TheAlgorithms/Java · error · IllegalArgumentException

Self-loops are not allowed

Error message

Self-loops are not allowed

What it means

Thrown by `WelshPowell.Graph.addEdge` when both endpoints are equal (`nodeA == nodeB`). Welsh-Powell graph coloring assumes a simple graph; a self-loop has no meaningful color assignment, so it is rejected. Vertices are validated for bounds first, then the self-loop check runs.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/graphs/WelshPowell.java:64

                throw new IllegalArgumentException("Number of vertices cannot be negative");
            }

            adjacencyLists = new HashSet[vertices];
            Arrays.setAll(adjacencyLists, i -> new HashSet<>());
        }

        /**
         * Adds an edge between two vertices in the graph.
         *
         * @param nodeA one end of the edge
         * @param nodeB the other end of the edge
         * @throws IllegalArgumentException if the vertices are out of bounds or if a self-loop is attempted
         */
        private void addEdge(int nodeA, int nodeB) {
            validateVertex(nodeA);
            validateVertex(nodeB);
            if (nodeA == nodeB) {
                throw new IllegalArgumentException("Self-loops are not allowed");
            }
            adjacencyLists[nodeA].add(nodeB);
            adjacencyLists[nodeB].add(nodeA);
        }

        /**
         * Validates that the vertex index is within the bounds of the graph.
         *
         * @param vertex the index of the vertex to validate
         * @throws IllegalArgumentException if the vertex is out of bounds
         */
        private void validateVertex(int vertex) {
            if (vertex < 0 || vertex >= getNumVertices()) {
                throw new IllegalArgumentException("Vertex " + vertex + " is out of bounds");
            }
        }

        /**

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Filter out self-loop edges `[v, v]` before calling makeGraph
2. Fix the data source so endpoints differ
3. If self-loops are meaningful in your model, use a graph type that supports them

Example fix

// before
int[][] edges = {{0, 0}, {0, 1}};
// after (removed self-loop)
int[][] edges = {{0, 1}};

for (int[] e : edges) {
    if (e[0] == e[1]) {
        throw new IllegalArgumentException("self-loop at " + e[0]);
    }
}

try {
    WelshPowell.makeGraph(n, edges);
} catch (IllegalArgumentException e) {
    // self-loop or bad vertex
}

When it happens

Trigger: Calling `makeGraph` with an edge `[v, v]`, or `addEdge(v, v)` directly — both endpoints identical.

Common situations: Data where a vertex is accidentally linked to itself (diagonal in an adjacency source); un-deduplicated edge endpoints; off-by-one that makes two distinct values coincide.

Related errors

• Number of vertices cannot be negative
• Vertex {vertex} is out of bounds
• Edge array must have exactly two elements
• 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/81e46100b994c631. Report an issue: GitHub.