Java method contains another method that throws exception

Question

I have public method add(String) that calls private method inspectSequence(String) to check whether String valid or not.

This method returns array if passed String is valid, if it's invalid then method throws IllegalArgumentException

the code is next

public void add(String sequence) throws IllegalArgumentException{
  inspectSequence(sequence);
}

private int[] inspectSequence(String sequence){
  int[] array;
  //some actions
  if(<some condition>) throw new IllegalArgumentException("description");
  return array;
}

So in cases, when invalid String were passed to add method output will be next:

java.lang.IllegalArgumentException: description
at inspectSequence
at add

But I don't want user to know about private inspectSequence method because this is realization detail, right?

So what I can do in that case? Is it a good idea to throw unchecked exception here?

And is a good idea to throw exception inside inspectSequence method or I should return null when supplied String isn't valid and then check returned result in add method and depending on it throw or not throw exception?

Answer 1

But I don't want user to know about private inspectSequence method because this is realization detail, right?

I'd say no. It's true that you don't want the user (which in that context means someone calling the code) to "know" about internal methods like inspectSequence(). With "know" I mean be able to call, depend upon etc.

Knowing that the exception might be thrown and under what circumstances is something that a caller should know about, knowing where exactly it is thrown isn't necessary but wouldn't hurt.

Of course you could catch that exception in the calling method and throw another one but that would just lose information and might make the code harder to debug/maintain since the information where the input was not accepted would be lost to the caller.

So what I can do in that case? Is it a good idea to throw unchecked exception here?

That depends on whether that exception should be handled at runtime or be fixed.

Suppose the caller needs to know that the sequence was invalid and should handle that information appropriately, e.g. display some information to the enduser. In that case it would be better to throw a checked exception that describes that case.

On the other hand, if the input violates the contract of the method, i.e. the input sequence should never be invalid (or otherwise it's a programming error) then an IllegalArgumentException would be ok - situations like passing null to a method that doesn't expect null parameters.

And is a good idea to throw exception inside inspectSequence method or I should return null when supplied String isn't valid and then check returned result in add method and depending on it throw or not throw exception?

I'd say no. Returning null and then handling that in the calling method might be a reasonable way in some cases (e.g. if you have different callers that handle null differently) but your case is none of those. It would make code more complex and thus harder to read and maintain, especially since null might have multiple meanings which you'd have to define in that case.

Answer 2

You can catch IllegalArgumentException and throw your own.

public void add(String sequence) throws MyCustomException {
    try {
        inspectSequence(sequence);
    } catch (IllegalArgumentException e) {
        throw new MyCustomException("more readable cause that hides internals");
    }
}