CODEWITHSUNDEEP
javaarraylist
AndroidDev
AndroidDev

Reputation: 16385

Java ArrayList Interface Type

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?

Upvotes: 0

Views: 743

Answers (3)

amit
amit

Reputation: 178421

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()

Upvotes: 2

Jon Skeet
Jon Skeet

Reputation: 1500065

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.)

Upvotes: 1

Guillaume Polet
Guillaume Polet

Reputation: 47608

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.

Upvotes: 1

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