Size of List changes when converted to Set

Question

I have an ArrayList:

List<MyClass> xxx = new ArrayList<MyClass>(functionThatGetsAList());

When I test xxx.size() I get 3, which is right. Then I try to cast:

Set<MyClass> yyy = new HashSet<MyClass>(xxx);

or alternatively:

Set<MyClass> yyy = new HashSet<MyClass>();
yyy.addAll(xxx);

In both cases yyy.size() is 1 and it only copies the first object. Why?

Show your actual code, what is in the array list? Set's remove duplicate entries. So if you had a list {1,1,1}, the set would be {1}. — turbo, Jan 24 '14 at 16:31
Looks like the instances of `MyClass` in your `List xxx` have the same hash code and are the same by the definition of its `equals` method. — Luiggi Mendoza, Jan 24 '14 at 16:32
I've got ten pounds that says you need to implement `equals()` on `MyClass` — Dan Temple, Jan 24 '14 at 16:47

zmf · Answer 1 · 2014-01-24T16:40:20.150

From the documentation for HashSet

boolean add(E e) Adds the specified element to this set if it is not already present.

Try adding those elements from the list to the set in a loop.

does set.add(l); return true for every iteration? If not I think its safe to assume that the things that you are trying to add to the set are the same thing as determined by hashcode. If this is the case, you'll need to override hashcode().

score 0 · Answer 2 · edited May 23 '17 at 11:56

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HashSet relies on the hashCode() method in Object. If you need a hash code, you need to override this method. When overriding hashCode(), one must also override equals(). See What issues should be considered when overriding equals and hashCode in Java?

edited May 23 '17 at 11:56

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answered Jan 24 '14 at 16:34

bstempi

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Size of List changes when converted to Set

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