列舉、內部類別與常用 API

/** 星期列舉：一組有名字的、型別安全的常數 */ enum Weekday MON, TUE, WED, THU, FRI, SAT, SUN

public class EnumBasicsDemo

public static void main(String[] args)

Weekday today = Weekday.SAT;

System.out.println("today = " + today); // 自動印常數名 System.out.println("名稱：" + today.name()); System.out.println("序號：" + today.ordinal()); // 從 0 起算 System.out.println("解析：" + Weekday.valueOf("MON"));

// values()：取得全部常數，順序同宣告順序 for (Weekday d : Weekday.values()) System.out.print(d + " "); System.out.println();

// enum 進 switch：case 直接寫常數名，不必加型別前綴 switch (today) case SAT, SUN -> System.out.println("週末：睡到自然醒"); case MON -> System.out.println("星期一：收心上工"); default -> System.out.println("平日：好好上課");

/** 行星列舉：每個常數自帶質量（公斤）與半徑（公尺） */ enum Planet

// 宣告常數時以括號呼叫建構子 MERCURY(3.303e23, 2.4397e6), EARTH(5.976e24, 6.37814e6), MARS(6.421e23, 3.3972e6);

private final double mass; // 質量 private final double radius; // 半徑

/** 萬有引力常數 */ private static final double G = 6.67300E-11;

/** enum 的建構子：只在載入常數時被呼叫 */ Planet(double mass, double radius) this.mass = mass; this.radius = radius;

/** 表面重力加速度：G 乘質量除以半徑平方 */ double surfaceGravity() return G * mass / (radius * radius);

/** 質量 otherMass 的物體在該行星上的重量（牛頓） */ double surfaceWeight(double otherMass) return otherMass * surfaceGravity();

public class PlanetDemo public static void main(String[] args) double myMass = 70.0; // 一位 70 公斤的太空旅人 for (Planet p : Planet.values()) System.out.printf("在 p, p.surfaceWeight(myMass));

/** 四則運算列舉：每個常數自帶計算邏輯 */ enum Operation

PLUS("+") @Override public double apply(double a, double b) return a + b; , MINUS("-") @Override public double apply(double a, double b) return a - b; , TIMES("*") @Override public double apply(double a, double b) return a * b; , DIVIDE("/") @Override public double apply(double a, double b) return a / b; ;

private final String symbol; // 運算符號

Operation(String symbol) this.symbol = symbol;

/** 抽象方法：每個常數都「必須」提供自己的版本 */ public abstract double apply(double a, double b);

public String getSymbol() return symbol;

public class OperationDemo public static void main(String[] args) double a = 12, b = 4; for (Operation op : Operation.values()) // op.apply(...) 依實際常數執行不同邏輯：多型！ System.out.printf(" a, op.getSymbol(), b, op.apply(a, b));

import java.util.EnumMap; import java.util.EnumSet; import java.util.Map;

/** EnumSet 與 EnumMap：搭配 EnumBasicsDemo 的 Weekday */ public class EnumCollectionsDemo

public static void main(String[] args)

// range：取宣告順序中的一段 EnumSet<Weekday> workdays = EnumSet.range(Weekday.MON, Weekday.FRI); // complementOf：取補集（其餘常數） EnumSet<Weekday> weekend = EnumSet.complementOf(workdays);

System.out.println("工作日：" + workdays); System.out.println("週末：" + weekend);

// EnumMap：鍵是列舉的 Map，建構時要提供類別資訊 Map<Weekday, String> plan = new EnumMap<>(Weekday.class); plan.put(Weekday.SAT, "爬山"); plan.put(Weekday.SUN, "寫程式"); System.out.println("週末計畫：" + plan);

/** 給匿名類別實作用的小介面 */ interface Greeter void greet();

/** 四種巢狀類別的最小示範 */ public class NestedDemo

private String outerField = "外部類別的欄位";

/** 一、靜態巢狀類別：不持有外部實體，最常用 */ static class StaticNested String hello() return "靜態巢狀類別：與外部實體無關";

/** 二、內部類別：隱含持有一份外部類別的 this */ class Inner String hello() // 可以直接讀外部類別的 private 欄位 return "內部類別讀到：" + outerField;

void localAndAnonymous()

// 三、區域類別：宣告在方法裡，只有本方法看得到 class Local String hello() return "區域類別：方法內的小幫手"; System.out.println(new Local().hello());

// 四、匿名類別：宣告與 new 一次完成，連名字都省了 Greeter greeter = new Greeter() @Override public void greet() System.out.println("匿名類別：一次性的實作"); ; greeter.greet();

public static void main(String[] args)

// 靜態巢狀類別：直接 new，不需要外部實體 System.out.println(new StaticNested().hello());

// 內部類別：必須先有外部實體，語法是 outer.new Inner() NestedDemo outer = new NestedDemo(); System.out.println(outer.new Inner().hello());

outer.localAndAnonymous();

import java.util.ArrayList; import java.util.Collections; import java.util.Comparator; import java.util.List;

/** 用匿名類別提供「依字串長度排序」的規則 */ public class AnonymousComparatorDemo

public static void main(String[] args)

List<String> cities = new ArrayList<>(List.of( "Kaohsiung", "Taipei", "Tainan", "Hsinchu"));

// new 介面名() ... ：宣告兼實體化，一步到位 Collections.sort(cities, new Comparator<String>() @Override public int compare(String a, String b) // 短的排前面；同長度維持原順序（穩定排序） return Integer.compare(a.length(), b.length()); );

System.out.println(cities);

import java.util.Random;

/** Math 常用方法與 Random 的正確用法 */ public class MathRandomDemo

public static void main(String[] args)

// — Math：static 方法直接呼叫，不必建立物件 — System.out.println("絕對值：" + Math.abs(-7)); System.out.println("四捨五入：" + Math.round(3.56)); System.out.println("無條件進位：" + Math.ceil(3.14)); System.out.println("無條件捨去：" + Math.floor(3.99)); System.out.println("次方：" + Math.pow(2, 10)); System.out.println("平方根：" + Math.sqrt(144)); System.out.println("較大值：" + Math.max(17, 23));

// — Random：擲骰子一萬次，驗證範圍公式 — Random rand = new Random(); int min = 7, max = 0; for (int i = 0; i < 10000; i++) // nextInt(6) 給 0 5，加 1 平移成骰子的 1 6 int dice = rand.nextInt(6) + 1; min = Math.min(min, dice); max = Math.max(max, dice); System.out.println("擲一萬次：最小 " + min + "、最大 " + max);

import java.time.Duration; import java.time.LocalDate; import java.time.LocalDateTime; import java.time.LocalTime; import java.time.Period; import java.time.format.DateTimeFormatter;

/** java.time 的建立、計算、格式化與解析 */ public class TimeDemo

public static void main(String[] args)

// 教學用固定日期讓輸出可重現；實務常用 LocalDate.now() LocalDate today = LocalDate.of(2026, 8, 14); LocalTime lunch = LocalTime.of(12, 30); LocalDateTime meeting = LocalDateTime.of(2026, 8, 14, 14, 0);

System.out.println("今天：" + today); System.out.println("星期：" + today.getDayOfWeek());

// 不可變：plus 與 minus 都回傳「新物件」，原值不動 LocalDate deadline = today.plusDays(30); System.out.println("30 天後截止：" + deadline); System.out.println("today 沒被改動：" + today);

// with 系列：換掉某個欄位，同樣回傳新物件 System.out.println("本月一號：" + today.withDayOfMonth(1));

// Period：以年月日計的日期差，算年齡最順手 LocalDate birthday = LocalDate.of(2004, 3, 21); Period age = Period.between(birthday, today); System.out.println("年齡：" + age.getYears() + " 歲又 " + age.getMonths() + " 個月");

// Duration：以時分秒計的時間差 Duration gap = Duration.between(lunch, meeting.toLocalTime()); System.out.println("午餐到會議相隔 " + gap.toMinutes() + " 分鐘");

// DateTimeFormatter：自訂顯示格式 DateTimeFormatter fmt = DateTimeFormatter.ofPattern("yyyy年MM月dd日 HH:mm"); System.out.println("格式化：" + meeting.format(fmt));

// 解析：字串轉回日期物件（ISO 格式可直接 parse） LocalDate parsed = LocalDate.parse("2026-10-10"); System.out.println("解析結果：" + parsed.getMonthValue() + " 月 " + parsed.getDayOfMonth() + " 日");

import java.time.LocalDateTime; import java.time.format.DateTimeFormatter; import java.util.ArrayList; import java.util.List;

/** 訂單：整合列舉狀態機、內部類別與日期時間 */ public class Order

/** 訂單狀態：合法轉移路徑寫死在列舉裡（巢狀 enum 隱含 static） */ enum OrderStatus

CREATED, PAID, SHIPPED, COMPLETED, CANCELLED;

/** 狀態機核心：只回答「能不能走到 target」 */ boolean canTransitionTo(OrderStatus target) // switch 運算式（Java 14）：涵蓋所有常數，不必 default return switch (this) case CREATED -> target == PAID || target == CANCELLED; case PAID -> target == SHIPPED || target == CANCELLED; case SHIPPED -> target == COMPLETED; case COMPLETED, CANCELLED -> false; // 終態 ;

/** 內部類別：訂單項目，離開訂單就沒有意義的小物件 */ class OrderItem

private final String product; private final int quantity; private final int unitPrice;

OrderItem(String product, int quantity, int unitPrice) this.product = product; this.quantity = quantity; this.unitPrice = unitPrice;

int subtotal() return quantity * unitPrice;

@Override public String toString() // 內部類別可直接使用外部訂單的 id 欄位 return "[" + id + "] " + product + " x" + quantity + " = " + subtotal() + " 元";

private final String id; // 訂單編號 private final LocalDateTime createdAt; // 下單時間 private final List<OrderItem> items = new ArrayList<>(); private OrderStatus status = OrderStatus.CREATED;

Order(String id, LocalDateTime createdAt) this.id = id; this.createdAt = createdAt;

void addItem(String product, int quantity, int unitPrice) items.add(new OrderItem(product, quantity, unitPrice));

/** 狀態轉移：先問狀態機，合法才放行 */ void changeStatus(OrderStatus target) if (!status.canTransitionTo(target)) System.out.println("不允許：" + status + " -> " + target); return; System.out.println("狀態轉移：" + status + " -> " + target); status = target;

void printSummary() DateTimeFormatter fmt = DateTimeFormatter.ofPattern("MM/dd HH:mm"); int total = 0; for (OrderItem item : items) System.out.println(item); total += item.subtotal(); System.out.println("總金額：" + total + " 元"); System.out.println("下單時間：" + createdAt.format(fmt)); // 簡化規則：下單後 3 天送達 System.out.println("預計送達：" + createdAt.plusDays(3).format(fmt));

public static void main(String[] args)

Order order = new Order("A001", LocalDateTime.of(2026, 8, 14, 10, 30)); order.addItem("機械鍵盤", 1, 2990); order.addItem("滑鼠墊", 2, 250);

order.printSummary(); order.changeStatus(OrderStatus.PAID); // 合法 order.changeStatus(OrderStatus.COMPLETED); // 不合法：還沒出貨 order.changeStatus(OrderStatus.SHIPPED); // 合法 order.changeStatus(OrderStatus.COMPLETED); // 合法