Why JUnit 5 default access modifier changed to package-private

This is JUnit 5 feature which produce a better encapsulation for test classes and methods

Make Jupiter tests package private #679

Test class mostly located in the same package of the class tested:

better way is to place the tests in a separate parallel directory structure with package alignment.

main/                          test/
   com/                           com/
      xyz/                           xyz/
         📜 SomeClass.java              🔨 SomeClassTests.java

This approach allows test code to access all the public and package visible members of the classes under test.

Why is the default access modifier in JUnit 5 package-private?

It's not the "default". There technically is no default. Rather, in JUnit Jupiter you have a choice: public, protected or package-private.

What is the benefit of changing it to package-private?

The benefit is that you don't have type public anymore. If your IDE automatically generates test methods and test classes for you that are public, feel free to leave them public.

But... if you are typing in the methods on your own, then just leave off public unless you are designing your test classes for subclassing from other packages, in which case you'd want to make your overrideable test methods either public or protected. And of course, interface default methods must be public.

Long story, short: we (the JUnit 5 team) believe in the principle "Less is more", meaning the less you have to type to achieve your goal, the better!