Java Enums — Methods, Fields, and Interfaces

import java.util.*;
import java.util.function.DoubleBinaryOperator;

// Enum with fields and methods
public enum HttpStatus {
    OK(200, "OK"),
    CREATED(201, "Created"),
    BAD_REQUEST(400, "Bad Request"),
    NOT_FOUND(404, "Not Found"),
    INTERNAL_ERROR(500, "Internal Server Error");

    private final int code;
    private final String reason;

    HttpStatus(int code, String reason) {
        this.code = code;
        this.reason = reason;
    }

    public int code() { return code; }
    public String reason() { return reason; }
    public boolean isError() { return code >= 400; }

    // Reverse lookup
    private static final Map<Integer, HttpStatus> BY_CODE = new HashMap<>();
    static { for (var s : values()) BY_CODE.put(s.code, s); }

    public static HttpStatus fromCode(int code) {
        return Optional.ofNullable(BY_CODE.get(code))
            .orElseThrow(() -> new IllegalArgumentException("Unknown: " + code));
    }
}

// Enum implementing interface (strategy pattern)
enum MathOp implements DoubleBinaryOperator {
    ADD {
        public double applyAsDouble(double a, double b) { return a + b; }
    },
    SUBTRACT {
        public double applyAsDouble(double a, double b) { return a - b; }
    },
    MULTIPLY {
        public double applyAsDouble(double a, double b) { return a * b; }
    };

    public double calculate(double a, double b) {
        return applyAsDouble(a, b);
    }
}

// Usage
class EnumDemo {
    public static void main(String[] args) {
        HttpStatus status = HttpStatus.fromCode(404);
        System.out.printf("%d %s (error: %b)%n", status.code(), status.reason(), status.isError());

        double result = MathOp.ADD.calculate(10, 5);
        System.out.println(result); // 15.0

        // EnumSet — efficient set of enum values
        EnumSet<HttpStatus> errors = EnumSet.of(HttpStatus.BAD_REQUEST, HttpStatus.NOT_FOUND);
        EnumMap<HttpStatus, String> messages = new EnumMap<>(HttpStatus.class);
        messages.put(HttpStatus.OK, "Success!");
    }
}