What is the purpose of this PR

• This PR fixes the error resulting from the flaky test: com.insightfullogic.java8.examples.chapter3.RefactorTest#allStringJoins
• The mentioned test may fail or pass without changes made to the source code when it is run in different JVMs due to HashSet's non-deterministic iteration order.
• The NonDex tool detects possible issues with "under-determined Java APIs." This means that this problem may come up if in the future someone tries to run the code in a different environment (JVM version, operating system, etc.). Fixing the test now will prevent developers from spending time on this in the future.

Why the tests fail

• This test fails because the test requests a set of strings returned from code in the Refactor class. This code returns a HashSet containing the string data. The test compares this result to a hard-coded string value. The problem is that the order of iteration is not guaranteed for HashSets. So, the test may fail as the given order can be different from the expected order.

Reproduce the test failure

• Run the tests with NonDex maven plugin. The commands to recreate the flaky test failures are:
  mvn edu.illinois:nondex-maven-plugin:2.1.1:nondex -Dtest=com.insightfullogic.java8.examples.chapter3.RefactorTest#allStringJoins

• To run the test without NonDex:
  mvn test -Dtest=com.insightfullogic.java8.examples.chapter3.RefactorTest#allStringJoins

Expected results

• This test should run successfully when run with NonDex.

Actual Result

• We get the following failure:

-------------------------------------------------------------------------------
Test set: com.insightfullogic.java8.examples.chapter3.RefactorTest
-------------------------------------------------------------------------------
Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.029 sec <<< FAILURE! - in com.insightfullogic.java8.examples.chapter3.RefactorTest
allStringJoins(com.insightfullogic.java8.examples.chapter3.RefactorTest)  Time elapsed: 0.028 sec  <<< FAILURE!
org.junit.ComparisonFailure: expected:<[[Acknowledgement, Resolution]]> but was:<[[Resolution, Acknowledgement]]>
	at org.junit.Assert.assertEquals(Assert.java:115)
	at org.junit.Assert.assertEquals(Assert.java:144)
	at com.insightfullogic.java8.examples.chapter3.RefactorTest.lambda$allStringJoins$0(RefactorTest.java:38)

Description of Fix

• I added some code to convert the HashSet into a list and then used Collections.sort() to sort it before comparing to the hard-coded string value.