What are test interfaces and default methods in JUnit 6?

// Define a reusable contract test as an interface
public interface ComparableContract<T extends Comparable<T>> {

    T createEqualValue();
    T createSmallerValue();
    T createLargerValue();

    @Test
    default void reflexive() {
        T val = createEqualValue();
        assertEquals(0, val.compareTo(createEqualValue()));
    }

    @Test
    default void symmetricForEqual() {
        T a = createEqualValue();
        T b = createEqualValue();
        assertEquals(0, a.compareTo(b));
        assertEquals(0, b.compareTo(a));
    }

    @Test
    default void smallerIsLessThanLarger() {
        assertTrue(createSmallerValue().compareTo(createLargerValue()) < 0);
    }
}

// Implementing classes get all contract tests for free:
class StringComparableTest implements ComparableContract<String> {
    @Override public String createEqualValue()   { return "hello"; }
    @Override public String createSmallerValue() { return "apple"; }
    @Override public String createLargerValue()  { return "zebra"; }
    // Also free to add class-specific @Test methods
    @Test void stringsAreCaseSensitive() {
        assertNotEquals(0, "Hello".compareTo("hello"));
    }
}

// Any class implementing ComparableContract gets all 3 contract tests.