Why does the Java ArrayList class return a boolean for add?

Question

My teacher gave me this analogy for a method I am writing. I still don't get why the add method returns a boolean?

Wouldn't it make more sense for it to return nothing?

Answer 1

This comes from Collection<E> interface, since the interface is used as an ancestor class for all kinds of collections, it makes sense to return a boolean to inform if add effectively changed the collection.

This is useful for collections like Set<T> which doesn't change if the element added was already present inside the collection. In case of a List<T> Java enforces by design to return true, so if you want to notify the caller that the operation didn't succeed you must find another way (eg. throw an exception).

This is clearly stated in Collection<E>.add documentation:

If a collection refuses to add a particular element for any reason other than that it already contains the element, it must throw an exception (rather than returning false). This preserves the invariant that a collection always contains the specified element after this call returns.

Basically the value returned doesn't indicate failure but if a successful add operation changed the collection. Any failure must be notified by throwing an exception.

Answer 2

The Javadoc for ArrayList.add(E) explains that it

Returns: true (as specified by Collection.add(E))

Which says

Returns: true if this collection changed as a result of the call

If it didn't return a boolean, then a caller would have to test the List or check an Exception or there would be no way to tell if the call succeeded. I think it was easier (and faster) to return a boolean.