Enums — Practice Tasks¶

Twelve exercises in enum design, strategy enums, EnumSet/EnumMap, and pattern matching.

Task 1 — Days of the week¶

Define enum Day { MONDAY, ..., SUNDAY } with: - boolean isWeekend() returning true for SAT/SUN - int hoursOfDaylight(Month m) (mock implementation) - A static EnumSet<Day> WEEKEND = EnumSet.of(SATURDAY, SUNDAY);

Task 2 — Strategy enum¶

Define enum Op with PLUS, MINUS, TIMES, DIVIDE. Each has an abstract int apply(int a, int b). Implement per-constant. Test:

Op.PLUS.apply(3, 4)   // 7
Op.DIVIDE.apply(10, 0)   // throws

Task 3 — Enum + interface¶

Define interface Operation { int apply(int a, int b); } and enum BasicOp implements Operation. Show that you can pass either a BasicOp constant or a custom lambda to a method expecting Operation.

Task 4 — Enum singleton¶

Implement a Database singleton via enum: - Hold a fake Connection field - Provide save(Object) and query(String) methods - Verify the same instance via Database.INSTANCE == Database.INSTANCE

Task 5 — Per-constant data¶

Define enum HttpStatus with constants and per-constant code and description fields. Add static factory from(int code) returning the corresponding constant or null.

Task 6 — EnumSet vs HashSet benchmark¶

Create a JMH benchmark comparing: - EnumSet.of(...) vs Set.of(...) for contains operations - EnumMap vs HashMap for get operations

Verify EnumSet/EnumMap are faster.

Task 7 — State machine with enum¶

Model a vending machine:

enum State {
    IDLE, COIN_INSERTED, ITEM_SELECTED, DISPENSING;
    abstract State on(Event e);
}

Task 8 — Switch with binding¶

Pattern-match a enum Op in a switch (Java 21+) with: - Guard: case PLUS when isCommutative() - Constants combined: case PLUS, MINUS -> "binary"

Task 9 — Enum from JSON¶

Using Jackson, serialize an enum Color { RED, GREEN, BLUE } as JSON. Test: - Default: "RED" - Custom via @JsonValue to lowercase: "red"

Task 10 — Don't store ordinal¶

Given an existing system that stores direction.ordinal() in DB: 1. Reorder constants and observe corruption 2. Refactor to store name() 3. Discuss migration path

Task 11 — Enum implementing Comparator¶

Define enum SortBy with BY_NAME, BY_AGE, BY_DATE — each implements Comparator<User>. Use users.sort(SortBy.BY_AGE).

Task 12 — Migrate enum to sealed records¶

Given:

enum Shape {
    CIRCLE(3.14), SQUARE(1.0), TRIANGLE(0.5);
    private final double factor;
    ...
}

Refactor to:

sealed interface Shape { ... }
record Circle(double radius) implements Shape { ... }
record Square(double side) implements Shape { ... }
record Triangle(double base, double h) implements Shape { ... }

Compare: how does the data model change? When is each preferable?

Validation¶

Memorize this: enums are for closed sets of labels with optional per-constant behavior. Use EnumSet/EnumMap for performance. Use enum + interface for SPI. Use sealed records when variants carry typed data. Don't use ordinal() for persistence.