Exceptions in method declaration in java

Question

I have the following method to implement for an ArrayList, but I'm not sure how to go about treating the exception. If the ArrayList is empty does it automatically throw that exception or do I need to write something in the method?

public T removeLast() throws EmptyCollectionException
{
    //TODO: Implement this.
}

ControlAltDel · Answer 1 · 2017-04-21T19:49:12.833

You haven't filled in the method, so we can't say for sure. If you used ArrayList.remove(0); on an empty list, it would give you a IndexOutOfBoundsException

In any case, it's never going to throw your custom exception: You need to throw this yourself. You can do this at the top of the method, like

public T removeLast() throws EmptyCollectionException
{
  if (myList.size() == 0) throw new EmptyCollectionException("List Is Empty");
  ... //otherwise...
}

score 2 · Accepted Answer · answered Apr 21 '17 at 19:42

2

An exception is an object that extends throwable. You would write that yourself

if(list.isEmpty()){
    throw new EmptyCollectionException("possibly a message here");
} else {
    //your logic here to return a T
}

answered Apr 21 '17 at 19:42

Owen

36
2

Correct, but the `else` is silly ;-) See ControlAltDel answer for a better alternative. – Paulo Mattos Apr 21 '17 at 19:47

score 0 · Answer 3 · answered Apr 21 '17 at 19:41

0

You need to throw the exception yourself if the ArrayList is empty.

The throws EmptyCollectionExceptio clause in the method signature is only a reminder, for the calling code, that removeLast() might throw an exception (and should be properly handled).

answered Apr 21 '17 at 19:41

Paulo Mattos

18,845
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Cardinal System · Answer 4 · 2017-04-21T19:51:45.550

0

EmptyCollectionException is not an existing exception. Define a new exception:

if(list.IsEmpty()){
throw new EmptyCollectionException("message");
}

or use IndexOutOfBoundsException instead, you can also use a try catch block:

try{

   //Whatever could cause the exception

}catch(IndexOutOfBoundsException e){
   //Handle the exception
   //e.printStackTrace();
}

edited Apr 21 '17 at 19:51

answered Apr 21 '17 at 19:41

Cardinal System

2,749
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1

Find me EmptyCollectionException in the JDK. How is it going to throw it? http://docs.oracle.com/javase/7/docs/api – ControlAltDel Apr 21 '17 at 19:44
@ControlAltDel using IndexOutOfBoundsException – Cardinal System Apr 21 '17 at 19:46

score 0 · Answer 5 · edited May 23 '17 at 12:10

First, you need to define your custom exception. That might be:

public class EmptyCollectionExceptionextends Exception {
    public EmptyCollectionException(String message) {
        super(message);
    }
}

Then you can throw the exception as in some of the other answers posted.

public T removeLast() throws EmptyCollectionException
{
  if (myList.size() == 0) throw new EmptyCollectionException("List Is Empty");
  ... //otherwise...
}

Exceptions in method declaration in java

5 Answers5