Graph Implementation in Java with simple program

Graphs are fundamental data structures used in computer science to represent networks of connected entities. Whether modeling social networks, computer networks, or even transportation systems, graphs provide an intuitive way to represent relationships. In this post, we’ll dive into how to implement a graph in Java using an adjacency list, a versatile representation suitable for a wide range of applications.

What is a Graph?

A graph is a collection of vertices (nodes) and edges (connections) that can represent various systems like networks, circuits, or relationships. Graphs can be:

Undirected: Edges do not have a direction, representing a bidirectional relationship.
Directed: Edges have a direction, representing a one-way relationship.
Weighted: Edges have weights, representing the cost or distance between vertices.

In this post, we will focus on undirected graphs without weights, implemented using an adjacency list.

Graph Representation: Adjacency List

An adjacency list is a collection of lists or arrays where each list corresponds to a vertex and contains all adjacent vertices. This representation is efficient in terms of space and allows quick traversal of adjacent nodes.

Benefits of Adjacency List:

Space-Efficient: Only stores actual edges, making it efficient for sparse graphs.
Flexible: Easy to extend and modify for more complex operations.

Java Implementation

Here’s a step-by-step guide to implementing a graph using an adjacency list in Java.

Step 1: Define the Graph Class

Start by defining the Graph class, including the number of vertices and an adjacency list:

import java.util.*;

public class Graph {
    private int vertices; // Number of vertices
    private LinkedList<Integer>[] adjList; // Adjacency list

    public Graph(int vertices) {
        this.vertices = vertices;
        adjList = new LinkedList[vertices];
        for (int i = 0; i < vertices; i++) {
            adjList[i] = new LinkedList<>();
        }
    }
}

Step 2: Add Edges

Implement a method to add edges to the graph. For undirected graphs, add each edge in both directions:

    // Add edge to the graph
    public void addEdge(int src, int dest) {
        adjList[src].add(dest); // Add edge from src to dest
        adjList[dest].add(src); // Add edge from dest to src (for undirected graph)
    }

Step 3: Print the Graph

Create a method to print the graph’s adjacency list for visualization:

    // Print the adjacency list of the graph
    public void printGraph() {
        for (int i = 0; i < vertices; i++) {
            System.out.print("Vertex " + i + ": ");
            for (Integer vertex : adjList[i]) {
                System.out.print(vertex + " ");
            }
            System.out.println();
        }
    }

Complete Graph Class

Combining the methods, here’s the complete Graph class:

import java.util.*;

// Graph class using adjacency list representation
public class Graph {
    private int vertices; // Number of vertices
    private LinkedList<Integer>[] adjList; // Adjacency list

    // Constructor
    public Graph(int vertices) {
        this.vertices = vertices;
        adjList = new LinkedList[vertices];
        for (int i = 0; i < vertices; i++) {
            adjList[i] = new LinkedList<>();
        }
    }

    // Add edge to the graph
    public void addEdge(int src, int dest) {
        adjList[src].add(dest); // Add edge from src to dest
        adjList[dest].add(src); // Add edge from dest to src (for undirected graph)
    }

    // Print the adjacency list of the graph
    public void printGraph() {
        for (int i = 0; i < vertices; i++) {
            System.out.print("Vertex " + i + ": ");
            for (Integer vertex : adjList[i]) {
                System.out.print(vertex + " ");
            }
            System.out.println();
        }
    }
}

Example Usage

Here’s a sample program to create a graph and add edges:

public class GraphExample {
    public static void main(String[] args) {
        // Create a graph with 5 vertices
        Graph graph = new Graph(5);

        // Add edges to the graph
        graph.addEdge(0, 1);
        graph.addEdge(0, 4);
        graph.addEdge(1, 2);
        graph.addEdge(1, 3);
        graph.addEdge(1, 4);
        graph.addEdge(2, 3);
        graph.addEdge(3, 4);

        // Print the graph's adjacency list
        graph.printGraph();
    }
}

Expected Output

Vertex 0: 1 4 
Vertex 1: 0 2 3 4 
Vertex 2: 1 3 
Vertex 3: 1 2 4 
Vertex 4: 0 1 3

This output shows the adjacency list for each vertex, representing the graph structure.

Conclusion

Implementing a graph using an adjacency list in Java is a straightforward process that provides a flexible and efficient way to manage and traverse graph structures. This implementation can serve as the foundation for more complex graph algorithms, such as depth-first search (DFS), breadth-first search (BFS), and shortest path algorithms.

By mastering the basics of graph representation, you’ll be well-prepared to tackle a wide range of problems and applications involving graphs, from network analysis to pathfinding in games.

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Published: [Date]

Tags: Graphs, Data Structures, Adjacency List, Java Programming, Algorithms

This guide covers the essential steps for implementing a graph using an adjacency list in Java. If you have any questions or need further clarifications, feel free to ask!