Question

I have many classes that implement a common interface. I use a definition like Array List mytypes, and add objects of that type to the ArrayList. Now i want to use contains method of the ArrayList class to see if this List contains a class i am adding.

If I implement hashcode and equals on the classes will the contains method know if a certain object already is in the ArrayList or not?

Answer 1

ArrayList.contains won’t use hashCode, but it will use equals, as documented:

Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

(This won’t check whether “this class” is already in the list – it will check whether an equal object is in the list.)