Java Hash Collection

Question

There is a class called HashSet in Java

For example, I'll add following int-shaped value to HashSet,

[input]

1,2,3,4,5,6,1,2,3,1,

[hash structure]

1,1,1
2,2
3,3
4
5
6

Is there the collection to become such a structure?

Answer 1

No, but it's easy enough to wrap around a HashMap.

public class Tally<T> {
  private final Map<T, Integer> count = new HashMap<T, Integer>();

  public void increment(T t) {
    Integer i = count.get(t);
    count.put(t, i == null ? 1 : i+1);
  }

  public void decrement(T t) {
    Integer i = count.get(t);
    if (i == null) {
      throw new IllegalArgumentException("not present");
    }
    if (i == 1) {
      count.remove(t);
    } else {
      count.put(t, i-1);
    }
  }

  public int get(T t) {
    Integer i = count.get(t);
    return i == null ? 0 : i;
  }
}

Answer 2

If I understand your question properly, you're looking for what's called a "Multiset". Java has no builtin Multisets, but you could build what you want with a HashMap<Integer, HashSet<Integer>>. There's also a number of third-party libraries for this, such as the Google Collections Library.