An example of graph in java

Main.java

import java.util.ArrayList;

//Definition of Node
class Node {
    String name; // Name of the node
    ArrayList<Node> connectedNodes = new ArrayList<>(); // All nodes connected to this node

    // Constructor, called when use `new Node("name")`
    Node(String n) {
        name = n;
    }

    // Tell system how to convert a Node object to a String (used in println)
    public String toString() {
        return "Node " + name;
    }

    // Connect two nodes
    // `static` means you can use this method without `**new** Node()`
    static void connect(Node n1, Node n2){
        n1.connectedNodes.add(n2); // Add node2 to the list of connected nodes in node1
        n2.connectedNodes.add(n1); // Add node1 to the list of connected nodes in node2
    }
}

public class Main {
    // The following code creates a graph like ↓ and print all connected nodes of node A
    // A------B
    // |      |
    // C------D
    public static void main(String[] args) {
        Node nodeA = new Node("A");
        Node nodeB = new Node("B");
        Node nodeC = new Node("C");
        Node nodeD = new Node("D");
        Node.connect(nodeA, nodeB);
        Node.connect(nodeB, nodeD);
        Node.connect(nodeD, nodeC);
        Node.connect(nodeC, nodeA);

        // Following code won't work:
        // When you use `new Node("A")`, you create a new node
        // `new Node("A")` and `new Node("A")` create two nodes have the same name, not one node.
        // Node.connect(new Node("A"), new Node("B"));
        // Node.connect(new Node("B"), new Node("D"));
        // Node.connect(new Node("D"), new Node("C"));
        // Node.connect(new Node("C"), new Node("A"));

        printConnectedNodes(nodeA);
    }

    static void printConnectedNodes(Node node) {
        for (Node connectedNode : node.connectedNodes) { //An alternative way to iterate through list/array
            System.out.println(connectedNode);
        }
    }
}