I should also add with my edit here that there was no such thing as a prince the way we use it (the German word Prinz as opposed to the German word Fuerst, there's a difference) in English to describe the child of a king. Emperor Justin had no sons. He was lining it up so that Justinian would get the crown but it was never the law in the Roman Empire that anyone, even an emperor, could simply declare who would get the crown after them, although they could be crowned co-emperor during their lifetime which is exactly what happened when Justinain was co-emperor for a few months in 527 CE from April to August. Thus the word prince is a very strange one to refer to Justinian, unless you are using Machievelli's meaning of a prince.
I should also add with my edit here that there was no such thing as a prince the way we use it (the German word Prinz as opposed to the German word Fuerst, there's a difference) in English to describe the child of a king. Emperor Justin had no sons. He was lining it up so that Justinian would get the crown but it was never the law in the Roman Empire that anyone, even an emperor, could simply declare who would get the crown after them, although they could be crowned co-emperor during their lifetime which is exactly what happened when Justinain was co-emperor for a few months in 527 CE from April to August. Thus the word prince is a very strange one to refer to Justinian, unless you are using Machievelli's meaning of a prince.