How to use Annotation Type FromDataPoints class of org.junit.experimental.theories package

Your getScore() returns Double, not double. Therefore compiler is confused: Should it convert both arguments to Object, or if it should convert only the Double to double?

    double a = 2.0;
    Double b = 2.0;
    // assertEquals(a,b); // fails to compile
    // the compiler is confused whether to use
    assertEquals((Object) a,(Object) b); // OK
    // or
    assertEquals(a,(double) b); // OK

Anyway, I would set the method to return primitive type double.

Most testing frameworks (the xUnit family) are based on the JUnit framework. The Assert family of functions in JUnit have the (expected, actual) format; it became a convention, and most of the other frameworks followed that convention.

Some frameworks (like TestNG or NUnit 2.4+ for .NET) reversed that order (with the use of a constraint-based model for NUnit) to increase readability ("make sure that the actual value is 56" feels more natural than "make sure that 56 is the actual value").

The bottom line is: stick to the convention of your framework. If you use JUnit, put the expected value first. If you use TestNG, put the actual value first. You're right, that makes no difference in the test results when you accidentally reverse the arguments. But it makes a big difference in the default message you get from a failing test. When your reversed assertEquals(ShouldBeTrueButReturnsFalse(), true) in JUnit fails, the default message says "expected [false] but found [true]", where it should have said "expected [true] but found [false]". This is confusing, to say the least, and you shouldn't have to deal with a possible misdirection of that message.

Some of the unit tests in the Github link you provide don't follow the convention and have the same problem. Don't do that. Stick to the convention of your framework.

It can be quite useful sometimes to get the latest release - if for example you release often your own dependencies.

You can get the latest version like

compile "junit:junit:+"

or better specify at least the major version like

compile "junit:junit:4.+"

TL-DR

• write plain unit tests for components that you can straightly test without loading a Spring container (run them in local and in CI build).

• write partial integration tests/slicing unit test for components that you cannot straightly test without loading a Spring container such as components related to JPA, controllers, REST clients, JDBC ... (run them in local and in CI build)

• write some full integration tests (end-to-end tests) for some high-level components where it brings values (run them in CI build).

---

3 main ways to test a component

• plain unit test (doesn't load a Spring container)
• full integration test (load a Spring container with all configuration and beans)
• partial integration test/ test slicing (load a Spring container with very restricted configurations and beans)

Can all components be tested in these 3 ways ?

In a general way with Spring any component can be tested in integration tests and only some kinds of components are suitable to be tested unitary(without container).
But note that with or without spring, unitary and integration tests are not opposed but complementary.

How to determine if a component can be plain tested (without spring) or only tested with Spring?

You recognize a code to test that doesn't have any dependencies from a Spring container as the component/method doesn't use Spring feature to perform its logical.
Take that FooService class :

@Service
public class FooService{

   private FooRepository fooRepository;
   
   public FooService(FooRepository fooRepository){
       this.fooRepository = fooRepository;
   }

   public long compute(...){
      List<Foo> foos = fooRepository.findAll(...);
       // core logic
      long result = 
           foos.stream()
               .map(Foo::getValue)
               .filter(v->...)
               .count();
       return result;
   }
}

FooService performs some computations and logic that don't need Spring to be executed.
Indeed with or without container the compute() method contains the core logic we want to assert.
Reversely you will have difficulties to test FooRepository without Spring as Spring Boot configures for you the datasource, the JPA context, and instrument your FooRepository interface to provide to it a default implementation and multiple other things.
Same thing for testing a controller (rest or MVC).
How could a controller be bound to an endpoint without Spring? How could the controller parse the HTTP request and generate an HTTP response without Spring? It simply cannot be done.

1)Writing a plain unit test

Using Spring Boot in your application doesn't mean that you need to load the Spring container for any test class you run.
As you write a test that doesn't need any dependencies from the Spring container, you don't have to use/load Spring in the test class.
Instead of using Spring you will instantiate yourself the class to test and if needed use a mock library to isolate the instance under test from its dependencies.
That is the way to follow because it is fast and favors the isolation of the tested component.
Here how to unit-test the FooService class presented above.
You just need to mock FooRepository to be able to test the logic of FooService.
With JUnit 5 and Mockito the test class could look like :

import org.mockito.junit.jupiter.MockitoExtension;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.junit.jupiter.api.extension.ExtendWith;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.BeforeEach;


@ExtendWith(MockitoExtension.class)
class FooServiceTest{

    FooService fooService;  

    @Mock
    FooRepository fooRepository;

    @BeforeEach 
    void init{
        fooService = new FooService(fooRepository);
    }

    @Test
    void compute(){
        List<Foo> fooData = ...;
        Mockito.when(fooRepository.findAll(...))
               .thenReturn(fooData);
        long actualResult = fooService.compute(...);
        long expectedResult = ...;
        Assertions.assertEquals(expectedResult, actualResult);
    }

}

2)Writing a full integration test

Writing an end-to-end test requires to load a container with the whole configuration and beans of the application.
To achieve that @SpringBootTest is the way :

The annotation works by creating the ApplicationContext used in your tests through SpringApplication

You can use it in this way to test it without any mock :

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.junit.jupiter.api.Test;

@SpringBootTest
public class FooTest {

   @Autowired
   Foo foo;

   @Test
   public void doThat(){
      FooBar fooBar = foo.doThat(...);
      // assertion...
   }    
   
}

But you can also mock some beans of the container if it makes sense :

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.mock.mockito.MockBean;
import org.springframework.boot.test.context.SpringBootTest;
import org.junit.jupiter.api.Test;
import org.mockito.Mockito;

@SpringBootTest
public class FooTest {

   @Autowired
   Foo foo;

   @MockBean
   private Bar barDep;

   @Test
   public void doThat(){
      Mockito.when(barDep.doThis()).thenReturn(...);
      FooBar fooBar = foo.doThat(...);
      // assertion...
   }    
   
}

Note the difference for mocking as you want to mock a plain instance of a Bar class (org.mockito.Mock annotation)and that you want to mock a Bar bean of the Spring context (org.springframework.boot.test.mock.mockito.MockBean annotation).

Full integration tests have to be executed by the CI builds

Loading a full spring context takes time. So you should be cautious with @SpringBootTest as this may make unit tests execution to be very long and generally you don't want to strongly slow down the local build on the developer's machine and the test feedback that matters to make the test writing pleasant and efficient for developers.
That's why "slow" tests are generally not executed on the developer's machines.
So you should make them integration tests (IT suffix instead of Test suffix in the naming of the test class) and make sure that these are executed only in the continuous integration builds.
But as Spring Boot acts on many things in your application (rest controllers, MVC controllers, JSON serialization/deserialization, persistence, and so for...) you could write many unit tests that are only executed on the CI builds and that is not fine either.
Having end-to-end tests executed only on the CI builds is ok but having also persistence, controllers or JSON tests executed only on the CI builds is not ok at all.
Indeed, the developer build will be fast but as drawback the tests execution in local will detect only a small part of the possible regressions...
To prevent this caveat, Spring Boot provides an intermediary way : partial integration test or the slice testing (as they call it) : the next point.

3)Writing a partial integration test focusing on a specific layer or concern thanks to slice testing

As explained in the point "Recognizing a test that can be plain tested (without spring))", some components can be tested only with a running container.
But why using @SpringBootTest that loads all beans and configurations of your application while you would need to load only a few specific configuration classes and beans to test these components?
For example why loading a full Spring JPA context (beans, configurations, in memory database, and so forth) to test the controller part?
And reversely why loading all configurations and beans associated to Spring controllers to test the JPA repository part?
Spring Boot addresses this point with the slice testing feature.
These are not as much as fast than plain unit tests (that is without container) but these are really much faster than loading a whole spring context. So executing them on the local machine is generally very acceptable.
Each slice testing flavor loads a very restricted set of auto-configuration classes that you can modify if needed according to your requirements.

Some common slice testing features :

• Auto-configured JSON Tests : @JsonTest

To test that object JSON serialization and deserialization is working as expected, you can use the @JsonTest annotation.

• Auto-configured Spring MVC Tests : @WebMvcTest

To test whether Spring MVC controllers are working as expected, use the @WebMvcTest annotation.

• Auto-configured Spring WebFlux Tests : @WebFluxTest

To test that Spring WebFlux controllers are working as expected, you can use the @WebFluxTest annotation.

• Auto-configured Data JPA Tests : @DataJpaTest

You can use the @DataJpaTest annotation to test JPA applications.

And you have still many other slice flavors that Spring Boot provides to you.
See the testing part of the documentation to get more details.
Note that if you need to define a specific set of beans to load that the built-in test slice annotations don't address, you can also create your own test slice annotation(https://spring.io/blog/2016/08/30/custom-test-slice-with-spring-boot-1-4).

4)Writing a partial integration test focusing on specific beans thanks to lazy bean initialization

Some days ago, I have encountered a case where I would test in partial integration a service bean that depends on several beans that themselves also depend on other beans. My problem was that two deep dependency beans have to be mocked for usual reasons (http requests and a query with large data in database).
Loading all the Spring Boot context looked an overhead, so I tried to load only specific beans. To achieve that, I annotation the test class with @SpringBootTest and I specified the classes attribute to define the configuration/beans classes to load.
After many tries I have gotten something that seemed working but I had to define an important list of beans/configurations to include.
That was really not neat nor maintainable.
So as clearer alternative, I chose to use the lazy bean initialization feature provided by Spring Boot 2.2 :

@SpringBootTest(properties="spring.main.lazy-initialization=true")
public class MyServiceTest { ...}

That has the advantage to load only beans used at runtime.
I don't think at all that using that property has to be the norm in test classes but in some specific test cases, that appears the right way.

The official junit solution to assert that two BigDecimal are matematically equal is to use hamcrest.

With java-hamcrest 2.0.0.0 we can use this syntax:

    // import static org.hamcrest.MatcherAssert.assertThat;
    // import org.hamcrest.Matchers;

    BigDecimal a = new BigDecimal("100")
    BigDecimal b = new BigDecimal("100.00")
    assertThat(a,  Matchers.comparesEqualTo(b));

Hamcrest 1.3 Quick Reference

new File(path).toURI().toURL();

Problem

I am afraid the test you have written is incorrect.

The main requirement is to share the same StringBuilder instance between different threads. Whereas you are creating a StringBuilder object for each thread.

The problem is that a new Threadsafe() initialises a new StringBuilder():

class Threadsafe {
    ...
    StringBuilder sb = new StringBuilder(str);
    ...
}
class MyThread1 implements Runnable {
    Threadsafe sf = new Threadsafe();
    ...
}
class MyThread2 implements Runnable {
    Threadsafe sf = new Threadsafe();
    ...
}

Explanation

To prove the StringBuilder class is not thread-safe, you need to write a test where n threads (n > 1) append some stuff to the same instance simultaneously.

Being aware of the size of all the stuff you are going to append, you will be able to compare this value with the result of builder.toString().length():

final long SIZE = 1000;         // max stream size

final StringBuilder builder = Stream
        .generate(() -> "a")    // generate an infinite stream of "a"
        .limit(SIZE)            // make it finite
        .parallel()             // make it parallel
        .reduce(new StringBuilder(), StringBuilder::append, (b1, b2) -> b1);
                                // put each element in the builder

Assert.assertEquals(SIZE, builder.toString().length());

Since it is actually not thread-safe, you may have trouble getting the result.

An ArrayIndexOutOfBoundsException may be thrown because of the char[] AbstractStringBuilder#value array and the allocation mechanism which was not designed for multithreading use.

Test

Here is my JUnit 5 test which covers both StringBuilder and StringBuffer:

public class AbstractStringBuilderTest {

    @RepeatedTest(10000)
    public void testStringBuilder() {
        testAbstractStringBuilder(new StringBuilder(), StringBuilder::append);
    }

    @RepeatedTest(10000)
    public void testStringBuffer() {
        testAbstractStringBuilder(new StringBuffer(), StringBuffer::append);
    }

    private <T extends CharSequence> void testAbstractStringBuilder(T builder, BiFunction<T, ? super String, T> accumulator) {
        final long SIZE = 1000;
        final Supplier<String> GENERATOR = () -> "a";

        final CharSequence sequence = Stream
                .generate(GENERATOR)
                .parallel()
                .limit(SIZE)
                .reduce(builder, accumulator, (b1, b2) -> b1);

         Assertions.assertEquals(
                SIZE * GENERATOR.get().length(),    // expected
                sequence.toString().length()        // actual
         );
    }

}

Results

AbstractStringBuilderTest.testStringBuilder: 
    10000 total, 165 error, 5988 failed, 3847 passed.

AbstractStringBuilderTest.testStringBuffer:
    10000 total, 10000 passed.

Use @BeforeEach instead of @Before and @AfterEach instead of @After.

First - I have to direct you to http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html -- she does an amazing job.

The basic idea is that you use

<T extends SomeClass>

when the actual parameter can be SomeClass or any subtype of it.

In your example,

Map<String, Class<? extends Serializable>> expected = null;
Map<String, Class<java.util.Date>> result = null;
assertThat(result, is(expected));

You're saying that expected can contain Class objects that represent any class that implements Serializable. Your result map says it can only hold Date class objects.

When you pass in result, you're setting T to exactly Map of String to Date class objects, which doesn't match Map of String to anything that's Serializable.

One thing to check -- are you sure you want Class<Date> and not Date? A map of String to Class<Date> doesn't sound terribly useful in general (all it can hold is Date.class as values rather than instances of Date)

As for genericizing assertThat, the idea is that the method can ensure that a Matcher that fits the result type is passed in.