According to the Cambridge Advanced Learner's Dictionary ,apple and pineapple can be countable or uncountabe.Then are these sentences correct? Any differences in meaning?
1a. I want six slices of (the/an) apple.
1b. I want six slices of apple.
2a.I like eating apple/pineapple.
2b.I like eating apples/pineapples.
1a/b- both correct. The only difference is that in 1b you're not specifying which apple, just the desired fruit (apple not pear).
2a/b Correct again. I can't see much difference. I'd probably use the plural if I ate a lot.