Java 开发中 for和Stream用法对比分析

List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "Anna");

for (String name : names) {
    if (name.startsWith("A")) {
        System.out.println(name.toUpperCase());
    }
}

names.stream()
     .filter(name -> name.startsWith("A"))
     .map(String::toUpperCase)
     .forEach(System.out::println);

for (String item : list) {
    System.out.println(item); 
}

for (File file : files) {
    if (file.length() == 0) {
        isEmpty = true;
        break; // 发现空文件就立即停止
    }
}

for (Order order : orders) {
    // 复杂的业务判断
    if (order.isValid() 
        && (order.isVip() || order.getAmount() > 1000)
        && !order.isCancelled()) {
        processOrder(order);
    }
}

int successCount = 0;
int failCount = 0;
List<Result> results = new ArrayList<>();
for (Task task : tasks) {
    try {
        Result result = executeTask(task);
        results.add(result);
        successCount++;
    } catch (Exception e) {
        failCount++;
        logger.error("任务执行失败", e);
    }
}

for (int i = 0; i < list.size(); i++) {
    if (i % 2 == 0) { // 每隔一个元素处理
        process(list.get(i));
    }
}

for (User user : users) {
    // 多个步骤，相互依赖
    Profile profile = buildProfile(user);
    validateProfile(profile);
    saveToDatabase(profile);
    sendNotification(user);
}

// 在游戏开发、算法竞赛等场景
for (int i = 0; i < MAX_ITERATIONS; i++) {
    // 极其简单的数学运算
    result += array[i] * factor;
}

for (int i = 0; i < list.size(); i++) {
    if (shouldRemove(list.get(i))) {
        list.remove(i); // 直接修改原集合
        i--; // 调整索引
    }
}

List<Product> results = products.stream()
    .filter(p -> p.getStock() > 0)      // 过滤有库存的
    .filter(p -> p.getPrice() < 100)    // 过滤价格小于100的
    .sorted(Comparator.comparing(Product::getPrice)) // 按价格排序
    .limit(10)                          // 只取前10个
    .collect(Collectors.toList());      // 收集结果

// 从员工列表中提取姓名
List<String> names = employees.stream()
    .map(Employee::getName)  // 提取姓名
    .collect(Collectors.toList());

// 复杂的统计一句搞定
DoubleSummaryStatistics stats = employees.stream()
    .mapToDouble(Employee::getSalary)
    .summaryStatistics();
System.out.println("平均工资: " + stats.getAverage());
System.out.println("最高工资: " + stats.getMax());
System.out.println("最低工资: " + stats.getMin());

// 按部门分组
Map<Department, List<Employee>> byDept = employees.stream()
    .collect(Collectors.groupingBy(Employee::getDepartment));
// 按条件分区
Map<Boolean, List<Employee>> partitioned = employees.stream()
    .collect(Collectors.partitioningBy(emp -> emp.getSalary() > 10000));

List<String> uniqueNames = employees.stream()
    .map(Employee::getName)
    .distinct()             // 去重
    .sorted()               // 排序
    .collect(Collectors.toList());

// 是否存在满足条件的元素
boolean hasManager = employees.stream()
    .anyMatch(emp -> "经理".equals(emp.getPosition()));
// 查找第一个满足条件的
Optional<Employee> firstRich = employees.stream()
    .filter(emp -> emp.getSalary() > 50000)
    .findFirst();

// 将姓名用逗号拼接
String nameStr = employees.stream()
    .map(Employee::getName)
    .collect(Collectors.joining(", "));
// 数字求和
double totalSalary = employees.stream()
    .mapToDouble(Employee::getSalary)
    .sum();

List<String> results = bigList.parallelStream()
    .map(this::expensiveOperation) // 耗时操作
    .collect(Collectors.toList());

// 错误示范！
int count = 0;
list.stream().forEach(item -> {
    if (item.isValid()) {
        count++; // 编译报错！count 必须是 final 或 effectively final
    }
});

long count = list.stream().filter(Item::isValid).count();