Question

I’d like to throw an exception in a method that is expecting a list that only has one element, and I’d like to throw an exception if the list has more than one. I am trying to determine if there is an appropriate existing Java exception to throw for this case. I tried eyeballing the list here but didn’t see anything that jumped out as correct.

In this case I am calling a method of an object, with the expectation that the method is referencing said list as an attribute of the object. So I am actually not passing the array, nor an index as an argument.

(edited for clarification)

Answer 1

Since you are trying to imply that given input is not a valid one you should use IllegalArgumentException with a message stating the reason for the same.

throw new IllegalArgumentException("Input specified has more elements then expected");

Or else you can have your own Exception which uniquely defines the condition.

Though I somehow feel since its the number of elements which is the reason for exception you can even use IndexOutOfBoundsException which makes sense in the given scenario.

throw new IndexOutOfBoundsException("Expected size is 1, submitted input has size " + list.size());

Edit: As per comment

Given that list is not part of method call but belongs to the object whose method is invoked, in this case list.size() > 1 signifies that object is not in a correct state and with that a customized version of IllegalStateException or IllegalStateException itself with a proper error message would be more appropriate.