Improve code quality and add test cases

[Kb-Tay]

No description provided.

[codecov]

Codecov Report

Merging #271 (018b906) into master (1477bcd) will increase coverage by 0.09%.
The diff coverage is 87.50%.

@@             Coverage Diff              @@
##             master     #271      +/-   ##
============================================
+ Coverage     75.58%   75.67%   +0.09%     
- Complexity      680      684       +4     
============================================
  Files           103      103              
  Lines          2183     2187       +4     
  Branches        223      223              
============================================
+ Hits           1650     1655       +5     
+ Misses          469      468       -1     
  Partials         64       64              

Files	Coverage Δ
...a/seedu/address/logic/commands/CompleteByDate.java	100.00% <100.00%> (ø)
.../seedu/address/logic/commands/CompleteByIndex.java	90.00% <100.00%> (ø)
.../seedu/address/logic/commands/CompleteCommand.java	100.00% <ø> (ø)
src/main/java/seedu/address/model/AddressBook.java	85.29% <100.00%> (+0.91%)	⬆️
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	100.00% <100.00%> (ø)
...a/seedu/address/model/person/UniquePersonList.java	90.47% <100.00%> (+0.47%)	⬆️
.../seedu/address/logic/commands/ScheduleCommand.java	59.37% <62.50%> (-1.92%)	⬇️

... and 1 file with indirect coverage changes

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!

[sopa301]

LGTM