant? 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?
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What is an enum in Java, and how does it differ from simply using a set of integer or String constants?

Beginner

Answer

An enum defines a fixed, type-safe set of named constants, where each constant is actually a full singleton instance of the enum type itself -- unlike int or String constants (which provide no compile-time type safety, allowing any arbitrary int/String value to be passed where a specific constant was intended), an enum's type system prevents passing an invalid value at all, since only the declared enum constants can exist as values of that type, catching a whole class of "invalid magic value" bugs at compile time rather than at runtime.
public enum Day {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

public void scheduleTask(Day day) { ... }

scheduleTask(Day.MONDAY);  // type-safe, only a valid Day constant can be passed
// scheduleTask(1);  // wouldn't even compile -- contrast with an int-constant-based approach
// where scheduleTask(99) would compile fine despite being a meaningless, invalid value
Real-world example A legacy codebase using int constants (STATUS_PENDING = 0, STATUS_APPROVED = 1) for an order's status is refactored to use an OrderStatus enum, immediately eliminating an entire class of bugs where an invalid or out-of-range integer could previously be silently passed and stored without any compile-time or even runtime detection.

Common follow-ups: Why can't you create a new instance of an enum type using 'new' the way you can with a regular class?;How does the compiler guarantee that only the declared enum constants can ever exist as values of that type?

OOP & Classes;Java Fundamentals: Syntax Data Types & Operators
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