Optional<T> reduce(BinaryOperator<T> accumulator)

T reduce(T identity, BinaryOperator<T> accumulator)

<U> U reduce(U identity,
            BiFunction<U, ? super T, U> accumulator,
            BinaryOperator<U> combiner)

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
Optional<Integer> max = numbers.stream()
                             .reduce(Integer::max);
max.ifPresent(System.out::println); // 輸出5

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
int sum = numbers.stream()
               .reduce(0, (a, b) -> a + b);
System.out.println(sum); // 輸出15

List<String> words = Arrays.asList("Hello", "World", "Java");
String combined = words.stream()
                     .reduce("", 
                            (partial, element) -> partial + " " + element,
                            String::concat);
System.out.println(combined.trim()); // 輸出"Hello World Java"

// 求和
int sum = numbers.stream().reduce(0, Integer::sum);
// 求積
int product = numbers.stream().reduce(1, (a, b) -> a * b);
// 求最大值
int max = numbers.stream().reduce(Integer.MIN_VALUE, Integer::max);

// 字符串連接
String concat = strings.stream().reduce("", String::concat);
// 拼接帶分隔符
String joined = strings.stream()
                     .reduce((a, b) -> a + ", " + b)
                     .orElse("");

// 計算員工總薪資
double totalSalary = employees.stream()
                           .reduce(0.0, 
                                  (sum, emp) -> sum + emp.getSalary(),
                                  Double::sum);

List<Order> orders = // 獲取訂單列表
BigDecimal total = orders.stream()
                       .map(Order::getAmount)
                       .reduce(BigDecimal.ZERO, BigDecimal::add);

List<Map<String, Integer>> maps = // 多個map的列表
Map<String, Integer> result = maps.stream()
                                .reduce(new HashMap<>(),
                                        (m1, m2) -> {
                                            m1.putAll(m2);
                                            return m1;
                                        });