What is the difference between the two literary devices an 'oxymoron' and a 'juxtaposition' and why are they used by writers?

Both devices display a similarity in the way that authors use them to emphasize a particular image or heighten an emotion. The difference lies in the fact that, with oxymoronic expressions, the contrast is created using specific words that generate a contradiction. Juxtapositions, on the other hand, are when larger images contradict each other. An example of an oxymoron could be the term a "cruel kindness" as each word connotes a different image. In literature an example could be the "marriage hearse" at the end of the poem 'London' by William Blake in which Blake uses this particular oxymoron to highlight ingrained corruption and hypocrisy in 17th century London. The device of a juxtaposition is more general in the way that it is a certain image surrounding characters, places or themes. An example of this can be seen in William Golding's 'Lord of the Flies' in which the main characters of Ralph and Jack clearly juxtapose each other as Ralph advocates for order and democracy whereas Jack chooses to embrace primatal human behaviors centered around chaos.