diff --git a/data-structures/src/main/java/com/baeldung/graph/Graph.java b/data-structures/src/main/java/com/baeldung/graph/Graph.java
index b20301f86f..16b7e04297 100644
--- a/data-structures/src/main/java/com/baeldung/graph/Graph.java
+++ b/data-structures/src/main/java/com/baeldung/graph/Graph.java
@@ -1,36 +1,36 @@
 package com.baeldung.graph;
 
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 import java.util.Stack;
 
 public class Graph {
 
-    private List<List<Integer>> neighbours;
-    private int n;
+    private Map<Integer, List<Integer>> adjVertices;
 
-    public Graph(int n) {
-        this.n = n;
-        this.neighbours = new ArrayList<List<Integer>>(n);
-        for (int i = 0; i < n; i++) {
-            this.neighbours.add(new ArrayList<Integer>());
-        }
+    public Graph() {
+        this.adjVertices = new HashMap<Integer, List<Integer>>();
+    }
+
+    public void addVertex(int vertex) {
+        adjVertices.putIfAbsent(vertex, new ArrayList<>());
     }
 
     public void addEdge(int src, int dest) {
-        this.neighbours.get(src)
-            .add(dest);
+        adjVertices.get(src).add(dest);
     }
 
     public void dfsWithoutRecursion(int start) {
         Stack<Integer> stack = new Stack<Integer>();
-        boolean[] isVisited = new boolean[n];
+        boolean[] isVisited = new boolean[adjVertices.size()];
         stack.push(start);
         while (!stack.isEmpty()) {
             int current = stack.pop();
             isVisited[current] = true;
             System.out.print(" " + current);
-            for (int dest : neighbours.get(current)) {
+            for (int dest : adjVertices.get(current)) {
                 if (!isVisited[dest])
                     stack.push(dest);
             }
@@ -38,14 +38,14 @@ public class Graph {
     }
 
     public void dfs(int start) {
-        boolean[] isVisited = new boolean[n];
+        boolean[] isVisited = new boolean[adjVertices.size()];
         dfsRecursive(start, isVisited);
     }
 
     private void dfsRecursive(int current, boolean[] isVisited) {
         isVisited[current] = true;
         System.out.print(" " + current);
-        for (int dest : neighbours.get(current)) {
+        for (int dest : adjVertices.get(current)) {
             if (!isVisited[dest])
                 dfsRecursive(dest, isVisited);
         }
@@ -53,7 +53,7 @@ public class Graph {
 
     public void topologicalSort(int start) {
         Stack<Integer> result = new Stack<Integer>();
-        boolean[] isVisited = new boolean[n];
+        boolean[] isVisited = new boolean[adjVertices.size()];
         topologicalSortRecursive(start, isVisited, result);
         while (!result.isEmpty()) {
             System.out.print(" " + result.pop());
@@ -62,10 +62,11 @@ public class Graph {
 
     private void topologicalSortRecursive(int current, boolean[] isVisited, Stack<Integer> result) {
         isVisited[current] = true;
-        for (int dest : neighbours.get(current)) {
+        for (int dest : adjVertices.get(current)) {
             if (!isVisited[dest])
                 topologicalSortRecursive(dest, isVisited, result);
         }
         result.push(current);
     }
+
 }
diff --git a/data-structures/src/test/java/com/baeldung/graph/GraphUnitTest.java b/data-structures/src/test/java/com/baeldung/graph/GraphUnitTest.java
index 837f83b494..249cb6e093 100644
--- a/data-structures/src/test/java/com/baeldung/graph/GraphUnitTest.java
+++ b/data-structures/src/test/java/com/baeldung/graph/GraphUnitTest.java
@@ -19,7 +19,13 @@ public class GraphUnitTest {
     }
 
     private Graph createDirectedGraph() {
-        Graph graph = new Graph(6);
+        Graph graph = new Graph();
+        graph.addVertex(0);
+        graph.addVertex(1);
+        graph.addVertex(2);
+        graph.addVertex(3);
+        graph.addVertex(4);
+        graph.addVertex(5);
         graph.addEdge(0, 1);
         graph.addEdge(0, 2);
         graph.addEdge(1, 3);