Java Collection - 如何根据哈希值创建一个集合

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");
    }

  }
}