Sweetness is a basic taste most commonly perceived when eating foods rich in certain substances (usually carbohydrates) called sugars. Simple sugars (monosaccharides) include glucose, fructose, and galactose. Compound sugars (disaccharides) are molecules made of two bonded monosaccharides; common examples are sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (two molecules of glucose). Chains of monosaccharides longer than two are not regarded as sugars and are called oligosaccharides or polysaccharides.
Coffee is often described in terms of sweetness and is common to see tasters describe a brew as “sweet” during Cupping, but it’s still not fully understood what chemical compound exactly we are detecting as sweet. Coffee beans do contain sucrose, but the roasting process degrades about 97% of it. The amount of sucrose that ends up in the brew is so low that it’s below the sensory threshold concentration, meaning it is impossible for us to perceive it.
There are some carbohydrates - arabinogalactans - that are usually present in the brew in high enough concentrations and maybe can be perceived as sweet, but their influence in the brew sweetness is still an hypothesis.
It’s also possible that the sweet flavor we sense actually comes from sweet aromas instead of from compounds on our taste buds.