I was wondering if in a Unit Test, is it better to instantiate Objects like Sets, Maps, LinkedLists or Arrays as a field of the Class and then in the @Before method that runs before each @Test, to clear them? OR is it better to simply instantiate the Objects in the @Before method but have them declare as a field of a class?

#1 e.g. Clearing/Emptying data structure and re-using

public class MyUnitTestExample1 {
   
   // Declare variable as field of class/member variable
   private Set<String> set = new HashSet<>();
   private Map<String, Integer> map = new HashMap<>();

   @Before
    public void setUp() {

       // Data structure should be cleared before each test, but only instantiated 1 time?
       set.clear();
       map.clear();
    }

#2 e.g. Instantiating new Object every test but declaring as field of Class such that it is accessible by every @Test in the Class

public class MyUnitTestExample2 {

   // Declare variable as field of class/member variable
   private Set<String> set;
   private Map<String, Integer> map;

   @Before
    public void setUp() {

    // Instantiate here
       set = new HashSet<>();
       map = new HashMap<>();
    }

... is one better than the other? More efficient? Faster? I would assume clearing the contents of the data structures and instantiating 1 time (or so I suppose it does this) is faster?

CodePudding user response：

1. I dont think performance matters for such cases. The clearing or new creation of a HashMap/Set and clearing the old object via garbage collection should both take minimal time. Even with thousands of entries it takes much longer to fill the map/set than clearing it.

2. What you should be more concerned about is using variables in a unit test class that are "global" for the unit tests of that class. I do not see the benefit. I would use a new set/map in each unit test to separate the test inside the class for another. Otherwise you could have sideeffects depending on the order of execution of the test. I had weird scenarios in the past where tests produced different results because the developer was running on Windows and the CI server was running on Linux.

CodePudding user response：

Clearing instance variables in @BeforeEach makes no sense - JUnit instantiates a new instance of your test class for each test.

public class MyUnitTestExample1 {

    private Set<String> set = new HashSet<>();

    @Test
    void test1() {
        System.out.println(this);
        System.out.println(System.identityHashCode(set));
    }

    @Test
    void test2() {
        System.out.println(this);
        System.out.println(System.identityHashCode(set));
    }
}

Output

com.sandbox.MyUnitTestExample1@3857f613
428566321
com.sandbox.MyUnitTestExample1@fa4c865
1004023029