euler 76 and 78 are very similar. i thought it would be a piece of cake. just have to use BigInteger instead of int
76

using BigInteger and adding one

this answer is correct
78

this returns 5997, and the System.out.println says 3000000
the correct answer is 55374
i tried changing
System.out.println(ways[j]);
to
System.out.println(ways[j].toString());
but get the same answer
i am guessing
if(ways[j].mod(limit) == BigInteger.ZERO)
is where the problem is, but not sure

I'd say you've nailed it. int is a primitive type and can be compared using ==. BigInteger is an object, for which the == operator compares references. You need to use the equals method instead.

if(ways[j].mod(limit).equals(BigInteger.ZERO))
hmmm...i thought that would fix it but i get the same answer. i tried this also
if(ways[j].remainder(limit).equals(BigInteger.ZERO))
same result

that difference is correct. #76 wants all combinations of 2 numbers that equal 100(up to 99 + 1).
#78 includes the case of all coins in one stack(100)
i might try it using long, but my guess is the number will be too big