TheAlgorithms/Java · error · IllegalArgumentException

Source vertex is out of bounds.

Error message

Source vertex is out of bounds.

What it means

Thrown by `DialsAlgorithm.run` when `source` is negative or `>= graph.size()`. Dial's algorithm is a single-source shortest-path method that indexes distances by vertex, so the source must reference an existing vertex in the adjacency list. The source is 0-indexed to match the list structure.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/graphs/DialsAlgorithm.java:63

        public int getWeight() {
            return weight;
        }
    }
    /**
     * Finds the shortest paths from a source vertex to all other vertices in a weighted graph.
     *
     * @param graph The graph represented as an adjacency list.
     * @param source The source vertex to start from (0-indexed).
     * @param maxEdgeWeight The maximum weight of any single edge in the graph.
     * @return An array of integers where the value at each index `i` is the
     * shortest distance from the source to vertex `i`. Unreachable vertices
     * will have a value of Integer.MAX_VALUE.
     * @throws IllegalArgumentException if the source vertex is out of bounds.
     */
    public static int[] run(List<List<Edge>> graph, int source, int maxEdgeWeight) {
        int numVertices = graph.size();
        if (source < 0 || source >= numVertices) {
            throw new IllegalArgumentException("Source vertex is out of bounds.");
        }

        // Initialize distances array
        int[] distances = new int[numVertices];
        Arrays.fill(distances, Integer.MAX_VALUE);
        distances[source] = 0;

        // The bucket queue. Size is determined by the max possible path length.
        int maxPathWeight = maxEdgeWeight * (numVertices > 0 ? numVertices - 1 : 0);
        List<Set<Integer>> buckets = new ArrayList<>(maxPathWeight + 1);
        for (int i = 0; i <= maxPathWeight; i++) {
            buckets.add(new HashSet<>());
        }

        // Add the source vertex to the first bucket
        buckets.get(0).add(source);

        // Process buckets in increasing order of distance

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Validate `0 <= source < graph.size()` before calling run
2. If your source is 1-based, subtract 1
3. Ensure the adjacency list has one entry per vertex so `graph.size()` matches the real vertex count

Example fix

// before
int[] d = DialsAlgorithm.run(graph, source, maxW);
// after
if (source < 0 || source >= graph.size()) throw new IllegalArgumentException("source");
int[] d = DialsAlgorithm.run(graph, source, maxW);

if (source < 0 || source >= graph.size()) {
    throw new IllegalArgumentException("source out of bounds: " + source);
}

try {
    DialsAlgorithm.run(graph, source, maxW);
} catch (IllegalArgumentException e) {
    // handle bad source
}

When it happens

Trigger: Calling `run(graph, source, maxEdgeWeight)` with a source outside `[0, graph.size())` — e.g. a 1-based source, or an adjacency list built with fewer entries than expected so `graph.size()` is smaller than `source`.

Common situations: 1-based vs 0-based confusion; an empty adjacency list (`graph.size()==0`) with source 0; source from config that exceeds vertex count after the graph was filtered.

Related errors

• Edge vertex out of range
• Incorrect source
• Vertex {vertex} is out of bounds
• Number of vertices must be positive
• Edges list must not be null or empty

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/978a801bde77be1f. Report an issue: GitHub.