Multiset.java

class Multiset<E> {

	private List<E> values;
	private List<Integer> frequency;
	private final String ERROR_MSG = "Count cannot be negative: ";

	public Multiset() {
		values = new ArrayList<>();
		frequency = new ArrayList<>();
	}
    
	public Multiset(Multiset<E> ms) {
        values = new ArrayList<>();
		frequency = new ArrayList<>();
	    ms.forEach((val)->{this.add(val);});
	}

	public int add(E element, int count) {
		if (count < 0) {
			throw new IllegalArgumentException(ERROR_MSG + count);
		}

		int index = values.indexOf(element);
		int prevCount = 0;

		if (index != -1) {
			prevCount = frequency.get(index);
			frequency.set(index, prevCount + count);
		}
		else if (count != 0) {
			values.add(element);
			frequency.add(count);
		}

		return prevCount;
	}

    public boolean add(E element) {
		return add(element, 1) >= 0;
	}

	boolean addAll(Collection<? extends E> c){
		for (E element: c)
			add(element, 1);

		return true;
	}

    public void addAll(E... arr) {

		for (E element: arr)
			add(element, 1);
	}

	public void forEach(Consumer<? super E> action)
	{
		List<E> all = new ArrayList<>();

		for (int i = 0; i < values.size(); i++)
			for (int j = 0; j < frequency.get(i); j++)
				all.add(values.get(i));

		all.forEach(action);
	}

	public boolean remove(Object element)
	{
		return remove(element, 1) > 0;
	}

	public int remove(Object element, int count)
	{
		if (count < 0) {
			throw new IllegalArgumentException(ERROR_MSG + count);
		}

		int index = values.indexOf(element);
		if (index == -1)
			return 0;

		int prevCount = frequency.get(index);

		if (prevCount > count) {
			frequency.set(index, prevCount - count);
		}
		else {
			values.remove(index);
			frequency.remove(index);
		}

		return prevCount;
	}

	public boolean contains(Object element) {
		return values.contains(element);
	}

	public boolean containsAll(Collection<?> c) {
		return values.containsAll(c);
	}

	public int setCount(E element, int count)
	{
		if (count < 0) {
			throw new IllegalArgumentException(ERROR_MSG + count);
		}

		if (count == 0)
			remove(element);

		int index = values.indexOf(element);
		if (index == -1)
			return add(element, count);

		int prevCount = frequency.get(index);
		frequency.set(index, count);

		return prevCount;
	}

	public int count(Object element) {
		int index = values.indexOf(element);

		return (index == -1) ? 0 : frequency.get(index);
	}

	public Set<E> elementSet() {
		return values.stream().collect(Collectors.toSet());
	}

	public boolean isEmpty() {
		return values.size() == 0;
	}

	public int size() {
		int size = 0;
		for (Integer i: frequency){
			size += i;
		}
		return size;
	}

	@Override
	public String toString() {
		StringBuilder sb = new StringBuilder("[");
		for (int i = 0; i < values.size(); i++) {
			sb.append(values.get(i));

			if (frequency.get(i) > 1)
				sb.append(" x ").append(frequency.get(i));

			if (i != values.size() - 1)
				sb.append(", ");
		}

		return sb.append("]").toString();
	}
}