Set and HashSet : HashSet « Collections « Java Tutorial

1. Java Tutorial
2. Collections
3. HashSet

1. A Set represents a mathematical set.
2. It is a Collection that, unlike List, does not allow duplicates.
3. There must not be two elements of a Set, say e1 and e2, such that e1.equals(e2).
4. The add method of Set returns false if you try to add a duplicate element.

import java.util.HashSet;
import java.util.Set;

public class MainClass {

  public static void main(String[] a) {
    Set set = new HashSet();
    set.add("Hello");
    if (set.add("Hello")) {
      System.out.println("addition successful");
    } else {
      System.out.println("addition failed");
    }

  }
}

addition failed

1. HashSet allows at most one null element.
2. HashSet is faster than other implementations of Set, TreeSet and LinkedHashSet.