Assign values to Array of Objects

Question

I am trying to create an array of objects. There are 10 nodes for which I am assigning a 'node_id'. I am also assigning values to other variables in the class. The for loop in 'networkInitialization()' method assigns values to 'node_id' in a sequential manner, but when I try to print it in main method it returns all '9'.

import java.util.*;
public class tman {
  static int N = 10;
  static int k = 5;
  static Node nodes[] = new Node[N]; 
  public static void main(String[] args) {
    networkInitialization();
    for(int i = 0; i<N; i++){
      System.out.println(nodes[i].node_id);
    }
  }
  public static void networkInitialization(){
    Random random = new Random();
    int next;
    double theta;
    System.out.println("Initializing the network");
    for(int i = 0; i<nodes.length; i++){
      nodes[i] = new Node();
      HashSet<Integer> used = new HashSet<Integer>();
  //Nodeid
      nodes[i].node_id = i;
  //        System.out.println(nodes[i].node_id);
  //Generating 'k' random neighbors list
      for(int j = 0; j<k; j++){
        next = random.nextInt(10);
        while (used.contains(next)) { //while we have already used the number
            next = random.nextInt(N); //generate a new one because it's already used
        }
        used.add(next);
//          System.out.println(next);
        nodes[i].neighbors[j] = next;
      }
//Calculating XCo and YCo
      theta = 3.14/2-(i-1)*3.14/(N-2);
      nodes[i].x_co = Math.cos(theta);
      nodes[i].y_co = Math.sin(theta);
      nodes[i].theta = theta;
//      System.out.println(nodes[0].x_co);
    }
  }
}
class Node{
  static int node_id;
  static double x_co;
  static double y_co;
  static double theta;
  static int k = 30;
  static int neighbors[] = new int[k];
  static Map<Integer, int[]> received_list = new HashMap<Integer, int[]>();
  int N;

  public static int getNodeId(){
    return node_id;
  }

  public static double getXCo(){
    return x_co;
  }

  public static double getYCo(){
    return y_co;
  }

  public static double getTheta(){
    return theta;
  }

  public static int[] getNeighbors(){
    return neighbors;
  }

  public static Map<Integer, int[]> getReceivedList(){
    return received_list;
  }

  public void setNodeId(int node_id){
    this.node_id = node_id;
  }

  public void setXCo(int x_co){
    this.x_co = x_co;
  }

  public void setYCo(int y_co){
    this.y_co = y_co;
  }

  public void setTheta(double theta){
    this.theta = theta;
  }

  public void setNeighbors(int neighbors[]){
    this.neighbors = neighbors;
  }
}

This is the output I am getting in main method.

Initializing the network
9
9
9
9
9
9
9
9
9
9

Anyone can help me on this ?

UPDATE : Looks like I have not understood static well enough. Removing all the static in the networkInitialization() method worked correctly. Thank you.

Answer 1

As was pointed out in the comments, the problem is in the use of static in the definition of Node. If a variable is static, it means that it will be the same for every instance of that class. To illustrate this, look at the code below:

Node a = new Node();
a.node_id = 1;

Node b = new Node();
b.node_id = 2;

If node_id is not static, it would behave as you would expect: Java creates a Node a, which has a field node_id that is set to 1 and then goes on to create another Node b which has its own field node_id that is naturally different from the node_id of node a.

However, if node_id is static, all nodes share the same field node_id! This means that Node a sets the shared node_id to one, but Node b accesses the same node_id and sets it to 2.

In your example, the last time the shared field node_id is called is when i = 9, so the shared node_id has a value of 9, regardless of which Node it is.

You said you were a beginner, so you probably won't use static much. However, static could be useful if, for example, you want to know how many nodes there are in total. In that case you can have a static field that keeps track of the number of nodes. Each time another node is called, you increase that number. Because it is shared, all nodes will know how many total nodes there are. If you want to know more about how to use static, see the Java tutorial.