double average = students.stream().mapToDouble(Student::getScore).average().orElse(0D);

这段Java代码使用了Java 8的Stream API来计算学生列表中成绩（假设为double类型）的平均值。`students` 是一个`Student`对象的集合，每个`Student`有一个名为`getScore()`的方法来获取其分数。

这里是逐步解释：

1. `students.stream()`：创建一个从`students`集合流化的操作。这允许我们对集合中的每个元素执行连续的操作。

2. `.mapToDouble(Student::getScore)`：将每个`Student`对象转换为其分数（double类型）。`Student::getScore`是一个方法引用，它表示调用`Student`对象的`getScore`方法并返回结果。

3. `.average()`：对所有映射后的分数进行平均计算。

4. `.orElse(0D)`：如果流为空（即没有学生），则返回默认值0.0（double类型）。`orElse()`是一个可选操作，当流中没有元素时提供备选值。

所以，整个表达式的结果就是`students`集合中所有学生的平均成绩，如果集合为空则返回0.0。示例中`average()`会返回一个`OptionalDouble`，但因为后面有`.orElse(0D)`，所以我们可以直接得到一个double类型的值。

相关问题

优化这段java代码 //通过域账号名称 调用userSdk获取 对应ID List<String> sendUserNameList = form.getOperationScoreDTOList() .stream().map(OperationScoreDTO::getSenderName).distinct().collect(Collectors.toList()); List<String> receiverUserNameList = form.getOperationScoreDTOList() .stream().map(OperationScoreDTO::getReceiverName).distinct().collect(Collectors.toList()); List<FindOneByUsernameVo> sendUserList = userServiceSdk.findByUsernames(sendUserNameList); List<FindOneByUsernameVo> receiverUserList = userServiceSdk.findByUsernames(receiverUserNameList); Map<String, Integer> senderMap = sendUserList .stream() .collect(Collectors.toMap(FindOneByUsernameVo::getUsername, FindOneByUsernameVo::getTeclibUserId)); Map<String, Integer> receiverMap = receiverUserList .stream() .collect(Collectors.toMap(FindOneByUsernameVo::getUsername, FindOneByUsernameVo::getTeclibUserId)); for (OperationScoreDTO operationScoreDTO : form.getOperationScoreDTOList()) { Integer senderId = senderMap.get(operationScoreDTO.getSenderName()); if (senderId == null) { throw new BizException("获取发送者失败"); } Integer receiverId = receiverMap.get(operationScoreDTO.getSenderName()); if (receiverId == null) { throw new BizException("获取接收者失败"); } Short score = operationScoreDTO.getScore(); String remark = operationScoreDTO.getRemark(); OperationScoreEntity os = new OperationScoreEntity(); os.setSendId(senderId); os.setReceiverId(receiverId); os.setEvent(score.toString()); os.setRemark(remark); os.setSendTime(LocalDate.now()); operationScoreRepository.save(os); String event = ScoreEventConstant.BONUS_SCORE; String module = ScoreModuleConstant.BONUS_SCORE; String payload = String.format("osId:%d;receiver:%d;remark:%s;score:%s", os.getId(), receiverId, remark, score); UserScoreEntity userScore = new UserScoreEntity(); userScore.setUserId(receiverId); userScore.setModule(module); userScore.setEvent(event); userScore.setPayload(payload); userScore.setOperationTime(operationScoreDTO.getOperationTime()); userScore.setScore(score); Integer totalScore = this.findTotalScoreByUserId(receiverId); userScore.setTotal(Math.max(totalScore + score, 0)); userScoreRepository.save(userScore); } return Boolean.TRUE;

在这段代码中，你可以考虑以下优化措施：

1. 将重复的代码提取为方法：将获取用户名列表和调用 `userServiceSdk.findByUsernames` 的逻辑提取为一个方法，这样可以减少重复代码。

2. 使用 `forEach` 替代 `for` 循环：使用流的 `forEach` 方法可以简化循环的书写，更加简洁和直观。

3. 使用批量插入：使用批量插入操作可以减少数据库交互次数，提高性能。

下面是优化后的代码示例：

List<String> sendUserNameList = form.getOperationScoreDTOList()
    .stream().map(OperationScoreDTO::getSenderName).distinct().collect(Collectors.toList());
List<String> receiverUserNameList = form.getOperationScoreDTOList()
    .stream().map(OperationScoreDTO::getReceiverName).distinct().collect(Collectors.toList());

Map<String, Integer> senderMap = getUserIdsByUsernames(sendUserNameList);
Map<String, Integer> receiverMap = getUserIdsByUsernames(receiverUserNameList);

form.getOperationScoreDTOList().forEach(operationScoreDTO -> {
    Integer senderId = senderMap.get(operationScoreDTO.getSenderName());
    if (senderId == null) {
        throw new BizException("获取发送者失败");
    }
    Integer receiverId = receiverMap.get(operationScoreDTO.getSenderName());
    if (receiverId == null) {
        throw new BizException("获取接收者失败");
    }
    Short score = operationScoreDTO.getScore();
    String remark = operationScoreDTO.getRemark();

    OperationScoreEntity os = new OperationScoreEntity();
    os.setSendId(senderId);
    os.setReceiverId(receiverId);
    os.setEvent(score.toString());
    os.setRemark(remark);
    os.setSendTime(LocalDate.now());
    operationScoreRepository.save(os);

    String event = ScoreEventConstant.BONUS_SCORE;
    String module = ScoreModuleConstant.BONUS_SCORE;
    String payload = String.format("osId:%d;receiver:%d;remark:%s;score:%s",
            os.getId(), receiverId, remark, score);

    UserScoreEntity userScore = new UserScoreEntity();
    userScore.setUserId(receiverId);
    userScore.setModule(module);
    userScore.setEvent(event);
    userScore.setPayload(payload);
    userScore.setOperationTime(operationScoreDTO.getOperationTime());
    userScore.setScore(score);
    Integer totalScore = this.findTotalScoreByUserId(receiverId);
    userScore.setTotal(Math.max(totalScore + score, 0));

    userScoreRepository.save(userScore);
});

return Boolean.TRUE;

private Map<String, Integer> getUserIdsByUsernames(List<String> usernames) {
    List<FindOneByUsernameVo> userList = userServiceSdk.findByUsernames(usernames);
    return userList.stream()
        .collect(Collectors.toMap(FindOneByUsernameVo::getUsername, FindOneByUsernameVo::getTeclibUserId));
}

通过上述优化，你可以减少代码重复，提高代码的可读性和维护性，并且减少数据库交互次数，提高性能。

Iterator<yuanshishujuceshijieguo> allrecords = Listrecords.iterator(); while (allrecords.hasNext()) { yuanshishujuceshijieguo ps1 = new yuanshishujuceshijieguo(); ps1 = allrecords.next(); int id = ps1.getId(); int uid = ps1.getUid(); int pid = ps1.getPid(); double score = ps1.getScore(); System.out.println("用户id=：" + uid + " 商品id=: " + pid + " 评分=：" + score); //request.getSession().setAttribute("yuanshishujuceshijieguo", ps1); //ps1.setScore(Double.(score,4)); psArray.add(ps1); }

这段代码的作用是从Listrecords列表中遍历所有的记录，并将每条记录的id、uid、pid和score等属性值取出来，然后使用System.out.println()方法将这些属性值打印出来。同时，将每条记录存储在名为psArray的ArrayList类型的数组中。具体的遍历过程请参考下面的示例代码：

Iterator<yuanshishujuceshijieguo> allrecords = Listrecords.iterator();
while (allrecords.hasNext()) {
    yuanshishujuceshijieguo ps1 = allrecords.next();
    int id = ps1.getId();
    int uid = ps1.getUid();
    int pid = ps1.getPid();
    double score = ps1.getScore();
    System.out.println("用户id=：" + uid + " 商品id=: " + pid + " 评分=：" + score);
    psArray.add(ps1); // 将记录存储在数组中
}

在上面的代码中，每次迭代器调用next()方法时，都会返回Listrecords列表中的下一个元素，存储在名为ps1的变量中，然后可以使用ps1对象的各种方法来获取记录中的属性值。获取完属性值后，使用System.out.println()方法将记录的属性值打印出来，并将该记录存储在psArray数组中。