Java ArrayList接口类型

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

from List.contains() :

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)).

So basically hashCode() is not relevant here, only equals()

EDIT : [better explicit then implicit], as mentioned in comments by @aiobee, equals() still needs to be overriden - according to the contract - but it will not have effect on the value returned by contains()

Answer 2

Implementing hashCode is not useful for that purpose but it is a goog practice to override both equals and hashCode simultaneously.

Yes, it will work, that is the purpose of the contains method.

Answer 3

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.)