Proper Unit Test Structure in Apex

ExampleController

public class ExampleController
   
   public Integer differenceInFooWidgetsAndStubCount(Id fooId, Id stubId){
      Foo__c foo = [
         SELECT
            Id,
            (SELECT Id FROM Widget__r)
         FROM
            Foo__c
         WHERE
            Id = :fooId
      ];
      
      Stub__c stub = [
         SELECT
            Id, Count_One__c, Count_Two__c, Use_Count_One__c
         FROM
            Stub__c
         WHERE
            Id = :stubId
      ];
      
      if(stub.Use_Count_One__c){
         return foo.Widget__r.size() - stub.Count_One__c;
      }else{
         return foo.Widget__r.size() - stub.Count_Two__c;
      }
   }
}

ExampleControllerTest

@isTest
public class ExampleControllerTest{
   static testMethod void testDifferenceInFooWidgetsAndStubCountOne(){
      Map<String, List<sObject>> objectMapping = TestUtilities.generateTestModel(true);
      Foo__c foo = objectMapping.get('Foo__c').get(0);
      Stub__c stub = objectMapping.get('Stub__c').get(0);
   
      Test.startTest();
      ExampleController controller = new ExampleController();
      Integer difference = controller.differenceInFooWidgetsAndStubCount(foo.Id, stub.Id);
      Test.stopTest();
      
      System.assertEquals(2, difference, 'The difference should be 2 when Count_One__c is used');
   }
   
   static testMethod void testDifferenceInFooWidgetsAndStubCountTwo(){
      Map<String, List<sObject>> objectMapping = TestUtilities.generateTestModel(false);
      Foo__c foo = objectMapping.get('Foo__c').get(0);
      Stub__c stub = objectMapping.get('Stub__c').get(0);
   
      Test.startTest();
      ExampleController controller = new ExampleController();
      Integer difference = controller.differenceInFooWidgetsAndStubCount(foo.Id, stub.Id);
      Test.stopTest();
      
      System.assertEquals(1, difference, 'The difference should be 1 when Count_Two__c is used');
   }
}

TestUtilities

@isTest
public class TestUtilities{
   public static Foo__c generateFoos(Integer count){
      List<Foo__c> foos = new List<Foo__c>();
      for(Integer i = 0; i<count; i++){
         foos.add(new Foo());
      }
      return foos;
   }
   
   public static List<Widget__c> generateWidgets(List<Foo__c> foos, Integer count){
      List<Widget__c> widgets = new List<Widget__c>();
      for(Foo__c foo:foos){
         for(Integer i = 0; i<count; i++){
            widgets.add(new Widget(
               Foo__c = foo.Id,
               Foo__r = foo
            ));
         }
      }
      return widgets;
   }
   
   public static List<Stub__c> generateStubs(Boolean useCountOne, Integer count){
      List<Stub__c> stubs = new List<Stub__c>();
      for(Integer i = 0; i<count; i++){
         stubs.add(new Stub__c(
            Count_One__c = 1,
            Count_Two__c = 2,
            Use_Count_One__c = useCountOne
         ));
      }
      return stubs;
   }
   
   public static Map<String, List<sObject>> generateTestModel(Boolean useCountOne){
      List<sObject> objectsToAdd = new List<sObject>();
      List<Foo__c> foos = generateFoos(1);
      List<Stub__c> stubs = generateStubs(useCountOne, 1);
      objectsToAdd.add(foos);
      objectsToAdd.add(stubs);
      insert objectsToAdd;
      objectsToAdd.clear();
      
      List<Widget__c> widgets = generateWidgets(foos.get(0), 3);
      objectsToAdd.add(widgets);
      insert objectsToAdd;
      objectsToAdd.clear();
      
      Map<String, List<sObject>> objectMapping = new Map<String, List<sObject>>();
      objectMapping.put('Foo__c', foos);
      objectMapping.put('Stub__c', stubs);
      objectMapping.put('Widget__c', widgets);
      return objectMapping;
   }
}

This code is from my blog article Proper Unit Test Structure in Apex