Why does the Java8 Stream GroupingBy return empty

Question

 @Transactional(isolation = Isolation.READ_UNCOMMITTED
            , propagation = Propagation.REQUIRED, rollbackFor = Exception.class)
    public void insertPhoneAndArea(List<RecordEntity> recordEntityList) {
        int tryCount = 0;
        OffsetTime startTime = OffsetTime.ofInstant(Instant.now(), ZoneId.systemDefault());
        Map<String, List<RecordEntity>> selfGroup = recordEntityList.stream().collect(Collectors.groupingBy(RecordEntity::getSelfNum));
        while (0==selfGroup.size()) {
            //There has a bug with Jdk Stream to groupBy in One elements ,So have a loop try
            selfGroup = recordEntityList.stream().collect(Collectors.groupingBy(RecordEntity::getSelfNum));
            tryCount++;
            if (tryCount > 20000) {
                break;
            }
            log.info("try to get SelfNum with Stream bug ! count in {} ",tryCount);
        }
}

Steps:

1. Invoking the method with name insertPhoneAndArea
2. Given the same data with variables, with "recordEntityList" in there, and it was not empty
3. When I executed the method, the selfGroup was empty and it's size was zero. So I feel strange with the result. It shouldn't be empty because the recordEntityList was not empty.
4. I turned on the Debug Mode with IDEA and I was using Alt+LeftClick to execute the code and the result was correct, it wasn't empty.
5. So I wrote a try count in the group code, as expected, that code in the loop was approximately random up to 30, and it was a correct result and it was out of the while loop.

I don't know what the bug is.

@Data
public class RecordEntity {

    private int id;
    /**
     * 通话时间
     */
    private Date recordTime;
    /**
     * 持续时长
     */
    private Integer duration;
    /**
     * 通话方式/ 1电话主叫、2电话被叫、3短信主叫、4短信被叫
     */
    private Integer recordMode;
    /**
     * 对方电话号码
     */
    private String dialNum;
    /**
     * 通话地点
     */
    private String recordAddress;
    /**
     * 通话类型/ 1本地、2异地
     */
    private Integer recordType;
    /**
     * 小区编号
     */
    private String area;
    /**
     * 基站号
     */
    private String baseStation;
    /**
     * 交换机号
     */
    private String exchange;
    /**
     * 本机号码
     */
    private String selfNum;
    /**
     * 机主姓名
     */
    private String username;
    /**
     * 机主身份证
     */
    private String idNum;
    /**
     * 通话记录id(唯一)
     */
    private String recordId;
    /**
     * 话单运营商
     */
    private Integer operator;
    /**
     * 对方号码类型（1-手机，2-固话，3-服务号）
     */
    private Integer dialNumType;
}

Answer 1

If Map<String, List<RecordEntity>> selfGroup = recordEntityList.stream().collect(Collectors.groupingBy(RecordEntity::getSelfNum)); sets selfGroup to an empty map, it must be because recordEntityList is empty.

Why the caller is passing an empty list to your method I obviously cannot tell.

PS You don’t need the Yoda condition, 0==selfGroup.size(). I’d recommend selfGroup.isEmpty().

Answer 2

any chance that selfNum is null? you can't group by null, actually, you'll get an exception. if not, try using phrase in another locale.