Unless Word#getNoun returns a String, you will be in the realms of StringBuilder#append(Object) which calls the toString method or adds “null” for a null.
What does your Word class represent? Is it a word, or a phrase?
Can you put all the words into your StringBuilder as they are, then iterate the StringBuilder looking for spaces, and capitalise the first char following. The Character class probably has a method to convert to upper case. Beware of ß in German which turns into two letters in upper case: SS.