Dijkstra in Java

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package dijkstra.java.simple;

import java.util.Arrays;

 * Used to perform the dijkstra Algorithm using adjacency matrices.
 * For a faster implementation, see @see ../fast/Dijkstra.java (using adjacency Lists)
public class Dijkstra {
     * Implementation of dijkstra using adjacency matrix.
     * This returns an array containing the length of the shortest path from the start node to each other node.
     * It is only guaranteed to return correct results if there are no negative edges in the graph. Positive cycles are fine.
     * This has a runtime of O(|V|^2) (|V| = number of Nodes), for a faster implementation see @see ../fast/Dijkstra.java (using adjacency lists)
     * @param graph an adjacency-matrix-representation of the graph where (x,y) is the weight of the edge or 0 if there is no edge.
     * @param start the node to start from.
     * @return an array containing the shortest distances from the given start node to each other node
    public static int[] dijkstra(int[][] graph, int start) {

        //This contains the distances from the start node to all other nodes
        int[] distances = new int[graph.length];
        //Initializing with a distance of "Infinity"
        Arrays.fill(distances, Integer.MAX_VALUE);
        //The distance from the start node to itself is of course 0
        distances[start] = 0;

        //This contains whether a node was already visited
        boolean[] visited = new boolean[graph.length];

        //While there are nodes left to visit...
        while (true) {

            // ... find the node with the currently shortest distance from the start node...
            int shortestDistance = Integer.MAX_VALUE;
            int shortestIndex = -1;
            for (int i = 0; i < graph.length; i++) {
                //... by going through all nodes that haven't been visited yet
                if (distances[i] < shortestDistance && !visited[i]) {
                    shortestDistance = distances[i];
                    shortestIndex = i;

            System.out.println("Visiting node " + shortestDistance + " with current distance " + shortestDistance);

            if (shortestIndex == -1) {
                // There was no node not yet visited --> We are done
                return distances;

            //...then, for all neighboring nodes....
            for (int i = 0; i < graph[shortestIndex].length; i++) {
                //...if the path over this edge is shorter...
                if (graph[shortestIndex][i] != 0 && distances[i] > distances[shortestIndex] + graph[shortestIndex][i]) {
                    //...Save this path as new shortest path.
                    distances[i] = distances[shortestIndex] + graph[shortestIndex][i];
                    System.out.println("Updating distance of node " + i + " to " + distances[i]);
            // Lastly, note that we are finished with this node.
            visited[shortestIndex] = true;
            System.out.println("Visited nodes: " + Arrays.toString(visited));
            System.out.println("Currently lowest distances: " + Arrays.toString(distances));


About the algorithm and language used in this code snippet:

Dijkstra's Algorithm

The Dijkstra algorithm is an algorithm used to solve the shortest path problem in a graph. This means that given a number of nodes and the edges between them as well as the “length” of the edges (referred to as “weight”), the Dijkstra algorithm is finds the shortest path from the specified start node to all other nodes. Nodes are sometimes referred to as vertices (plural of vertex) - here, we’ll call them nodes.

Description of the Algorithm

The basic principle behind the Dijkstra algorithm is to iteratively look at the node with the currently smallest distance to the source and update all not yet visited neighbors if the path to it via the current node is shorter. In more detail, this leads to the following Steps:

  1. Initialize the distance to the starting node as 0 and the distances to all other nodes as infinite
  2. Set all nodes to “unvisited”
  3. While we haven’t visited all nodes:

    1. Find the node with currently shortest distance from the source (for the first pass, this will be the source node itself)
    2. For all nodes next to it that we haven’t visited yet, check if the currently smallest distance to that neighbor is bigger than if we were to go via the current node
    3. If it is, update the smallest distance of that neighbor to be the distance from the source to the current node plus the distance from the current node to that neighbor

In the end, the array we used to keep track of the currently shortest distance from the source to all other nodes will contain the (final) shortest distances.

Example of the Algorithm

Consider the following graph: Graph for the Dijkstra shortest path algorithm

The steps the algorithm performs on this graph if given node 0 as a starting point, in order, are:

  1. Visiting node 0
  2. Updating distance of node 1 to 3
  3. Updating distance of node 2 to 1
  4. Visited nodes: 0
  5. Currently lowest distances: [0, 3, 1, Infinite, Infinite, Infinite]
  6. Visiting node 1 with current distance 1

    • Updating distance of node 3 to 5
    • Visited nodes: 0, 2
    • Currently lowest distances: [0, 3, 1, 5, Infinite, Infinite]
  7. Visiting node 3 with current distance 3

    • Updating distance of node 4 to 4
    • Visited nodes: 0, 1, 2
    • Currently lowest distances: [0, 3, 1, 5, 4, Infinite]
  8. Visiting node 4 with current distance 4

    • Updating distance of node 5 to 5
    • Visited nodes: 0, 1, 2, 4
    • Currently lowest distances: [0, 3, 1, 5, 4, 5]
  9. Visiting node 5 with current distance 5

    • No distances to update
    • Visited nodes: 0, 1, 2, 3, 4
    • Currently lowest distances: [0, 3, 1, 5, 4, 5]
  10. Visiting node 5 with current distance 5

    • No distances to update
    • Visited nodes: 0, 1, 2, 3, 4, 5

All nodes visited Final lowest distances: [0, 3, 1, 5, 4, 5]

Runtime of the Algorithm

The runtime complexity of Dijkstra depends on how it is implemented. If a min-heap is used to determine the next node to visit, and adjacency is implemented using adjacency lists, the runtime is O(|E| + |V|log|V|) (|V| = number of Nodes, |E| = number of Edges). If a we simply search all distances to find the node with the lowest distance in each step, and use a matrix to look up whether two nodes are adjacent, the runtime complexity increases to O(|V|^2).

Space of the Algorithm

The space complexity of Dijkstra depends on how it is implemented as well and is equal to the runtime complexity.


The Java Logo

Java™ is a compiled language used for many purposes, ranging from embedded systems, UI-applications to web servers.

Getting to “Hello World” in Java

The most important things first - here’s how you can run your first line of code in Java.

  1. Download and install the latest version of Java from java.com. You can also download an earlier version if your use case requires it.
  2. Open a terminal, make sure the javac and javac commands are working, and that the command your’re going to be using is referring to the version you just installed by running java -version. If you’re getting a “command not found” error (or similar), try restarting your command line, and, if that doesn’t help, your computer. If the issue persists, here are some helpful StackOverflow questions for each platform:

  3. As soon as that’s working, copy the following snippet into a file named HelloWorld.java:

    class HelloWorld {
    public static void main(String[] args) {
        // Paste any following code snippets here.
        System.out.println("Hello World");
  4. Change directory by typing cd path/to/HelloWorld, then run javac HelloWorld.java to compile the file (which creates the bytecode), then run java HelloWorld (without the .java ending). This should print “Hello World” to your Terminal.

That’s it! Notice that the entry barrier is a little higher with Java than it is with e.g. Python - but Java is much faster and, in my experience, tends to have fewer bugs in large projects due to strong typing and other factors.

Fundamentals in Java

To understand algorithms and technologies implemented in Java, one first needs to understand what basic programming concepts look like in this particular language. Each of the following snippets should be surrounded by the boilerplate code of the hello world example and should be compiled and run using the commands mentioned above.

Variables and Arithmetic

Variables in Java are statically typed, meaning the content of a variable needs to be specified when writing the code. The datatype for whole numbers, for example is int. Numbers with decimal places are typed float or double depending on the required precision. The type for text ist String.

int number = 5;
double decimalNumber = 3.25;
double result = number * decimalNumber;
String callout = "The number is ";
// In this instance, the values are concatenated rather than added because one of them is a String.
System.out.println(callout + result);


Arrays in Java are real arrays (as opposed to e.g. Python where they’re implemented as lists). The implications of that are that the size needs to be set when they are created and cannot be changed, but also that they are more efficient in Java than they are in Python.

int[] integers = new int[5];
integers[3] = 12; // Assigning values to positions in the array
// integers[4] is 0, integers[6] would give IndexOutOfBoundsException
String[] strings = {"Hello", "World"}; // Array initialization with initial values
System.out.println(strings[0] + integers[3]); // Prints "Hello12"


Just like most programming languages, Java can do if-else statements. Additionally, Java can also do switch-case statements.

int value = 5;
if(value == 5){
    System.out.println("Value is 5");
} else if(value < 5){
    System.out.println("Value is less than 5");
} else {
    System.out.println("Value is something else");

switch (value){
    case 1:
        System.out.println("Value is 1");
        break; // Don't go further down the cases
    case 2:
        System.out.println("Value is 2");
        break; // Don't go further down the cases
    case 3:
        System.out.println("Value is 3");
        break; // Don't go further down the cases
    case 4:
        System.out.println("Value is 4");
        break; // Don't go further down the cases
    case 5:
        System.out.println("Value is 5");
        break; // Don't go further down the cases
        System.out.println("Value is something else");

The above Java code will print “Value is 5” twice.


Java supports for, while as well as do while loops. break and continue statements are also supported. The below example illustrates the differences:

int value = 2;
for (int i = 0; i < value; i++) {
while (value > 0) {
do {
} while (value > 0);

This will print the following to the terminal:


Note the last 0: it is printed because in the do-while-loop, compared to the while-loop. the code block is executed at least once before the condition is checked.


Functions in Java can be part of a class, or of an object of a class. For more information on object oriented programming I recommend the w3schools course. Here is a minimal example of a function as part of a class (also called a static function):

class HelloWorld {
    public static void main(String[] args) {
        System.out.println(addNumbers(3, 4));

    public static int addNumbers(int numberOne, int numberTwo) {
        return numberOne + numberTwo;

And here’s an example of calling a function of an object of a class:

class HelloWorld {
    public static void main(String[] args) {
        System.out.println(new HelloWorld().addNumbers(3, 4));

    public int addNumbers(int numberOne, int numberTwo) {
        return numberOne + numberTwo;

Note how the first example uses the static keyword, and the second example needs to instantiate on object of the class before in can call the function of that object. These are some of the differences in class methods and object functions.


Java requires the use of curly brackets ({}) to surround code blocks in conditions, loops, functions etc.; It also requires semicolons at then end of statements. While this can lead to some annoying syntax errors, it also means the use of whitespace for preferred formatting (e.g. indentation of code pieces) does not affect the code.

Advanced Knowledge of Java

Java was first released in 1995 and is multi-paradigm, meaning while it is primarily object-oriented, it also has functional and reflective elements. It’s statically typed, but offers some amount of dynamic typing in recent versions. For more information, Java has a great Wikipedia) article.