import java.io.*;
import java.util.Scanner;

public class floyd {
    
    public static void main(String[] args) throws FileNotFoundException {
        if(args.length != 1) {
            System.err.println("Usage: floyd <graph-file>");
            return;
        }

        String inputFile = args[0];
        Scanner scanner = new Scanner(new File(inputFile));
        PrintWriter writer = new PrintWriter("output.txt");

        // To skip non-integer lines such as "Problem1 Amatrix: n = 7"
        while(scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();
            // Try to extract ‘n’ from lines like "Problem1 Amatrix: n = 7"
            if(line.startsWith("Problem") && line.contains("n =")) {
                int n = Integer.parseInt(line.split("n =")[1].trim());
                // Now we know ‘n’, we can proceed with the adjacency matrix
                int[][] dist = new int[n][n];
                int[][] next = new int[n][n];

                // Read the matrix
                for(int i = 0; i < n; i++) {
                    if(scanner.hasNextLine()) {
                        line = scanner.nextLine().trim();
                        String[] parts = line.split("\\s+");
                        for(int j = 0; j < n; j++) {
                            dist[i][j] = Integer.parseInt(parts[j]);
                            if(dist[i][j] != 0 && i != j) {
                                next[i][j] = j;
                            }
                        }
                    }
                }

                for (int k = 0; k < n; ++k) {
                    for (int i = 0; i < n; ++i) {
                        for (int j = 0; j < n; ++j) {
                            if (dist[i][j] > dist[i][k] + dist[k][j]) {
                                dist[i][j] = dist[i][k] + dist[k][j];
                                next[i][j] = next[i][k];
                            }
                        }
                    }
                }
    
                writer.println("Problem" + ": n = " + n);
                writer.println("Pmatrix:");
                for (int i = 0; i < n; i++) {
                    for (int j = 0; j < n; j++) {
                        writer.print(next[i][j] + "  ");
                    }
                    writer.println();
                }
    
                // Reconstructing paths
                for (int i = 0; i < n; i++) {
                    for (int j = 0; j < n; j++) {
                        if (i != j) {
                            int u = i;
                            String path = "V" + (i+1);
                            while(u != j){
                                u = next[u][j];
                                path += " V" + (u+1);
                            }
                            writer.println("V" + (i+1) + "-V" + (j+1) + ": shortest path and length");
                            writer.println(path + ": " + dist[i][j]);
                        }
                    }
                }
            }
       }
       scanner.close();
       writer.close();
    }
}
./floyd graphfile.txt
Exception in thread "main" java.lang.IllegalStateException: Scanner closed
        at java.base/java.util.Scanner.ensureOpen(Scanner.java:1154)
        at java.base/java.util.Scanner.findWithinHorizon(Scanner.java:1786)
        at java.base/java.util.Scanner.hasNextLine(Scanner.java:1615)
        at floyd.main(floyd.java:17) The input file graphfile.txt is Problem1 Amatrix: n = 7
0  6  5  4  6  3  6  
6  0  6  4  5  5  3  
5  6  0  3  1  4  6  
4  4  3  0  4  1  4  
6  5  1  4  0  5  5  
3  5  4  1  5  0  3  
6  3  6  4  5  3  0  

Problem2 Amatrix: n = 6
0  1  2  1  3  4  
1  0  3  2  2  3  
2  3  0  3  3  6  
1  2  3  0  3  5  
3  2  3  3  0  5  
4  3  6  5  5  0 and the output is Problem: n = 7
Pmatrix:
0  1  2  3  4  5  6  
0  0  2  3  4  5  6  
0  1  0  3  4  5  6  
0  1  2  0  4  5  6  
0  1  2  3  0  5  6  
0  1  2  3  4  0  6  
0  1  2  3  4  5  0  
V1-V2: shortest path and length
V1 V2: 6
V1-V3: shortest path and length
V1 V3: 5
V1-V4: shortest path and length
V1 V4: 4
V1-V5: shortest path and length
V1 V5: 6
V1-V6: shortest path and length
V1 V6: 3
V1-V7: shortest path and length
V1 V7: 6
V2-V1: shortest path and length
V2 V1: 6
V2-V3: shortest path and length
V2 V3: 6
V2-V4: shortest path and length
V2 V4: 4
V2-V5: shortest path and length
V2 V5: 5
V2-V6: shortest path and length
V2 V6: 5
V2-V7: shortest path and length
V2 V7: 3
V3-V1: shortest path and length
V3 V1: 5
V3-V2: shortest path and length
V3 V2: 6
V3-V4: shortest path and length
V3 V4: 3
V3-V5: shortest path and length
V3 V5: 1
V3-V6: shortest path and length
V3 V6: 4
V3-V7: shortest path and length
V3 V7: 6
V4-V1: shortest path and length
V4 V1: 4
V4-V2: shortest path and length
V4 V2: 4
V4-V3: shortest path and length
V4 V3: 3
V4-V5: shortest path and length
V4 V5: 4
V4-V6: shortest path and length
V4 V6: 1
V4-V7: shortest path and length
V4 V7: 4
V5-V1: shortest path and length
V5 V1: 6
V5-V2: shortest path and length
V5 V2: 5
V5-V3: shortest path and length
V5 V3: 1
V5-V4: shortest path and length
V5 V4: 4
V5-V6: shortest path and length
V5 V6: 5
V5-V7: shortest path and length
V5 V7: 5
V6-V1: shortest path and length
V6 V1: 3
V6-V2: shortest path and length
V6 V2: 5
V6-V3: shortest path and length
V6 V3: 4
V6-V4: shortest path and length
V6 V4: 1
V6-V5: shortest path and length
V6 V5: 5
V6-V7: shortest path and length
V6 V7: 3
V7-V1: shortest path and length
V7 V1: 6
V7-V2: shortest path and length
V7 V2: 3
V7-V3: shortest path and length
V7 V3: 6
V7-V4: shortest path and length
V7 V4: 4
V7-V5: shortest path and length
V7 V5: 5
V7-V6: shortest path and length
V7 V6: 3
 which is incorrect. The expected output is Problem1: n = 7
Pmatrix: 
0  0  0  6  3  0  6  
0  0  5  0  0  4  0  
0  5  0  0  0  0  5  
6  0  0  0  3  0  6  
3  0  0  3  0  3  0  
0  4  0  0  3  0  0  
6  0  5  6  0  0  0 

V1-Vj: shortest path and length
V1 V1: 0
V1 V2: 6
V1 V3: 5
V1 V6 V4: 4
V1 V3 V5: 6
V1 V6: 3
V1 V6 V7: 6

V2-Vj: shortest path and length
V2 V1: 6
V2 V2: 0
V2 V5 V3: 6
V2 V4: 4
V2 V5: 5
V2 V4 V6: 5
V2 V7: 3

V3-Vj: shortest path and length
V3 V1: 5
V3 V5 V2: 6
V3 V3: 0
V3 V4: 3
V3 V5: 1
V3 V6: 4
V3 V5 V7: 6

V4-Vj: shortest path and length
V4 V6 V1: 4
V4 V2: 4
V4 V3: 3
V4 V4: 0
V4 V3 V5: 4
V4 V6: 1
V4 V6 V7: 4

V5-Vj: shortest path and length
V5 V3 V1: 6
V5 V2: 5
V5 V3: 1
V5 V3 V4: 4
V5 V5: 0
V5 V3 V6: 5
V5 V7: 5

V6-Vj: shortest path and length
V6 V1: 3
V6 V4 V2: 5
V6 V3: 4
V6 V4: 1
V6 V3 V5: 5
V6 V6: 0
V6 V7: 3

V7-Vj: shortest path and length
V7 V6 V1: 6
V7 V2: 3
V7 V5 V3: 6
V7 V6 V4: 4
V7 V5: 5
V7 V6: 3
V7 V7: 0

Problem2: n = 6
Pmatrix:
0  0  0  0  2  2  
0  0  1  1  0  0  
0  1  0  1  0  2  
0  1  1  0  0  2  
2  0  0  0  0  2  
2  0  2  2  2  0  

V1-Vj: shortest path and length
V1 V1: 0
V1 V2: 1
V1 V3: 2
V1 V4: 1
V1 V2 V5: 3
V1 V2 V6: 4

V2-Vj: shortest path and length
V2 V1: 1
V2 V2: 0
V2 V1 V3: 3
V2 V1 V4: 2
V2 V5: 2
V2 V6: 3

V3-Vj: shortest path and length
V3 V1: 2
V3 V1 V2: 3
V3 V3: 0
V3 V1 V4: 3
V3 V5: 3
V3 V1 V2 V6: 6

V4-Vj: shortest path and length
V4 V1: 1
V4 V1 V2: 2
V4 V1 V3: 3
V4 V4: 0
V4 V5: 3
V4 V1 V2 V6: 5

V5-Vj: shortest path and length
V5 V2 V1: 3
V5 V2: 2
V5 V3: 3
V5 V4: 3
V5 V5: 0
V5 V2 V6: 5

V6-Vj: shortest path and length
V6 V2 V1: 4
V6 V2: 3
V6 V2 V1 V3: 6
V6 V2 V1 V4: 5
V6 V2 V5: 5
V6 V6: 0