TheAlgorithms/Java · error · IllegalArgumentException

adjacencyList size must equal vertexCount

Error message

adjacencyList size must equal vertexCount

What it means

TarjanBridges.findBridges requires the adjacencyList size to exactly equal vertexCount because each vertex 0..vertexCount-1 must have a corresponding list entry it can index. A mismatch means the graph representation is internally inconsistent — the algorithm would either skip vertices or hit IndexOutOfBoundsException mid-DFS. The check fails fast up front instead of letting the DFS corrupt state.

Source

Thrown at src/main/java/com/thealgorithms/graph/TarjanBridges.java:57

     * an integer in the range {@code [0, vertexCount)}. For each undirected edge (u, v),
     * v must appear in {@code adjacencyList.get(u)} and u must appear in
     * {@code adjacencyList.get(v)}.</p>
     *
     * @param vertexCount   the total number of vertices in the graph (must be non-negative)
     * @param adjacencyList the adjacency list representation of the graph; must contain
     *                      exactly {@code vertexCount} entries (one per vertex)
     * @return a list of bridge edges, where each bridge is represented as an {@code int[]}
     *         of length 2 with {@code edge[0] < edge[1]}; returns an empty list if no bridges exist
     * @throws IllegalArgumentException if {@code vertexCount} is negative, or if
     *                                  {@code adjacencyList} is null or its size does not match
     *                                  {@code vertexCount}
     */
    public static List<int[]> findBridges(int vertexCount, List<List<Integer>> adjacencyList) {
        if (vertexCount < 0) {
            throw new IllegalArgumentException("vertexCount must be non-negative");
        }
        if (adjacencyList == null || adjacencyList.size() != vertexCount) {
            throw new IllegalArgumentException("adjacencyList size must equal vertexCount");
        }

        List<int[]> bridges = new ArrayList<>();

        if (vertexCount == 0) {
            return bridges;
        }

        BridgeFinder finder = new BridgeFinder(vertexCount, adjacencyList, bridges);

        // Run DFS from every unvisited vertex to handle disconnected graphs
        for (int i = 0; i < vertexCount; i++) {
            if (!finder.visited[i]) {
                finder.dfs(i, -1);
            }
        }

        return bridges;

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Verify adjacencyList.size() == vertexCount immediately before the call, e.g. assert adjacencyList.size() == vertexCount or an explicit if-check.
2. Ensure every vertex in [0, vertexCount) has an entry — add empty lists for isolated vertices (Collections.nCopies(vertexCount, List.of()) as a starter).
3. If you build the list by appending, derive vertexCount from the list: findBridges(adjacencyList.size(), adjacencyList).
4. If the list legitimately has extra slots (e.g. 1-based indexing), strip the unused slot or shift to 0-based before calling.

Example fix

// before
List<List<Integer>> adj = new ArrayList<>();
for (int v = 0; v <= n; v++) adj.add(neighbors(v)); // off-by-one
TarjanBridges.findBridges(n, adj);

// after
List<List<Integer>> adj = new ArrayList<>();
for (int v = 0; v < n; v++) adj.add(neighbors(v)); // exactly n entries
TarjanBridges.findBridges(adj.size(), adj);

if (adjacencyList == null || adjacencyList.size() != vertexCount) {
    throw new IllegalArgumentException(
        "adjacencyList size (" + (adjacencyList == null ? "null" : adjacencyList.size())
        + ") must equal vertexCount (" + vertexCount + ")");
}
TarjanBridges.findBridges(vertexCount, adjacencyList);

When it happens

Trigger: Calling findBridges(vertexCount, adjacencyList) where adjacencyList.size() != vertexCount — e.g. vertexCount=5 but the list has 4 or 6 entries, or passing a null adjacencyList. Also triggered when a graph builder adds/removes vertices after the count was captured.

Common situations: Building the adjacency list in a loop with an off-by-one (e.g. adding vertices 0..n-1 but initializing the list with n+1 slots), deserializing a graph where isolated vertices were omitted as empty lists, or refactoring vertex indexing (0-based vs 1-based) without updating the count.

Related errors

• Matrix must be square
• Weights matrix must not be null or empty
• Weights matrix must be square
• Weights must be -1 (no edge) or >= 0
• k must be >= 1

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/4bf2f1089ef18fc5. Report an issue: GitHub.