do I need to update or insert a list, set or map in testclass

Question

When ik create a list, set or map and add records to these do I need to update / insert after adding records. I did a test system debug on a testclass with only a return.

My debug returns a list with Ids, but I want to make shure I understand the concept

 public static List<Account> createAccounts(Integer numAccts) {        
        List<Account> accList = new List<Account>();

at this point isnt accList empty? as accList = null?

    for(Integer i=0;i<numAccts;i++) {
        Account a = new Account(Name='TestAccount' +'-'+ i);
        accList.add(a);
    }

Do I not need to update or insert accList as a dml action at this point or is the a difference with a class or testclass?

return accList;
}

Answer 1

Your code is correct, but you still need to run the DML action to insert the list of Accounts to the database, to be able then to perform other selection queries for testing purposes or if you are simply writing an Apex class.

To do that (in a test class), you need to run

Test.startTest();
insert accList;
Test.endTest();

after the for loop.

You may want to use the Database class to handle the database action results after running the dml operation in a test class.

Answer 2

Yes you need to create data so that your query can fetch them and perform the calculation in code.

Get Started with Apex Unit Tests

I suggest you to check this trailhead module to get better understanding of test class and apex testing.