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[source].length - 1;

        int[] topological_sort = new int [number_of_nodes + 1];

```
        int pos = 1;
```
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        int j ;
```

        int visited[] = new int[number_of_nodes + 1];

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        int element = source;
```
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        int i = source;
```
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        visited[source] = 1;
```
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        stack.push(source);
```
```
 
```
```
        while (!stack.isEmpty())
```
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        {
```
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            element = stack.peek();
```

            while (i <= number_of_nodes)

```
            {
```

                if (adjacency_matrix[element][i] == 1 && visited[i] == 1)

```
                {
```

                    if (stack.contains(i))

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                    {
```

                        System.out.println("TOPOLOGICAL SORT NOT POSSIBLE");

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                        return null;
```
```
                    }
```
```
                }  
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                if (adjacency_matrix[element][i] == 1 && visited[i] == 0)

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                {
```
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                    stack.push(i);
```
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                    visited[i] = 1;
```
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                    element = i;
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                    i = 1;
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                    continue;
```
```
                }
```
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                i++;
```
```
            }
```
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            j = stack.pop();	
```

            topological_sort[pos++] = j;

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            i = ++j;
```
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        }
```
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        return topological_sort;
```
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    }	
```
```
 
```

    public static void main(String...arg)

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    {
```
```
        int number_no_nodes, source;
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        Scanner scanner = null;
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        int topological_sort[]  = null;
```
```
        try 
```
```
        {
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            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-- )

```
            {
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                if (topological_sort[i] != 0)

                    System.out.print(topological_sort[i]+"\t");

```
            } 	
```

        }catch(InputMismatchException inputMismatch)

```
         {
```

             System.out.println("Wrong Input format");

         }catch(NullPointerException nullPointer)

```
         {
```
```
         }
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        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

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