我正在尝试在一个图表上执行呼吸优先搜索，我从一个文本文件“Input.txt”的邻接列表创建我在第162行得到一个NullPointerException，其中显示“root.visited = true; “我无法弄清楚为什么会这样。这是我的代码。
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<ArrayList<Integer>> adjacency = new ArrayList<ArrayList<Integer>>();
adjacency.add(new ArrayList<Integer>());
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 < numbers.length; i++)
values[i] += Integer.parseInt(numbers[i]);
// Adding values to Matrix
for (int i = 0; i < 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<Integer>());
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 < row.length; i++) {
g.addVertex(n);
if (root = true) {
}
}
}
// create the edges specified in this row
for (int i = 0; i < 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 & errors
Graph MyGraph = new Graph();
MyGraph.dfs();
}
public static class Graph {
public Vertex root;
public ArrayList<Vertex> vertices = new ArrayList<Vertex>();
public ArrayList<Vertex> dfsArrList = new ArrayList<Vertex>();
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 < size) {
if (adjMatrix[index][j] == 1
&& vertices.get(j).visited == false) {
return vertices.get(j);
}
j++;
}
return null;
}
public Iterator<Vertex> dfs() {
Stack<Vertex> s = new Stack<Vertex>();
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 < 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;
}
}
}
答案 0 :(得分：1)
看看这段代码（在main的支撑部分之后略显奇怪）：
Graph MyGraph = new Graph();
MyGraph.dfs();
dfs方法以：
Stack<Vertex> s = new Stack<Vertex>();
s.push(this.root);
root.visited = true;
您希望MyGraph.root在这里期待什么？它怎么可能是null以外的东西？ （请注意，您在MyGraph中声明了两个main变量。由于某种原因，第一个变量没有用于任何事情。）
我强烈建议您重新构建此代码：
main方法重构为为了避免引入新的范围 - 主要方法是有效的：
public static void main(String[] args) {
{
// Code
}
}
很奇怪。
答案 1 :(得分：0)
Vertex root未初始化，因此root.visited在运行时变为null.visited，导致NullPointerException。
Vertex root应按setRootVertex方法设置值。
答案 2 :(得分：0)
当您调用
MyGraph.dfs（）时;
public static class Graph {
public Vertex root;
您的root未初始化。