TheAlgorithms/Java · error · RuntimeException

Queue is full

Error message

Queue is full

What it means

Thrown by PriorityQueue.insert(int) as a RuntimeException("Queue is full") when isFull(). This is an array-backed max-heap whose capacity is fixed at construction (size+1 slots, with slot 0 unused). Once the heap holds the configured number of items, further inserts are rejected. Note the generic RuntimeException, not a more specific type.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/queues/PriorityQueues.java:114

            }

            // If not exchange the value of parent with child
            int temp = queueArray[pos];
            queueArray[pos] = queueArray[current];
            queueArray[current] = temp;
            pos = current; // Exchange parent position to child position in the array
        }
    }

    /**
     * Inserts an element in it's appropriate place
     *
     * @param value Value to be inserted
     */
    public void insert(int value) {
        // Print overflow message if the capacity is full
        if (isFull()) {
            throw new RuntimeException("Queue is full");
        } else {
            queueArray[++nItems] = value;
            swim(nItems); // Swim up the element to its correct position
        }
    }

    /**
     * Dequeue the element with the max priority from PQ
     *
     * @return The element removed
     */
    public int remove() {
        if (isEmpty()) {
            throw new RuntimeException("Queue is Empty");
        } else {
            int max = queueArray[1]; // By definition of our max-heap, value at queueArray[1] pos is
                                     // the greatest

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Guard with isFull() (or track nItems against capacity) before insert().
2. Size the constructor capacity to the peak number of concurrent items.
3. Remove lower-priority elements before inserting new ones to keep occupancy below capacity.
4. Catch RuntimeException (or the more general Exception) around insert since the type is not specific.

Example fix

// before
pq.insert(value); // throws RuntimeException when full

// after
if (pq.isFull()) {
    pq.remove(); // evict lowest priority to make room
}
pq.insert(value);

if (!pq.isFull()) {
    pq.insert(value);
} else {
    pq.remove(); // evict lowest priority
    pq.insert(value);
}

null

try { pq.insert(value); } catch (RuntimeException e) { /* capacity handling */ }

When it happens

Trigger: Calling insert(value) more than the configured capacity times without remove() calls. Capacity is the size passed to the constructor (default 11); the array is allocated as size+1 but usable capacity equals size.

Common situations: Fixed-capacity priority queue overwhelmed by ingest rate; capacity mis-sized because the +1 internal slot led to an off-by-one in mental model; feed loop with no removal step; RuntimeException not caught because callers only expected checked exceptions.

Related errors

• Queue is Empty
• MinPriorityQueue is full. Cannot insert new element.
• Queue is full
• initialCapacity < 1
• Cannot extract from empty heap

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/18947f2febee03bd. Report an issue: GitHub.