Java Program to Create a Random Graph using Random Edge Generation

This is a java program to generate a random graph by generating random number of edges. One important thing to note here is, that we need to decide minimum and maximum number of nodes such that all edges get accommodated. Minimum number of vertices is positive solution to n(n-1) = 2e, where e is number of edges and maximum number of vertices is e+1.

Here is the source code of the Java Program to Create a Random Graph Using Random Edge Generation. The Java program is successfully compiled and run on a Windows system. The program output is also shown below.

//This is a java program to generate a random graph using random edge generation

```
import java.util.HashMap;
```
```
import java.util.LinkedList;
```
```
import java.util.List;
```
```
import java.util.Map;
```
```
import java.util.Random;
```
```
import java.util.Scanner;
```
```
 
```
```
public class Random_Edges_Graph 
```
```
{
```

    private Map<Integer, List<Integer>> adjacencyList;

```
 
```
```
    public Random_Edges_Graph(int v) 
```
```
    {
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        adjacencyList = new HashMap<Integer, List<Integer>>();

```
        for (int i = 1; i <= v; i++)
```

            adjacencyList.put(i, new LinkedList<Integer>());

```
    }
```
```
 
```

    public void setEdge(int to, int from)

```
    {
```

        if (to > adjacencyList.size() || from > adjacencyList.size())

            System.out.println("The vertices does not exists");

```
 
```

        List<Integer> sls = adjacencyList.get(to);

```
        sls.add(from);
```

        List<Integer> dls = adjacencyList.get(from);

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        dls.add(to);
```
```
    }
```
```
 
```

    public List<Integer> getEdge(int to)

```
    {
```
```
        if (to > adjacencyList.size()) 
```
```
        {
```

            System.out.println("The vertices does not exists");

```
            return null;
```
```
        }
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```
        return adjacencyList.get(to);
```
```
    }
```
```
 
```

    public static void main(String args[])

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    {
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        System.out.println("Enter the number of edges: ");

```
 
```

        Scanner sc = new Scanner(System.in);

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        int e = sc.nextInt();
```
```
        try 
```
```
        {
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            int minV = (int) Math.ceil((1 + Math.sqrt(1 + 8 * e)) / 2);

```
            int maxV = e + 1;
```
```
 
```

            Random random = new Random();

            int v = Math.abs(random.nextInt(maxV - minV) + minV);

            System.out.println("Random graph has "+v+" vertices");

```
 
```

            Random_Edges_Graph reg = new Random_Edges_Graph(v);

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            int count = 1, to, from;
```
```
            while (count <= e) 
```
```
            {
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                to = Math.abs(random.nextInt(v + 1 - 1) + 1);

                from = Math.abs(random.nextInt(v + 1 - 1) + 1);

```
 
```
```
                reg.setEdge(to, from);
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```
                count++;
```
```
            }
```
```
 
```
```
            System.out
```

                    .println("THe Adjacency List Representation of the random graph is: ");

```
 
```

            for (int i = 1; i <= v; i++)

```
            {
```

                System.out.print(i + " -> ");

                List<Integer> edgeList = reg.getEdge(i);

                if (edgeList.size() == 0)

                    System.out.print("null");

```
                else 
```
```
                {
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                    for (int j = 1;; j++)

```
                    {
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                        if (j != edgeList.size())

                            System.out.print(edgeList.get(j - 1) + " -> ");

```
                        else {
```

                            System.out.print(edgeList.get(j - 1));

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                            break;
```
```
                        }
```
```
                    }
```
```
                }
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```
                System.out.println();
```
```
            }
```
```
        } 
```
```
        catch (Exception E) 
```
```
        {
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            System.out.println("Something went wrong");

```
        }
```
```
        sc.close();
```
```
    }
```
```
 
```
```
}
```

Output:

$ javac Random_Edges_Graph.java
$ java Random_Edges_Graph
 
Enter the number of edges: 
5
Random graph has 4 vertices
THe Adjacency List Representation of the random graph is: 
1 -> 4 -> 4
2 -> 3 -> 3
3 -> 2 -> 4 -> 2
4 -> 1 -> 1 -> 3

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