Why does List.add(E) return boolean while List.Add(int, E) returns void?

Question

Looking at the javadoc I saw the that an ArrayList has an overloaded add method:

public boolean add(E e)

Appends the specified element to

Answer 1

Because it's an extra information which doesn't cost anything and can be useful in certain situations. For example:

Set set = new HashSet();
set.add("foobar");
boolean wasAlreadyThere = set.add("foobar");

Otherwise you would have to do

boolean wasAlreadyThere = set.contains("foobar");
set.add("foobar");

which requires twice the work (first you have to lookup, then lookup again to add).