HashSet : Java Glossary


A wrapper around HashMap to implement the Set interface. Instead of put you used add. Instead of get you use contains.

You’d think HashSet would be a useful tool for arranging unique Objects. However, it is useless for that purpose because it will only tell you if an equivalent Object is already in the HashSet. It won’t divulge a reference to the canonical Object itself. To arrange uniqueness, you need a HashMap with key and value referencing the same canonical unique Object.

In other words,

HashSet.contains tells you if there is an Object in the HashSet Collection that matches yours, as measured by equals. It does not tell you if your Object is the exact same Object as the one in the Collection. Further, HashSet will not give you a reference to its Object. However, if you use HashMap instead, you can get a reference to the Collection’s Object.
Using HashSets Tips
Exporting data from a HashSet Learning More
Comparing HashSets for Duplicates Links

Using HashSets

Two other techniques for dealing with sets include EnumSet and BitSet.

Here is how you can initialise a HashSet in a single line.

Exporting data from a HashSet

Here is how to export data from a HashSet and sort it.

// exporting data from a HashSet and sorting it.
HashSet<String> h = ...;

final String[] exported = h.toArray( new String[ h.size ]  );
Arrays.sort( exported );
for ( String elt : exported )
  out.println( elt );

Comparing HashSets for Duplicates


HashSet does not have a constructor that takes an array of initial values, but it does have one that takes a Collection. So you can initialise by using Arrays.asList to convert like this:



A HashSet is a fairly heavy duty solution. If you have only a few elements there are lighter weight ways of discovering if something belongs in a set.

Learning More

Oracle’s Javadoc on HashSet class : available:
Oracle’s Javadoc on Set interface : available:
Oracle’s Javadoc on Collections.unmodifiableSet : available:
Oracle’s Javadoc on Collections.emptySet : available:

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