Problem

As the kata is currently, it has the following sample unit test:

@Test
public void singleArg() {
    assertEquals(greetings.greetingForAllFriends("Bilal"), "Hello, Bilal!");
}

As the method signature is only defined for String[] and not String, the comparison will always fail.

Suggested Fix

@Test
public void singleArg() {
    assertEquals(greetings.greetingForAllFriends("Bilal"), new String[] { "Hello, Bilal!" });
}