doc: improve readme for java version and example

[JacksonTian]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.06%. Comparing base (3360475) to head (2390908).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #923      +/-   ##
============================================
- Coverage     90.07%   90.06%   -0.02%     
+ Complexity     1650     1649       -1     
============================================
  Files           123      123              
  Lines          4767     4749      -18     
  Branches        537      537              
============================================
- Hits           4294     4277      -17     
+ Misses          347      346       -1     
  Partials        126      126              

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