Java Program to Find MST (Minimum Spanning Tree) using Prim’s Algorithm

This Java program is to find MST using Prim’s algorithm.In computer science, Prim’s algorithm is a greedy algorithm that finds a minimum spanning tree for a connected weighted undirected graph. This means it finds a subset of the edges that forms a tree that includes every vertex, where the total weight of all the edges in the tree is minimized.

Here is the source code of the Java program to find MST using prim’s algorithm. 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;
 
public class Prims
{
    private boolean unsettled[];
    private boolean settled[];
    private int numberofvertices;
    private int adjacencyMatrix[][];
    private int key[];
    public static final int INFINITE = 999;
    private int parent[];
 
    public Prims(int numberofvertices)
    {
        this.numberofvertices = numberofvertices;
        unsettled = new boolean[numberofvertices + 1];
        settled = new boolean[numberofvertices + 1];
        adjacencyMatrix = new int[numberofvertices + 1][numberofvertices + 1];
        key = new int[numberofvertices + 1];
        parent = new int[numberofvertices + 1];
    }
 
    public int getUnsettledCount(boolean unsettled[])
    {
        int count = 0;
        for (int index = 0; index < unsettled.length; index++)
        {
            if (unsettled[index])
            {
                count++;
            }
        }
        return count;
    }
 
    public void primsAlgorithm(int adjacencyMatrix[][])
    {
        int evaluationVertex;
        for (int source = 1; source <= numberofvertices; source++)
        {
            for (int destination = 1; destination <= numberofvertices; destination++)
            {
                this.adjacencyMatrix[destination] = adjacencyMatrix[destination];
            }
        }
 
        for (int index = 1; index <= numberofvertices; index++)
        {
            key[index] = INFINITE;
        }
        key[1] = 0;
        unsettled[1] = true;
        parent[1] = 1;
 
        while (getUnsettledCount(unsettled) != 0)
        {
            evaluationVertex = getMimumKeyVertexFromUnsettled(unsettled);
            unsettled[evaluationVertex] = false;
            settled[evaluationVertex] = true;
            evaluateNeighbours(evaluationVertex);
        }
    } 
 
    private int getMimumKeyVertexFromUnsettled(boolean[] unsettled2)
    {
        int min = Integer.MAX_VALUE;
        int node = 0;
        for (int vertex = 1; vertex <= numberofvertices; vertex++)
        {
            if (unsettled[vertex] == true && key[vertex] < min)
            {
                node = vertex;
                min = key[vertex];
            }
        }
        return node;
    }
 
    public void evaluateNeighbours(int evaluationVertex)
    {
 
        for (int destinationvertex = 1; destinationvertex <= numberofvertices; destinationvertex++)
        {
            if (settled[destinationvertex] == false)
            {
                if (adjacencyMatrix[evaluationVertex][destinationvertex] != INFINITE)
                {
                    if (adjacencyMatrix[evaluationVertex][destinationvertex] < key[destinationvertex])
                    {
                        key[destinationvertex] = adjacencyMatrix[evaluationVertex][destinationvertex];
                        parent[destinationvertex] = evaluationVertex;
                    }
                    unsettled[destinationvertex] = true;
                }
            }
        }
    }
 
    public void printMST()
    {
        System.out.println("SOURCE  : DESTINATION = WEIGHT");
        for (int vertex = 2; vertex <= numberofvertices; vertex++)
        {
            System.out.println(parent[vertex] + "\t:\t" + vertex +"\t=\t"+ adjacencyMatrix[parent[vertex]][vertex]);
        }
    }
 
    public static void main(String... arg)
    {
        int adjacency_matrix[][];
        int number_of_vertices;
        Scanner scan = new Scanner(System.in);
 
        try
        {
            System.out.println("Enter the number of vertices");
            number_of_vertices = scan.nextInt();
            adjacency_matrix = new int[number_of_vertices + 1][number_of_vertices + 1];
 
            System.out.println("Enter the Weighted Matrix for the graph");
            for (int i = 1; i <= number_of_vertices; i++)
            {
                for (int j = 1; j <= number_of_vertices; j++)
                {
                    adjacency_matrix[i][j] = scan.nextInt();
                    if (i == j)
                    {
                        adjacency_matrix[i][j] = 0;
                        continue;
                    }
                    if (adjacency_matrix[i][j] == 0)
                    {
                        adjacency_matrix[i][j] = INFINITE;
                    }
                }
            }
 
            Prims prims = new Prims(number_of_vertices);
            prims.primsAlgorithm(adjacency_matrix);
            prims.printMST();
 
        } catch (InputMismatchException inputMismatch)
        {
            System.out.println("Wrong Input Format");
        }
        scan.close();
    }
}

$javac Prims.java
$java Prims
 
Enter the number of vertices
5
Enter the Weighted Matrix for the graph
0 4 0 0 5
4 0 3 6 1 
0 3 0 6 2
0 6 6 0 7
5 1 2 7 0
SOURCE  : DESTINATION   =    WEIGHT
1       :      2        =       4
5       :      3        =       2
2       :      4        =       6
2       :      5        =       1