Java Stream.reduce归约操作详解

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

List words = Arrays.asList("Hello", " ", "world", "!");
String sentence = words.stream().reduce("", String::concat);
System.out.println(sentence); // 输出 "Hello world!"

List emptyList = Collections.emptyList();
Optional result = emptyList.stream().reduce(Integer::max);
if (result.isPresent()) {
    System.out.println("最大值：" + result.get());
} else {
    System.out.println("列表为空");
}

List amounts = Arrays.asList(
    BigDecimal.valueOf(100),
    BigDecimal.valueOf(200),
    BigDecimal.valueOf(300)
);

BigDecimal total = amounts.parallelStream()
    .reduce(BigDecimal.ZERO,
            BigDecimal::add,
            BigDecimal::add);