code: import java.util.Arrays;
  import java.util.Collections;
  import java.util.Set;
  import java.util.TreeSet;
  
  public class Sets {
      public static void main(String[] args){
          Set<Integer> a = new TreeSet<>();
          //TreeSet sorts on natural ordering (or an optional comparator)
          //other options: HashSet (hashcode)
          //               LinkedHashSet (insertion order)
          //               EnumSet (optimized for enum values)
          //others at: http://download.oracle.com/javase/7/docs/api/java/util/Set.html
          Set<Integer> b = new TreeSet<>();
          Set<Integer> c = new TreeSet<>();
          Set<Integer> d = new TreeSet<>();
  
          a.addAll(Arrays.asList(1, 2, 3, 4, 5));
          b.addAll(Arrays.asList(2, 3, 4, 5, 6, 8));
          c.addAll(Arrays.asList(2, 3, 4));
          d.addAll(Arrays.asList(2, 3, 4));
          System.out.println("a: " + a);
          System.out.println("b: " + b);
          System.out.println("c: " + c);
          System.out.println("d: " + d);
  
          System.out.println("2 in a: " + a.contains(2));
          System.out.println("6 in a: " + a.contains(6));
  
          Set<Integer> ab = new TreeSet<>();
          ab.addAll(a);
          ab.addAll(b);
          System.out.println("a union b: " + ab);
  
          Set<Integer> a_b = new TreeSet<>();
          a_b.addAll(a);
          a_b.removeAll(b);
          System.out.println("a - b: " + a_b);
  
          System.out.println("c subset of a: " + a.containsAll(c));
          //use a.conatins() for single elements
  
          System.out.println("c = d: " + c.equals(d));
          System.out.println("d = c: " + d.equals(c));
  
          Set<Integer> aib = new TreeSet<>();
          aib.addAll(a);
          aib.retainAll(b);
          System.out.println("a intersect b: " + aib);
  
          System.out.println("add 7 to a: " + a.add(7));
          System.out.println("add 2 to a again: " + a.add(2));
  
          //other noteworthy things related to sets:
          Set<Integer> empty = Collections.EMPTY_SET; //immutable empty set
          //empty.add(2);  would fail
          empty.isEmpty(); //test if a set is empty
          empty.size();
          Collections.disjoint(a, b); //returns true if the sets have no common elems (based on their .equals() methods)
          Collections.unmodifiableSet(a); //returns an immutable copy of a
      }
  }
  