Java: Can I make an ArrayList of objects?

Question

I have the following thing to do:

I've made the Node and Edge class (I hope I got them right) and I'm stuck on the GraphModel class. Here is my code so far:

Node.java (ignore the getNr)

import java.awt.Rectangle;

public class Node {
    private String name;
    private Rectangle rectangle;


    public Node(int x, int y, int width, int height, String name) {
        this.rectangle =new Rectangle(x, y, width, height);
        this.name = name;
    }

    public Node() {
        this.rectangle = new Rectangle (0,0,0,0); //gonna change the parameters later
        this.name = "default name";
    }

    public int getNr() {
        return nr;
    }

    public String getName() {
        return name;
    }
}

Edge.java:

public class Edge {
    private Node nodeA;
    private Node nodeB;

    public Edge(Node nodeA, Node nodeB) {
        this.nodeA = nodeA;
        this.nodeB = nodeB;
    }

    public Node getNodeA() {
        return nodeA;
    }

    public Node getNodeB() {
        return nodeB;
    }
}

Now, I hope they are correct but if you notice there is something not right please let me know. Now about the GraphModel, I used a code that makes a graph with a given number of nodes (which is not good because I want to increase/decrease the N (number of nodes, you will see in the code) when I add/remove a node) . Also, the nodes are represented by integers, I wonder how can I make them be represented by the actual node class that I've created (because I will later have to make a GUI for that). I've tried to use ArrayList<ArrayList<Node>> but I have no idea what should I do next. Here is the code GraphModel.java:

import java.util.*;

public class GraphModel {
    private int numNode;
    private Node node;

    /* Add edge for undirected graph (I don't understand why these give opposite errors like java do you want an int or a node????
    static void addEdge(ArrayList<ArrayList<Node>> gr, Edge edge) {   <---- Initially made list of Node type but I'm noob and it doesn't work
        gr.get(edge.getNodeA().getNr()).add(edge.getNodeB().getNr());
        gr.get(edge.getNodeB()).add(edge.getNodeA());

    }
    */
    static void addEdge(ArrayList<ArrayList<Integer>> gr, int s, int d) {
        gr.get(s).add(d);
        gr.get(d).add(s);

    }

    /*next todo
    static void delEdge(ArrayList<ArrayList<Integer>> gr,
                        int s, int d)
     */


        public static void makeGraph(){
        int NR=4; //I've tried to make it dynamic but I've no idea how (like, adjust it's size when adding edges (will do)
        ArrayList<ArrayList<Integer>> gr = new ArrayList<ArrayList<Integer>>(NR);
        for (int i = 0; i < NR; i++)
            gr.add(new ArrayList<Integer>());


        GraphModel.addEdge(gr,0,1);
        GraphModel.addEdge(gr,0,2);
        GraphModel.addEdge(gr,0,3);
        GraphModel.addEdge(gr,1,2);
        GraphModel.addEdge(gr,1,3);
        printGraph(gr);
    }

    static void printGraph(ArrayList<ArrayList<Integer>> gr) {
        for (int i = 0; i < gr.size(); i++) {
            System.out.println("\nNode " + i + ":");
            for (int j = 0; j < gr.get(i).size(); j++) {
                System.out.print(" -> " + gr.get(i).get(j));
            }
            System.out.println();
        }
    }
}

Hope that makes sense, if not feel free to ask me anything. Also keep in mind that I am a beginner in Object oriented programming, started it like 1 week ago. Thank you very much for your time!

jnrdn0011 · Accepted Answer · 2020-06-05 14:52:01Z

You can do this

For GraphModel class

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class GraphModel {
    Set<Node> nodes;
    Set<Edge> edges;

    public GraphModel() {
        this.nodes = new HashSet<>();
        this.edges = new HashSet<>();
    }

    // add node
    public void addNode(Node node) {
        nodes.add(node);
    }

// add edge
public void addEdge(Edge edge) {
    edges.add(edge);
}

// remove node
public void removeNode(Node node) {
    nodes.remove(node);
    // remove all edges who are connected with this node
    List<Edge> edgesToRemove = new ArrayList<>();
    // get all edges who are connected to this node
    for(Edge edge: edges){
        if(edge.getNodeA().equals(node) || edge.getNodeB().equals(node)){
            edgesToRemove.add(edge);
        }
    }
    // remove edges
    edges.removeAll(edgesToRemove);
}

// remove an edge
public void removeEdge(Edge edge) {
    edges.remove(edge);
}

public static void main(String[] args) {
    GraphModel graph = new GraphModel();
    Node A = new Node(0, 0, 1, 1, "A");
    Node B = new Node(1, 1, 1, 1, "B");
    graph.addNode(A);
    graph.addNode(B);
    Edge edge = new Edge(A, B);
    graph.addEdge(edge);
    System.out.println(graph);
    graph.removeNode(A);
    System.out.println(graph);
}

@Override
public String toString() {
    return "GraphModel{" +
            "nodes=" + nodes +
            ", edges=" + edges +
            '}';
}
}

For Node class

import java.awt.*;

public class Node {
private String name;
private Rectangle rectangle;


public Node(int x, int y, int width, int height, String name) {
    this.rectangle = new Rectangle(x, y, width, height);
    this.name = name;
}

public Node() {
    this.rectangle = new Rectangle(0, 0, 0, 0); //gonna change the parameters later
    this.name = "default name";
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public Rectangle getRectangle() {
    return rectangle;
}

public void setRectangle(Rectangle rectangle) {
    this.rectangle = rectangle;
}

@Override
public boolean equals(Object o) {
    Node node = (Node) o;
    return this.name.equals(node.getName());
}

@Override
public String toString() {
    return "Node{" +
            "name='" + name + '\'' +
            ", x=" + rectangle.x +
            ", y=" + rectangle.y +
            ", width=" + rectangle.width +
            ", height=" + rectangle.height +
            '}';
}
}

and for Edge class

public class Edge {
private Node nodeA;
private Node nodeB;

public Edge(Node nodeA, Node nodeB) {
    this.nodeA = nodeA;
    this.nodeB = nodeB;
}

public Node getNodeA() {
    return nodeA;
}

public Node getNodeB() {
    return nodeB;
}

@Override
public String toString() {
    return "Edge{" +
            "nodeA=" + nodeA +
            ", nodeB=" + nodeB +
            '}';
}
}

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