Enum — tập giá trị cố định, type-safe thay cho magic number

// ANTI-PATTERN: int constants
static final int TRANG_THAI_MOI        = 1;
static final int TRANG_THAI_XU_LY      = 2;
static final int TRANG_THAI_HOAN_TAT   = 3;

void xuLy(int trangThai) {
    if (trangThai == 1) { ... }
    // van co the truyen 999 -- compiler khong phat hien!
}

public enum TrangThai {
    MOI, DANG_XU_LY, HOAN_TAT, HUY_BO;
}

TrangThai t = TrangThai.MOI;  // compiler chi chap nhan gia tri trong enum

void xuLy(TrangThai trangThai) {  // type-safe: chi nhan TrangThai
    if (trangThai == TrangThai.MOI) { ... }
    // khong the truyen int 999 -- compile error
}

TrangThai t = TrangThai.DANG_XU_LY;

// Switch expression (Java 14+)
String label = switch (t) {
    case MOI        -> "Moi tao";
    case DANG_XU_LY -> "Dang xu ly";
    case HOAN_TAT   -> "Da xong";
    case HUY_BO     -> "Da huy";
};
// Compiler bao loi neu quen 1 case -- exhaustive check

public enum PhanLoaiBmi {
    GAY("Gay", 0, 18.5),
    BINH_THUONG("Binh thuong", 18.5, 25),
    THUA_CAN("Thua can", 25, 30),
    BEO_PHI("Beo phi", 30, Double.MAX_VALUE);

    private final String nhan;
    private final double min;
    private final double max;

    PhanLoaiBmi(String nhan, double min, double max) {
        this.nhan = nhan;
        this.min  = min;
        this.max  = max;
    }

    public String getNhan() { return nhan; }

    // Static factory: tu BMI -> PhanLoaiBmi
    public static PhanLoaiBmi tuBmi(double bmi) {
        for (PhanLoaiBmi pl : values()) {
            if (bmi >= pl.min && bmi < pl.max) return pl;
        }
        return BEO_PHI;
    }
}

// Dung:
double bmi = 22.4;
PhanLoaiBmi pl = PhanLoaiBmi.tuBmi(bmi);
System.out.println(pl.getNhan());  // "Binh thuong"

TrangThai t = TrangThai.DANG_XU_LY;

t.name()      // "DANG_XU_LY" -- ten khai bao
t.ordinal()   // 1 -- vi tri 0-indexed trong enum (MOI=0, DANG_XU_LY=1...)
t.toString()  // "DANG_XU_LY" -- mac dinh giong name()

TrangThai[] all = TrangThai.values();  // mang tat ca gia tri
TrangThai t2 = TrangThai.valueOf("MOI");  // tu String -> enum; IllegalArgumentException neu sai

public class OrderDemo {

    public enum TrangThai {
        PENDING("Cho xu ly"),
        PROCESSING("Dang xu ly"),
        SHIPPED("Da giao hang"),
        DELIVERED("Da nhan hang"),
        CANCELLED("Da huy");

        private final String moTa;

        TrangThai(String moTa) {
            this.moTa = moTa;
        }

        public String getMoTa() { return moTa; }

        public boolean canCancel() {
            return this == PENDING || this == PROCESSING;
        }
    }

    public static void main(String[] args) {
        TrangThai t = TrangThai.PROCESSING;

        System.out.println("Trang thai: " + t.getMoTa());
        System.out.println("Co the huy: " + t.canCancel());

        // switch expression -- exhaustive
        String icon = switch (t) {
            case PENDING    -> "CLOCK";
            case PROCESSING -> "GEAR";
            case SHIPPED    -> "TRUCK";
            case DELIVERED  -> "CHECK";
            case CANCELLED  -> "X";
        };
        System.out.println("Icon: " + icon);

        // Built-in methods
        System.out.println(t.name());                      // PROCESSING
        System.out.println(TrangThai.values().length);     // 5
        System.out.println(TrangThai.valueOf("SHIPPED"));  // SHIPPED
    }
}