TheAlgorithms/Java · error · IllegalArgumentException

Edge array must have exactly two elements

Error message

Edge array must have exactly two elements

What it means

Thrown by `WelshPowell.makeGraph` when an inner edge array does not have exactly two elements. Each edge must be a `{src, dest}` pair; arrays of other lengths (1, 3, 0) are malformed and rejected before `addEdge` is called.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/graphs/WelshPowell.java:114

         */
        int getNumVertices() {
            return adjacencyLists.length;
        }
    }

    /**
     * Creates a graph with the specified number of vertices and edges.
     *
     * @param numberOfVertices the total number of vertices
     * @param listOfEdges a 2D array representing edges where each inner array contains two vertex indices
     * @return a Graph object representing the created graph
     * @throws IllegalArgumentException if the edge array is invalid or vertices are out of bounds
     */
    public static Graph makeGraph(int numberOfVertices, int[][] listOfEdges) {
        Graph graph = new Graph(numberOfVertices);
        for (int[] edge : listOfEdges) {
            if (edge.length != 2) {
                throw new IllegalArgumentException("Edge array must have exactly two elements");
            }
            graph.addEdge(edge[0], edge[1]);
        }
        return graph;
    }

    /**
     * Finds the coloring of the given graph using the Welsh-Powell algorithm.
     *
     * @param graph the input graph to color
     * @return an array of integers where each index represents a vertex and the value represents the color assigned
     */
    public static int[] findColoring(Graph graph) {
        int[] colors = initializeColors(graph.getNumVertices());
        Integer[] sortedVertices = getSortedNodes(graph);
        for (int vertex : sortedVertices) {
            if (isBlank(colors[vertex])) {
                boolean[] usedColors = computeUsedColors(graph, vertex, colors);

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Strip edge data to exactly two indices per edge (drop the weight column)
2. Validate each inner array's length is 2 before calling makeGraph
3. Fix the upstream parsing that produced wrong-length arrays

Example fix

// before (weighted triples)
int[][] edges = {{0, 1, 5}, {1, 2, 3}};
// after (weights not used by Welsh-Powell)
int[][] edges = {{0, 1}, {1, 2}};

for (int[] e : listOfEdges) {
    if (e.length != 2) {
        throw new IllegalArgumentException("edge must have length 2: " + Arrays.toString(e));
    }
}

try {
    WelshPowell.makeGraph(n, edges);
} catch (IllegalArgumentException e) {
    // malformed edge array
}

When it happens

Trigger: Passing `listOfEdges` containing an array of length != 2, e.g. `{{0,1,5}}` (weighted triple), `{{0}}`, or `{}`.

Common situations: Passing weighted edge data `{u,v,w}` to a function that expects unweighted pairs; jagged/malformed input; mis-parsing CSV rows into wrong-length arrays.

Related errors

• Number of vertices cannot be negative
• Self-loops are not allowed
• 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/7cf22029b7ad4ae8. Report an issue: GitHub.