The expression "It's a matter of sad joy" illustrates an oxymoron.
An oxymoron is a figure of speech that combines two contradictory or opposite terms in a single phrase or expression. In this case, the words "sad" and "joy" are two antonyms or opposite terms that are juxtaposed in the same sentence, creating a contradictory effect.
The phrase implies that there are two conflicting emotions present at the same time. The word "joy" suggests happiness or delight, while the word "sad" suggests a feeling of sorrow or grief. It is possible to experience both emotions simultaneously, and this phrase acknowledges that complexity.
Therefore, the phrase "It's a matter of sad joy" is an example of an oxymoron, which highlights the contradictions between opposing concepts, in this case, sadness and joy.