Question

I have the following.

HashSet<String> set1 = new HashSet<String>();
HashSet<String> set2 = new HashSet<String>();
String strB1 = "B";
String strB2 = "B";

set1.add( "A" );
set1.add( strB1 );
set2.add( strB2 );

set1.removeAll( set2 ); 

Will set1 end up containing only “A”, or will it contain “B” as well?

Answer 1

HashSet uses object equality (Object.equals), not identity (“reference equals”).

Additionally, HashSet uses Object.hashCode for the hashing function.

Unfortunately, to “know” this, takes a little bit of “reading into” of the documentation and knowing how a hash is implemented. From the documentation for HashSet.contains:

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

The more general Set documentation says:

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.

With very few exceptions, such as IdentityHashMap, the data-structures in Java work on equality and not identity.

Thus, to answer the question, HashSet works on the “String values”.

(This example can be particularly misleading due to String intern’ing, but that doesn’t change the above as identity implies equality even if the converse is not true.)