wrong output Node data occurs while using for-loop

Question

I want to put a Node in each array, and the Node will contain the data from my input.txt file. So I used for-loop to get data from input.txt file to EdgeArray.

But the result shows like 3 0 -4 (enter) 3 0 -4 (enter) 3 0 -4 (enter) 3 0 -4 (enter) 3 0 -4 (enter)

My Input file has the data like 4 5 (enter) 0 1 -3 (enter) 1 2 -2 (enter) 0 2 0(enter) 3 2 -1 (enter) 3 0 -4 (enter).

So the result must be like 0 1 -3 (enter) 1 2 -2 (enter) 0 2 0 (enter) 3 2 -1 (enter) 3 0 -4 (enter)

What's wrong and What should I do to fix this?

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

public class Assignment51 {
    public static void main(String[] args) throws IOException {
        BufferedReader in;
        try {
            in = new BufferedReader(new FileReader("input.txt"));

            String graphGuide;
            graphGuide = in.readLine();

            String divider[] = graphGuide.split(" "); //공백을 구분자로 지정하기
            int NodeNum = Integer.parseInt(divider[0]); //Node갯수
            int EdgeNum = Integer.parseInt(divider[1]); //Edge갯수

            Node empty = new Node();

            Node [] NodeArray = new Node [NodeNum];
            for(int i = 0; i < NodeNum; i++){
                NodeArray [i] = empty; 
            }

            Node [] EdgeArray = new Node [EdgeNum];
            for(int i = 0; i < EdgeNum; i++){
                EdgeArray [i] = empty; 
            }

            for(int i = 0; i < EdgeNum; i++){
                String temp;
                temp = in.readLine();

                String divider2[] = temp.split(" ");
                int v1 = Integer.parseInt(divider2[0]);
                int v2 = Integer.parseInt(divider2[1]);
                int weight = Integer.parseInt(divider2[2]);

                EdgeArray[i].VertexA = v1;
                EdgeArray[i].VertexB = v2;
                EdgeArray[i].weight = weight;
            }

            for(int k = 0; k < EdgeNum; k++){
                System.out.println(EdgeArray[k].VertexA + " " + EdgeArray[k].VertexB + " " + EdgeArray[k].weight);
            }

            KruskalsAlgorithm(EdgeArray, EdgeNum);

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}

Answer 1

            Node empty = new Node();

        Node [] NodeArray = new Node [NodeNum];
        for(int i = 0; i < NodeNum; i++){
            NodeArray [i] = empty; 
        }

        Node [] EdgeArray = new Node [EdgeNum];
        for(int i = 0; i < EdgeNum; i++){
            EdgeArray [i] = empty; 
        }

this place. you can't initialize like this.NodeArray [i] = empty. because it will refer to the same object. chage to NodeArray [i] = new Node();

Answer 2

You need to understand that when you initialize an object, like Node empty = new Node(); this object is created in the heap, and the reference empty points to It.

When you make a loop like that:

Node [] EdgeArray = new Node [EdgeNum];
    for(int i = 0; i < EdgeNum; i++){
        EdgeArray [i] = empty; 
    }

It means that you initialize each element in the array with that same object that empty refers to. So, later, when you write something like this:

EdgeArray[i].VertexA = v1;
EdgeArray[i].VertexB = v2;
EdgeArray[i].weight = weight;

What actually happens is, that EdgeArray[i].VertexA is referencing to the empty object, that is stored in the heap, so It is equivalent to empty.VertexA , empty.VertexB and so on.

Each iteration of the loop you change the same object, and that's why you get the output

3 0 -4 (enter) 3 0 -4 (enter) 3 0 -4 (enter) 3 0 -4 (enter) 3 0 -4 (enter)

because, these were the values that you entered to empty object in the last iteration of the loop.

I hope this was helpful and that now you know what to do to fix your problem (create new object Node() for each element in the array).