Java ArrayList Interface Type

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?

That depends on the ArrayList implementation.

SLaks
– SLaks

2012-03-06 14:28:17 +00:00
Commented Mar 6, 2012 at 14:28 — SLaks
– SLaks, Commented Mar 6, 2012 at 14:28

amit · Accepted Answer · 2012-03-06 14:34:49Z

2

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()

edited Mar 6, 2012 at 14:34

answered Mar 6, 2012 at 14:29

amit

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2 Comments

aioobe Over a year ago

hashCode should still, according to the contract, be overridden as the OP says.

amit Over a year ago

@aioobe: yes, it does, I fully agree on this - but it is not relevant to the return value of contains(). Anyway, I added it to the answer, since it is indeed woths mentioning explicitly.

Jon Skeet · Accepted Answer · 2012-03-06 14:29:04Z

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.)

answered Mar 6, 2012 at 14:29

Jon Skeet

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2 Comments

AndroidDev Over a year ago

But The type of the ArrayList is an Interface. Will the JVM call the equals method on the class that implements the Interface ????

Jon Skeet Over a year ago

@user365019: Yes, just as it's documented.

Guillaume Polet · Accepted Answer · 2012-03-06 14:29:04Z

1

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.

answered Mar 6, 2012 at 14:29

Guillaume Polet

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