// Java implementation of Dijkstra's Algorithm 
// using Priority Queue 
import java.util.*; 
public class DPQ { 
	private int dist[]; 
	private Set<Integer> settled; 
	private PriorityQueue<Node> pq; 
	private int V; // Number of vertices 
	List<List<Node> > adj; 

	public DPQ(int V) 
	{ 
		this.V = V; 
		dist = new int[V]; 
		settled = new HashSet<Integer>(); 
		pq = new PriorityQueue<Node>(V, new Node()); 
	} 

	// Function for Dijkstra's Algorithm 
	public void dijkstra(List<List<Node> > adj, int src) 
	{ 
		this.adj = adj; 

		for (int i = 0; i < V; i++) 
			dist[i] = Integer.MAX_VALUE; 

		// Add source node to the priority queue 
		pq.add(new Node(src, 0)); 

		// Distance to the source is 0 
		dist[src] = 0; 
		while (settled.size() != V) { 

			// remove the minimum distance node 
			// from the priority queue 
			int u = pq.remove().node; 

			// adding the node whose distance is 
			// finalized 
			settled.add(u); 

			e_Neighbours(u); 
		} 
	} 

	// Function to process all the neighbours 
	// of the passed node 
	private void e_Neighbours(int u) 
	{ 
		int edgeDistance = -1; 
		int newDistance = -1; 

		// All the neighbors of v 
		for (int i = 0; i < adj.get(u).size(); i++) { 
			Node v = adj.get(u).get(i); 

			// If current node hasn't already been processed 
			if (!settled.contains(v.node)) { 
				edgeDistance = v.cost; 
				newDistance = dist[u] + edgeDistance; 

				// If new distance is cheaper in cost 
				if (newDistance < dist[v.node]) 
					dist[v.node] = newDistance; 

				// Add the current node to the queue 
				pq.add(new Node(v.node, dist[v.node])); 
			} 
		} 
	} 

	// Driver code 
	public static void main(String arg[]) 
	{ 
		int V = 5; 
		int source = 0; 

		// Adjacency list representation of the 
		// connected edges 
		List<List<Node> > adj = new ArrayList<List<Node> >(); 

		// Initialize list for every node 
		for (int i = 0; i < V; i++) { 
			List<Node> item = new ArrayList<Node>(); 
			adj.add(item); 
		} 

		// Inputs for the DPQ graph 
		adj.get(0).add(new Node(1, 9)); 
		adj.get(0).add(new Node(2, 6)); 
		adj.get(0).add(new Node(3, 5)); 
		adj.get(0).add(new Node(4, 3)); 

		adj.get(2).add(new Node(1, 2)); 
		adj.get(2).add(new Node(3, 4)); 

		// Calculate the single source shortest path 
		DPQ dpq = new DPQ(V); 
		dpq.dijkstra(adj, source); 

		// Print the shortest path to all the nodes 
		// from the source node 
		System.out.println("The shorted path from node :"); 
		for (int i = 0; i < dpq.dist.length; i++) 
			System.out.println(source + " to " + i + " is "
							+ dpq.dist[i]); 
	} 
} 

// Class to represent a node in the graph 
class Node implements Comparator<Node> { 
	public int node; 
	public int cost; 

	public Node() 
	{ 
	} 

	public Node(int node, int cost) 
	{ 
		this.node = node; 
		this.cost = cost; 
	} 

	@Override
	public int compare(Node node1, Node node2) 
	{ 
		if (node1.cost < node2.cost) 
			return -1; 
		if (node1.cost > node2.cost) 
			return 1; 
		return 0; 
	} 
} 
 
by

Java online compiler

Write, Run & Share Java code online using OneCompiler's Java online compiler for free. It's one of the robust, feature-rich online compilers for Java language, running the Java LTS version 17. Getting started with the OneCompiler's Java editor is easy and fast. The editor shows sample boilerplate code when you choose language as Java and start coding.

Taking inputs (stdin)

OneCompiler's Java online editor supports stdin and users can give inputs to the programs using the STDIN textbox under the I/O tab. Using Scanner class in Java program, you can read the inputs. Following is a sample program that shows reading STDIN ( A string in this case ).

import java.util.Scanner;
class Input {
    public static void main(String[] args) {
    	Scanner input = new Scanner(System.in);
    	System.out.println("Enter your name: ");
    	String inp = input.next();
    	System.out.println("Hello, " + inp);
    }
}

Adding dependencies

OneCompiler supports Gradle for dependency management. Users can add dependencies in the build.gradle file and use them in their programs. When you add the dependencies for the first time, the first run might be a little slow as we download the dependencies, but the subsequent runs will be faster. Following sample Gradle configuration shows how to add dependencies

apply plugin:'application'
mainClassName = 'HelloWorld'

run { standardInput = System.in }
sourceSets { main { java { srcDir './' } } }

repositories {
    jcenter()
}

dependencies {
    // add dependencies here as below
    implementation group: 'org.apache.commons', name: 'commons-lang3', version: '3.9'
}

About Java

Java is a very popular general-purpose programming language, it is class-based and object-oriented. Java was developed by James Gosling at Sun Microsystems ( later acquired by Oracle) the initial release of Java was in 1995. Java 17 is the latest long-term supported version (LTS). As of today, Java is the world's number one server programming language with a 12 million developer community, 5 million students studying worldwide and it's #1 choice for the cloud development.

Syntax help

Variables

short x = 999; 			// -32768 to 32767
int   x = 99999; 		// -2147483648 to 2147483647
long  x = 99999999999L; // -9223372036854775808 to 9223372036854775807

float x = 1.2;
double x = 99.99d;

byte x = 99; // -128 to 127
char x = 'A';
boolean x = true;

Loops

1. If Else:

When ever you want to perform a set of operations based on a condition If-Else is used.

if(conditional-expression) {
  // code
} else {
  // code
}

Example:

int i = 10;
if(i % 2 == 0) {
  System.out.println("i is even number");
} else {
  System.out.println("i is odd number");
}

2. Switch:

Switch is an alternative to If-Else-If ladder and to select one among many blocks of code.

switch(<conditional-expression>) {    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
...    
    
default:     
 //code to be executed when all the above cases are not matched;    
} 

3. For:

For loop is used to iterate a set of statements based on a condition. Usually for loop is preferred when number of iterations is known in advance.

for(Initialization; Condition; Increment/decrement){  
    //code  
} 

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while(<condition>){  
 // code 
}  

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {
  // code 
} while (<condition>); 

Classes and Objects

Class is the blueprint of an object, which is also referred as user-defined data type with variables and functions. Object is a basic unit in OOP, and is an instance of the class.

How to create a Class:

class keyword is required to create a class.

Example:

class Mobile {
    public:    // access specifier which specifies that accessibility of class members 
    string name; // string variable (attribute)
    int price; // int variable (attribute)
};

How to create a Object:

Mobile m1 = new Mobile();

How to define methods in a class:

public class Greeting {
    static void hello() {
        System.out.println("Hello.. Happy learning!");
    }

    public static void main(String[] args) {
        hello();
    }
}

Collections

Collection is a group of objects which can be represented as a single unit. Collections are introduced to bring a unified common interface to all the objects.

Collection Framework was introduced since JDK 1.2 which is used to represent and manage Collections and it contains:

  1. Interfaces
  2. Classes
  3. Algorithms

This framework also defines map interfaces and several classes in addition to Collections.

Advantages:

  • High performance
  • Reduces developer's effort
  • Unified architecture which has common methods for all objects.
CollectionDescription
SetSet is a collection of elements which can not contain duplicate values. Set is implemented in HashSets, LinkedHashSets, TreeSet etc
ListList is a ordered collection of elements which can have duplicates. Lists are classified into ArrayList, LinkedList, Vectors
QueueFIFO approach, while instantiating Queue interface you can either choose LinkedList or PriorityQueue.
DequeDeque(Double Ended Queue) is used to add or remove elements from both the ends of the Queue(both head and tail)
MapMap contains key-values pairs which don't have any duplicates. Map is implemented in HashMap, TreeMap etc.