You have provided an item of data that wasn't of the type expected, and Java isn't aware of a way to convert between them (although there might be). If you are being told what looks like a char is a String, then see "<char>" Vs '<char>'.
String foo = new String("bar"); boolean bob = foo.startsWith(5); The startsWith method needs a String, not an int Code that would work: boolean bob = foo.startsWith("5"); or boolean bob = foo.startsWith("b");
Check any parameters you're passing are of the right type. For example, the method doStuff(short foo) wouldn't work if you passed a String.