wow!

f.trim(); would also work in this case - I find it easier to read than substring, though substring is more efficient probably.

I don't get why there are three spaces in the assertion of the "hello world" part of the basic test - given the description I would much rather expect two.

//Modified copy of the "basic" unit test from the task.
public class SpacifyTest {
@Test
public void basicTest() {
assertEquals("h e l l o| | | |w o r l d",Spacify.spacify("hello world")); //three | | on the left side of the assertion?!
assertEquals("1 2 3 4 5",Spacify.spacify("12345"));
}
}