J.8. Sets

A Set is an unordered Collection of unique elements (i.e., no duplicate elements). The collections framework contains several Set implementations, including HashSet and TreeSet. HashSet stores its elements in a hash table, and TreeSet stores its elements in a tree. Hash tables are presented in Section J.9.

Figure J.8 uses a HashSet to remove duplicate strings from a List. Recall that both List and Collection are generic types, so line 16 creates a List that contains String objects, and line 20 passes a Collection of Strings to method printNonDuplicates.

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 1   // Fig. J.8: SetTest.java
 2   // HashSet used to remove duplicate values from an array of strings.
 3   import java.util.List;
 4   import java.util.Arrays;
 5   import java.util.HashSet;
 6   import java.util.Set;
 7   import java.util.Collection;
 8
 9   public class SetTest
10   {
11      public static void main( String[] args )
12      {
13         // create and display a List< String >
14         String[] colors = { "red", "white", "blue", "green", "gray",
15            "orange", "tan", "white", "cyan", "peach", "gray", "orange" };
16         List< String > list = Arrays.asList( colors );
17         System.out.printf( "List: %s ", list );
18
19         // eliminate duplicates then print the unique values
20         printNonDuplicates( list );
21      } // end main
22
23      // create a Set from a Collection to eliminate duplicates
24      private static void printNonDuplicates( Collection< String > values )
25      {
26         // create a HashSet
27         Set< String > set = new HashSet< String >( values );
28
29         System.out.print( " Nonduplicates are: " );
30
31         for ( String value : set )
32            System.out.printf( "%s ", value );
33
34         System.out.println();
35      } // end method printNonDuplicates
36   } // end class SetTest

Fig. J.8 | HashSet used to remove duplicate values from an array of strings.

Method printNonDuplicates (lines 24–35) takes a Collection argument. Line 27 constructs a HashSet<String> from the Collection<String> argument. By definition, Sets do not contain duplicates, so when the HashSet is constructed, it removes any duplicates in the Collection. Lines 31–32 output elements in the Set.