How can we test package-private class?

Question

I am reading book Effective Java, in Item 13: Minimize the accessibility of classes and members, it mentioned that:

To facilitate testing, you may be tempted to make a class, interface, or member more accessible. This is fine up to a point. it is acceptable to make a private member of a public class package-private in order to test it, but it is not acceptable to raise the accessibility any higher than that. In other words, it is not acceptable to make a class, interface, or member a part of packages's exported API to facilitate testing.

I know we should encapsulate members, hiding information from clients, we can test them by accessing the class with setters and getters, but How should I understand make class package-private accessible, if so How to test it?

Answer 1

Basically it means that your tester class should reside in the same package as your tested class. That would make your tested class and all package protected members and methods accessible to your testing class. The classes may be located under different roots: your tested class may be under src/main/java/myrootpackage/first/second/MyClass.java and your testing class may be located under src/test/java/myrootpackage/first/second/MyClassTester.java.

Answer 2

I guess what the author means is that instead of

package my.package;

class TestableClass {
     private String property;
}

You can change the visibility to

package my.package;

class TestableClass {
    String property;
}

With the second implementation you can access the property from a test in the package my.package like

package my.package;

class TestableClassTest {
    // ...
}

At the same time, a subclass cannot access the property - that's the benefit comapred to protected.