Where does it go wrong??

Question

In this application, i want the user to enter a number, but somehow the conversion of the value entered doesn't work. When I start the application and enter any value I get java.lang.NumberFormatException.How should I change the code??
import java.io.*;
import java.lang.*;
import java.awt.*;
public class Assignment5
public static void main( String args[] ) throws IOException
final int maxvert = 20; // Maximun number of vertices allowed
int n; // Number of vertex in this instance
char Ans = 'n'; // Use for loop
int w[][]; // Inputted graph represented by matrix
int s[][] = new int [maxvert][maxvert]; // Minimum spanning tree graph
int nearest[] = new int [maxvert]; // Index of vertex nearest to vi
int distance[] = new int [maxvert]; // Weight on edge between vi and the vertex
int near = 0; // The index for which distance[i] is the smallest
int tmp_nearest, min;
File f_in;
FileInputStream input;
int len;
int f_len, max, ch, edge;
String str;
String oneChar;
String infile = "in.dat";
String st = new String("");
do
w = new int [maxvert][maxvert]; // Inputted graph represented by matrix
System.out.println("Prims Algorithm!");
System.out.println("This Algorithm finds the Minimum Spanning Tree of a graph.");
System.out.println("How many vetices do you want (Max = "+ maxvert +" ) ?");
while((ch = System.in.read()) != '\n')
oneChar = String.valueOf( (char)ch );
st = st.concat(oneChar);
n = Integer.parseInt(st);
System.out.println("The vertices = "+n);
System.out.println("Enter graph in matrix form, ");
System.out.println("(0 means a loop, -1 means on edge)");
for (int x = 0; x < n; x++)
for (int y = 0; y < n; y++)
System.out.print("(V "+x+", V "+y+") : ");
st = new String("");
while((ch = System.in.read()) != '\n')
oneChar = String.valueOf( (char)ch );
st = st.concat(oneChar);
w[x][y] = Integer.parseInt(st);
if (w[x][y] == -1) // We use 1000 instead of infinity or -1
w[x][y] = 1000;
System.out.println();
System.out.println();
//int minus_one = -1;
for (int x = 0; x < n; x++) //Formatted output, so it looks neat and lined-up
System.out.print(" ");
for (int y = 0; y < n; y++)
if ((w[x][y] >= 100) && (w[x][y] <= 999))
System.out.print(+w[x][y]);
if ((w[x][y] >= 10) && (w[x][y] <= 99))
System.out.print(" "+w[x][y]);
if ((w[x][y] >= 0) && (w[x][y] <= 9))
System.out.print(" "+w[x][y]);
if (w[x][y] == 1000)
System.out.print(" -1");
System.out.println();
System.out.println("Is this correct ?");
//st = new String("");
Ans = (char)System.in.read();
}while((Ans != 'y') && (Ans != 'Y'));
for (int i = 0; i < n; i++) // initialize Spanning tree array
for (int j = 0; j < n; j++)
if (i == j)
s[i][j] = 0;
else
s[i][j] = 1000;
for (int i = 0; i < n; i++)
nearest[i] = 0;
// For all vertices, initialize vi to be the
// to be the nearest vertex in the set.
distance[i] = w[0];
// initialize the distance from the set
// to be the weight on the edge to v1.
for (int z = 1; z <= n - 1; z++)
min = 1000;
for (int i = 1; i < n; i++) // Check each vertex to see which on is the nearest
if ((distance[i] >= 0) && (distance[i] < min))
min = distance[i];
near = i;
tmp_nearest = nearest[near];
s[near][tmp_nearest] = w[near][tmp_nearest]; //Add it to the set
distance[near] = -1;
for (int i = 1; i < n; i++)
if (w[i][near] < distance[i])
distance[i] = w[i][near]; // Update its distance
nearest[i] = near;
System.out.println();
System.out.println("The minimum spanning tree is :");
for (int x = 0; x < n; x++)
System.out.print(" ");
for (int y = 0; y < n; y++)
if ((s[x][y] >= 100) && (s[x][y] <= 999))
System.out.print(+s[x][y]);
if ((s[x][y] >= 10) && (s[x][y] <= 99))
System.out.print(" "+s[x][y]);
if ((s[x][y] >= 0) && (s[x][y] <= 9))
System.out.print(" "+s[x][y]);
if (s[x][y] == 1000)
System.out.print(" -1");
System.out.println();
System.out.println();

Answer 1

Usen = Integer.parseInt(st.trim());rather than "st" itself. You are forgetting that in DOS (which you are probably using?) the line separator ir \r\n and the \r makes parseInt fail.
I would also suggest making the code a little bit more readable using a BufferedReader to read the whole line as a String for you instead of doing it all yourself...