Error while redirecting the input from file

Question

Here's my code:

import java.util.Scanner;

class Graph{
boolean [][]array;
int N;
Graph (){
    array = new boolean [1001][1001];
    N=0;
}

void read_graph() {
    Scanner sc = new Scanner(System.in);
    N = sc.nextInt();
    sc.nextLine();

    String str;
    String []substr;

    for (int K=1; K<=N; K++){
        str = sc.nextLine();
        substr = str.split(" ");
        for (int I=0; I<substr.length; I++)
            System.out.println(substr[0]+" "+substr[I]);
    }   
}

void query(){
Scanner sc = new Scanner(System.in);
    int P, Q;
    int counter = 0;
    boolean flag = true;
    while (flag){
    counter++;
    P = sc.nextInt();
    Q = sc.nextInt();
    sc.nextLine();
    if ( P == Q && P == 0 )
        flag =false;
    else {
        if (Q == 1)
            System.out.println("DFS done");
        else
            System.out.println("Bfs done");      
                  }
    }
    }       
}
class demo{
public static void main( String [] args ){
    Graph G= new Graph();
    Scanner sc = new Scanner(System.in);
    int numGraphs = sc.nextInt();
    while (numGraphs>0){
        G.read_graph();
        G.query();
        numGraphs--;
        }
    }
}

Here's the Input data:

1
6
1 2 3 4
2 2 3 6
3 2 1 2
4 1 1
5 0
6 1 2
5 1
1 0
1 0
0 0

When I give this input data with keyboard it works fine but when I saved this input to file and redirected this as input(in linux using '<'), it throws error message.

Exception in thread "main" java.util.NoSuchElementException
at java.util.Scanner.throwFor(Scanner.java:855)
at java.util.Scanner.next(Scanner.java:1478)
at java.util.Scanner.nextInt(Scanner.java:2108)
at java.util.Scanner.nextInt(Scanner.java:2067)
at Graph.read_graph(b.java:13)
at demo.main(b.java:56)

Help me in pointing out the mistake.

Answer 1

Don't create a Scanner object in every method. Pass the first Scanner object you have created around.

Here is a list of changes that should fix the issue:

--- demo-old.java   2012-01-25 23:12:54.000000000 +0530
+++ demo.java   2012-01-25 23:13:45.000000000 +0530
@@ -10,4 +10,3 @@

-void read_graph() {
-    Scanner sc = new Scanner(System.in);
+void read_graph(Scanner sc) {
     N = sc.nextInt();
@@ -26,4 +25,3 @@

-void query(){
-Scanner sc = new Scanner(System.in);
+void query(Scanner sc){
     int P, Q;
@@ -53,4 +51,4 @@
     while (numGraphs>0){
-        G.read_graph();
-        G.query();
+        G.read_graph(sc);
+        G.query(sc);
         numGraphs--;

Answer 2

Why are you creating 3 scanners? It is possible that it is choking in the lines

1) P = sc.nextInt();
2) Q = sc.nextInt();

because the input with only 1 int is being read in line 1 and then line 2 is trying to scan the nextInt() for an empty line.

I have no idea why this would work when inputing by hand, unless the input is in a different order.

Answer 3

You should not use < to redirect input. You need to use scanner class to read from file.

File file = new File("data.txt");
Scanner scanner = new Scanner(file);
while (scanner.hasNextLine()) {
//logic
}