I think both mean the same thing. Basically, those words mirror each other.
What they have in common is "heur", etymologically meaning "destiny/chance/hasard/luck".
"bon-heur" and "mal-heur" match each other, as good-hasard or bad-hasard would match.
So, "heur-eux" is derived from the same concept.
Yes, thanks Sitesurf. The reason why I asked is because in English 'joy' and 'happiness' seem to be synonyms. Various online dictionaries seem to agree. For example, both Macmillan and Oxford define 'joy' as 'a feeling of great happiness'. http://www.macmillandictionary.com/dictionary/british/joy And Merriam-Webster defines 'joy' as: 'a state of happiness' http://www.merriam-webster.com/dictionary/joy Also: Collins Thesaurus (among several others) gives 'joy' as synonym for 'happiness': http://www.collinsdictionary.com/dictionary/english-thesaurus/happiness
In French, "la joie" is generally a lighthearted, temporary emotion, unless it is used in a religious sense (therefore rarer).
"le bonheur" is a deeper and lasting feeling.
That is the reason why Duo wanted to stress the difference by not accepting "la joie" as a synonymous.
Synonymous means similar, not "the same". There are nuances of difference between words listed as synonyms, and the context is important in understanding which to use. "Joy" is a feeling of extreme elevation, extreme happiness, profound through it may be, and is usually short lived, it simply cannot be sustained; while the word "happiness" connotes a more lasting state of being--more than mere contentment, it is deep and satisfying--and will not exhaust you as joy would.
"A sentiment of happiness" says the app ... when would we even say that English? A feeling of happiness, a happy sentiment refering to a gesture, a sentiment of goodwill or good fortune are expressions used. But a sentiment of happiness is awkward sounding because you're either happy or you aren't. IMO