Update the ArrayMultiSet class AND/OR its inner-class Iterator so that it has a working fail-fast Iterator.

package edu.buffalo.cse116; import java.util.Arrays; import java.util.Collection; import java.util.ConcurrentModificationException; import java.util.Iterator; import java.util.NoSuchElementException; /** * Class which implements the concept of a multiset -- an unorganized collection which does not limited the number of * times an instance can be added. * * @author Carl Alphonce * @author Matthew Hertz * @param  Type of data contained with the instance. */ public class ArrayMultiSet implements Collection { /** Array in which the elements in this multiset are stored. */ private E[] _store; /** * Array indices below this amount contain the active elements in this collection. */ private int _size; /** * Modification counter used to preserve the fail-fast nature of its iterators. */ private long _modCount; /** * Create a new empty multiset. */ public ArrayMultiSet() { _modCount = 0; clear(); } /** * Remove all of the elements within the instance and invalidate any current iterators. */ @SuppressWarnings("unchecked") @Override public void clear() { _store = (E[]) (new Object[16]); _size = 0; // maintains the class invariant } /** * Update the multiset so that it includes all of the elements from before the call AND the given element. * * @param e Item to be added to this collection. */ @Override public boolean add(E e) { // Check if we do not have enough space in the underlying array to store the // new element if (_size == _store.length) { // We do not have space, so create a new larger space (doubling the size // is the most time efficient) _store = Arrays.copyOf(_store, _store.length * 2); } // Add the element to the store _store[_size] = e; // Finally, we can increase _size, since this change will no longer violate // any class invariants. _size += 1; return true; } /** * Return true if at least one element in the multiset is equal to the given object. When {@code obj} is null, it must * use the {@code ==} operator to perform these checks, but when {@code obj} is not null, the {@link Object#equals} * method is used. * * @param obj Object (or null) for which we will search * @return {@code true} if {@code obj} was found; {@code false} if a match could not be found. */ @Override public boolean contains(Object obj) { // Only scan through _size, since those are the only "real" entries for the // multiset. for (int i = 0; i < _size; i++ ) { // When obj is null, we need to use == if ((obj == null) && (_store[i] == null)) { return true; } // Otherwise, we use .equals() to find a match else if ((obj != null) && obj.equals(_store[i])) { return true; } // No else clause, since the match could be at a higher index! } // Checked all VALID indices, so the result must be: return false; } @Override public int size() { return _size; } /** * Remove the element found at the given index. This method also acts to maintain the class invariants.
 * Precondition: {@code i} is a valid index within {@code _store}. * * @param i Index of the element to remove from the multiset. */ private void removeAtIndex(int i) { // We do not need to check i, since we made that a precondition (assumption) // for this method. // Slide elements at highest index down to fill in "hole" we are creating _store[i] = _store[_size - 1]; // Update which is the first unused index in the array _size -= 1; // Finally set the newly unused index to null thus avoiding a space leak _store[_size] = null; } /** * Removes a single instance of the given object, if one can be found in the multiset. The method returns {@code true} * if a match was found (and removed) and {@code false} if no match was found. Normally, this uses * {@link Object#equals} to check if there is a match, but uses {@code ==} when {@code obj} is {@code null}. * * @param obj Object (or null) which we want to remove * @return {@code true} if {@code obj} was found and an instance removed; {@code false} if a match could not be found. */ @Override public boolean remove(Object obj) { for (int i = 0; i < _size; i++ ) { if ((obj == null) && (_store[i] == null)) { removeAtIndex(i); return true; } else if ((obj != null) && obj.equals(_store[i])) { removeAtIndex(i); return true; } // No else clause, since a match may still exist at a higher index! } // Checked all VALID indices, so the result must be: return false; } @Override public boolean isEmpty() { return _size == 0; } @Override public Iterator iterator() { return new MultiSetIterator(); } /** * Instances of this class are used as the iterators for a multiset. We must keep this as an inner-class, because the * multiset definition does not include any methods to access its elements. * * @author Carl Alphonse * @author Matthew Hertz */ private class MultiSetIterator implements Iterator { /** * The index of the _store entry which will be returned by the next call to next() */ private int _cursor; /** * In keeping with the fail-fast convention, the iterator is only valid while _modCount remains on this version. */ private long _collectionVersion; /** * Create a new instance of this class that will go through the (valid) entries in the store. */ public MultiSetIterator() { _cursor = 0; } public boolean hasNext() { } public E next() { } public void remove() { throw new UnsupportedOperationException(); } } /* * The remaining methods are part of the Collection interface, but are beyond what is necessary for CSE 116. * Students who want a complete Multiset implementation should investigate Google's "Guava" library. */ @Override public Object[] toArray() { throw new UnsupportedOperationException(); } @Override public  T[] toArray(T[] a) { throw new UnsupportedOperationException(); } @Override public boolean containsAll(Collection c) { throw new UnsupportedOperationException(); } @Override public boolean addAll(Collection c) { throw new UnsupportedOperationException(); } @Override public boolean removeAll(Collection c) { throw new UnsupportedOperationException(); } @Override public boolean retainAll(Collection c) { throw new UnsupportedOperationException(); } }