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Sets

  • A set is a collection that cannot contain duplicate elements.
  • This interface models the mathematical set abstraction and is used to represent sets.

The Java platform contains three general-purpose Set implementations.

  • HashSet
  • TreeSet
  • LinkedHashSet

HashSet

  • HashSet stores its elements in a hash table
  • is the best performing implementation
  • however makes no guarantees concerning the order of iteration

TreeSet

  • TreeSet stores its elements in a red-black tree
  • orders its elements based on their values
  • it is substantially slower than HashSet

LinkedHashSet

  • This type is implemented as a hash table with a linked list running through it
  • orders its elements based on the order it is inserted ⇒ insertion order

Operations

Sort

A set isn't an ordered collection, so sorting is not available.

Manual sorting of a set

//key is id of employee
Map<Integer, Employee> map = new HashMap<Integer, Employee>();
Set<Integer> set = map.keySet();
Integer[] ids  =  set.toArray(new Integer[set.size()]);
Arrays.sort(ids);

for (Integer key : ids) {
  //do sth
}