Databases
StackOverflow2013
Postsdbo
PONullPointerException Java BFS
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PONullPointerException Java BFS
    primarykey
    data
    text
    <p>I am trying to perform a Breath First Search on a graph that I am creating from an adjacency list from a text file "Input.txt" I am getting a NullPointerException on line 162 where it says "root.visited = true;" I cannot figure out why this is. Here is what code i have.</p> <pre><code> import java.io.BufferedReader; import java.io.FileNotFoundException; import java.io.FileReader; import java.io.IOException; import java.util.ArrayList; import java.util.Iterator; import java.util.LinkedList; import java.util.Queue; import java.util.Stack; public class BFS { public static void main(String[] args) { { Graph g = new Graph(); // Making Adjacency Matrix ArrayList&lt;ArrayList&lt;Integer&gt;&gt; adjacency = new ArrayList&lt;ArrayList&lt;Integer&gt;&gt;(); adjacency.add(new ArrayList&lt;Integer&gt;()); String input = "input.txt"; Graph MyGraph = new Graph(); boolean root = true; Vertex n = null; int j = 0; try { // Reading in the Input File to the Adjacency Matrix BufferedReader reader = new BufferedReader( new FileReader(input)); String line = reader.readLine(); while (line != null) { // Getting numbers from each line String[] numbers = line.split(","); int[] values = new int[numbers.length]; for (int i = 0; i &lt; numbers.length; i++) values[i] += Integer.parseInt(numbers[i]); // Adding values to Matrix for (int i = 0; i &lt; values.length; i++) { adjacency.get(j).add(values[i]); System.out.print(" " + adjacency.get(j).get(i)); // output // of // matrix } System.out.print("\n"); // Progressing through file adjacency.add(new ArrayList&lt;Integer&gt;()); line = reader.readLine(); j++; } // read each line for (int currRow = 0; (line = reader.readLine()) != null; currRow++) { String[] row = line.split(","); // insert vertices if (currRow == 0) { for (int i = 0; i &lt; row.length; i++) { g.addVertex(n); if (root = true) { } } } // create the edges specified in this row for (int i = 0; i &lt; row.length; i++) { // edge exists between indices 'currRow' and col 'i'. if (Integer.parseInt(row[i]) == 1) { g.connectVertex(currRow, i); } } } reader.close(); } catch (FileNotFoundException e) { System.out.println(e); } catch (IOException e) { System.out.println(e); } /* DFS Algorithm */ // schtuff (use stack? use index of node for unique number?) // System.out.print("Visited nodes (in order): "); } // catch exceptions &amp; errors Graph MyGraph = new Graph(); MyGraph.dfs(); } public static class Graph { public Vertex root; public ArrayList&lt;Vertex&gt; vertices = new ArrayList&lt;Vertex&gt;(); public ArrayList&lt;Vertex&gt; dfsArrList = new ArrayList&lt;Vertex&gt;(); public int[][] adjMatrix; int size; public void setRootVertex(Vertex n) { this.root = n; } public Vertex getRootVertex() { return this.root; } public void addVertex(Vertex n) { vertices.add(n); } public void removeVertex(int loc) { vertices.remove(loc); } public void connectVertex(int vertexStart, int vertexEnd) { if (adjMatrix == null) { size = vertices.size(); adjMatrix = new int[size][size]; } int startIndex = vertices.indexOf(vertexStart) + 1; int endIndex = vertices.indexOf(vertexEnd) + 1; adjMatrix[startIndex][endIndex] = 1; adjMatrix[endIndex][startIndex] = 1; } public void removeEdge(Vertex v1, Vertex v2) { int startIndex = vertices.indexOf(v1); int endIndex = vertices.indexOf(v2); adjMatrix[startIndex][endIndex] = 1; adjMatrix[endIndex][startIndex] = 1; } public int countVertices() { int ver = vertices.size(); return ver; } private Vertex getUnvisitedChildNode(Vertex n) { int index = vertices.indexOf(n); int j = 0; while (j &lt; size) { if (adjMatrix[index][j] == 1 &amp;&amp; vertices.get(j).visited == false) { return vertices.get(j); } j++; } return null; } public Iterator&lt;Vertex&gt; dfs() { Stack&lt;Vertex&gt; s = new Stack&lt;Vertex&gt;(); s.push(this.root); root.visited = true; printVertex(root); while (!s.isEmpty()) { Vertex n = s.peek(); Vertex child = getUnvisitedChildNode(n); if (child != null) { child.visited = true; dfsArrList.add(child); s.push(child); } else { s.pop(); } } clearVertices(); return dfsArrList.iterator(); } private void clearVertices() { int i = 0; while (i &lt; size) { Vertex n = vertices.get(i); n.visited = false; i++; } } private void printVertex(Vertex n) { System.out.print(n.vertexName + " "); } } public class Vertex { public char vertexName; public boolean visited = false; public Vertex(char l) { this.vertexName = l; } } } </code></pre>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USxyz
    1. USCfox7
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. COAnd where is the exception stack trace ? Provide only relevant code
      singulars
      1. PONullPointerException Java BFS
      1. USHardik Mishra
    2. COFirst thing to make it easier to track down changes to your variables: make them private. Why would you bother with setters and getters at all while almost all your variables are public? Next: you never appear to be calling setRootVertex...
      singulars
      1. PONullPointerException Java BFS
      1. USJon Skeet
    3. COYour `root` is null.
      singulars
      1. PONullPointerException Java BFS
      1. USShark
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload