Hutool — 39

If you are looking for "Hutool 39," you are likely looking for the modern, active version of the library (specifically versions like 5.8.11+ which require JDK 1.8+). Hutool is a popular Java utility library that simplifies JDK standard operations.

import cn.hutool.json.JSONUtil;
import cn.hutool.json.JSONObject;
import cn.hutool.json.JSONArray;
7. CollUtil.isEmpty(coll)
Null-safe emptiness check. hutool 39
21. DateUtil.between(date1, date2, DateUnit.DAY)
Calculates difference in days, hours, minutes, or seconds. If you are looking for "Hutool 39," you
Conclusion
Hutool 3.9 is a "just works" utility library. It does not chase trends but solves real Java frustrations with concise, well-tested code. If you are maintaining a Java 8 codebase and need a Swiss army knife for common tasks, Hutool 3.9 delivers without bloat. DateUnit.DAY)  
Calculates difference in days
Are There Downsides?
Yes – no library is perfect.