TheAlgorithms/Java · error · IllegalArgumentException

Item not found in the heap

Error message

Item not found in the heap

What it means

Thrown by GenericHeap.updatePriority(T) when the item is not a key in the internal HashMap<T,Integer> index. The heap tracks element positions by equality via the map, so an item that was never added, was already removed, or is a different instance whose equals() does not match a stored key fails the containsKey check. Note GenericHeap does not expose a public contains() method, so callers must track membership themselves.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/heaps/GenericHeap.java:144

     * @param j index of the second item
     */
    private void swap(int i, int j) {
        T ith = this.data.get(i);
        T jth = this.data.get(j);
        this.data.set(i, jth);
        this.data.set(j, ith);
        map.put(ith, j);
        map.put(jth, i);
    }

    /**
     * Updates the priority of the specified item by restoring the heap property.
     *
     * @param item the item whose priority is to be updated
     */
    public void updatePriority(T item) {
        if (!map.containsKey(item)) {
            throw new IllegalArgumentException("Item not found in the heap");
        }
        int index = map.get(item);
        upHeapify(index);
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Only call updatePriority on the exact object reference previously passed to add() and not yet removed.
2. Maintain your own Set<T> of live items and check contains() before calling updatePriority.
3. Ensure inserted objects have stable equals/hashCode (do not mutate fields used by hashCode after insertion).

Example fix

// before
heap.updatePriority(item);

// after
if (liveItems.contains(item)) {
    heap.updatePriority(item);
}

// GenericHeap has no public contains(); track membership yourself
Set<T> live = new HashSet<>();
// on add: live.add(item);  on remove: live.remove(returned);
if (live.contains(item)) {
    heap.updatePriority(item);
}

try {
    heap.updatePriority(item);
} catch (IllegalArgumentException e) {
    // item not in heap; re-insert or skip
}

When it happens

Trigger: Calling updatePriority on an item never inserted via add(); on an item already returned by remove() (it is dropped from the map); on a newly constructed object that is equal-but-distinct when equals/hashCode are inconsistent with storage.

Common situations: Dijkstra/A*-style relaxation where a node was already extracted; stale references kept after removal; mutable objects whose hashCode changed after insertion (map lookup then misses).

Related errors

• Cannot insert null into the heap.
• Heap is empty
• initialCapacity < 1
• Element not in queue
• Input list cannot be null

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/1634f97a6bd23ce5. Report an issue: GitHub.