Fix

Author: Hardvan

src/main/java/com/thealgorithms/datastructures/graphs/KahnsAlgorithm.java

@@ -103,6 +103,7 @@ void calculateInDegree() {
      * (u, v), vertex u appears before vertex v in the ordering.
      *
      * @return an ArrayList of vertices in topological order
+     * @throws IllegalStateException if the graph contains a cycle
      */
     ArrayList<E> topSortOrder() {
         calculateInDegree();
@@ -115,10 +116,13 @@ ArrayList<E> topSortOrder() {
         }
 
         ArrayList<E> answer = new ArrayList<>();
+        int processedVertices = 0;
 
         while (!q.isEmpty()) {
             E current = q.poll();
             answer.add(current);
+            processedVertices++;
+
             for (E adjacent : graph.getAdjacents(current)) {
                 inDegree.put(adjacent, inDegree.get(adjacent) - 1);
                 if (inDegree.get(adjacent) == 0) {
@@ -127,6 +131,10 @@ ArrayList<E> topSortOrder() {
             }
         }
 
+        if (processedVertices != graph.getVertices().size()) {
+            throw new IllegalStateException("Graph contains a cycle, topological sort not possible");
+        }
+
         return answer;
     }
 }

src/test/java/com/thealgorithms/datastructures/graphs/KahnsAlgorithmTest.java

@@ -72,9 +72,7 @@ void testSingleNodeGraph() {
         graph.addEdge("a", "a"); // self-loop
 
         TopologicalSort<String> topSort = new TopologicalSort<>(graph);
-        ArrayList<String> result = topSort.topSortOrder();
 
-        String[] expectedOrder = {};
-        assertArrayEquals(expectedOrder, result.toArray());
+        assertThrows(IllegalStateException.class, () -> topSort.topSortOrder());
     }
 }