HashSet : Java Glossary

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,

Using HashSets

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

Initialisation

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:

Tips

• To create an effective case-insensitive HashSet, just convert all keys to lower case before feeding them to the HashSet. It is on my to-do list to create CaseInsensitiveHashSet that does this for you.

Alternatives

• An || if chain 

  // or chain alternative to a HashSet
  if ( a == 10 || a == 100 || a == 1000 )
     {
     out.println( "is power of 10" );
     }

• linear search 
• Boolean bit array or BitSet.

  rem BitSet alternative to HashSet
  
  BitSet bitSet = new BitSet ( 10 ); // indexed 0..9
  bitSet.set( 0 );
  bitSet.set( 5 );
  
  if ( bitSet.get( index ) )
    {
    out.println( index + " is in the set" );
    }

• binary search
• EnumSet
• Syntactic sugar: a new keyword in that lets you say:
  if ( index in { 32, 64, 128, 256 } )
  It would be implemented as an if-chain, linear search, BitSet, binary search, EnumSet or HashSet, at the compiler/runtime’s discretion.

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