Java Program for Topological Sorting in Graphs

This Java program,to perform the topological Sort on a given graph by the DFS method.The topological sort is performed on a directed acyclic graph.

Here is the source code of the Java program to perform the Topological Sort on the graph. The Java program is successfully compiled and run on a Linux system. The program output is also shown below.

import java.util.InputMismatchException;
import java.util.Scanner;
import java.util.Stack;
 
public class TopologicalSort
{
    private Stack<Integer> stack;
 
    public TopologicalSort() 
    {
        stack = new Stack<Integer>();
    }
 
    public int [] topological(int adjacency_matrix[][], int source) throws NullPointerException
    {
        int number_of_nodes = adjacency_matrix.length - 1;
        int[] topological_sort = new int [number_of_nodes + 1];
        int pos = 1;
        int j ;
        int visited[] = new int[number_of_nodes + 1];
        int element = source;
        int i = source;
        visited = 1;
        stack.push(source);
 
        while (!stack.isEmpty())
        {
            element = stack.peek();
            while (i <= number_of_nodes)
            {
                if (adjacency_matrix[element][i] == 1 && visited[i] == 1)
                {
                    if (stack.contains(i))
                    {
                        System.out.println("TOPOLOGICAL SORT NOT POSSIBLE");
                        return null;
                    }
                }  
                if (adjacency_matrix[element][i] == 1 && visited[i] == 0)
                {
                    stack.push(i);
                    visited[i] = 1;
                    element = i;
                    i = 1;
                    continue;
                }
                i++;
            }
            j = stack.pop();	
            topological_sort[pos++] = j;
            i = ++j;
        }
        return topological_sort;
    }	
 
    public static void main(String...arg)
    {
        int number_no_nodes, source;
        Scanner scanner = null;
        int topological_sort[]  = null;
        try 
        {
            System.out.println("Enter the number of nodes in the graph");
            scanner = new Scanner(System.in);
            number_no_nodes = scanner.nextInt();
 
            int adjacency_matrix[][] = new int[number_no_nodes + 1][number_no_nodes + 1];
            System.out.println("Enter the adjacency matrix");
	    for (int i = 1; i <= number_no_nodes; i++)
                for (int j = 1; j <= number_no_nodes; j++)
                    adjacency_matrix[i][j] = scanner.nextInt();
 
            System.out.println("Enter the source for the graph");
            source = scanner.nextInt();
 
            System.out.println("The Topological sort for the graph is given by ");
            TopologicalSort toposort = new TopologicalSort();
            topological_sort = toposort.topological(adjacency_matrix, source);
            System.out.println();
            for (int i = topological_sort.length - 1; i > 0; i-- )
            {
                if (topological_sort[i] != 0)
                    System.out.print(topological_sort[i]+"\t");
            } 	
        }catch(InputMismatchException inputMismatch)
         {
             System.out.println("Wrong Input format");
         }catch(NullPointerException nullPointer)
         {
         }
        scanner.close();
    }
}

$javac TopologicalSort.java
$java TopologicalSort
Enter the number of nodes in the graph
4
Enter the adjacency matrix
0 1 0 0
0 0 1 1
0 0 0 0
0 0 0 0
Enter the source for the graph
1
The Topological sort for the graph is given by 
1	2	4	3