Add test class JoinerTest.java

[barret-yzh]

Fixes #1328
Add test class JoinerTest.java

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.71%. Comparing base (1e81da1) to head (9584902).
⚠️ Report is 20 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #1590      +/-   ##
=============================================
- Coverage      34.33%   31.71%   -2.63%     
- Complexity       849      875      +26     
=============================================
  Files            262      300      +38     
  Lines           5953     6666     +713     
  Branches         561      647      +86     
=============================================
+ Hits            2044     2114      +70     
- Misses          3717     4355     +638     
- Partials         192      197       +5     

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