Can you help me fix the exception? I have an Exception in thread "main" java.lang.NullPointerException

import java.util.List;

public class Scheduler {

static double idleTime = 0;

static Queue queue = new FIFOQueue();

public static void executeProcesses() {

double currentTime = 0;

System.out.println("Simulation starts with default parameters.");

while (!queue.isEmpty()) {

Process process = queue.poll();

if (process.getArrivalTime() > currentTime) {

idleTime = process.getArrivalTime() - currentTime;

currentTime = process.getArrivalTime();

}

System.out.printf("process %d started at time %f. ", process.getId(), currentTime);

do {

double currentBurstTime = process.getNextBurstTime();

System.out.printf("process %d has CPU burst %f. ", process.getId(), currentBurstTime);

currentTime += currentBurstTime;

} while (process.hasMoreBurst());

System.out.printf("process %d completes and terminates at %f. ", process.getId(), currentTime);

}

System.out.println("Simulation summary");

System.out.printf("CPU idle time: %f ", idleTime);

System.out.printf("CPU idle percentage: %f ", idleTime / currentTime);

System.out.println("Signature name: " + queue.getSignature());

}

public static void main(String[] args) {

ProcessFactory factory = new ProcessFactory();

List processList = factory.batchGenerate();

processList.forEach(p -> queue.offer(p));

executeProcesses();

}

}

package csc3502.processsimulator;

public interface Queue {

void offer(Process p);

Process poll();

boolean isEmpty();

String getSignature();

}

import java.util.ArrayList;

import java.util.List;

import java.util.Random;

public class ProcessFactory {

private int id = 1;

private double arrivalTime = 0;

private int priority = 1;

// setting parameters

private double maxBurst = 60;

private double minBurst = 10;

private double mean = 50;

private double stdDeviance = 30;

private int numOfProcess = 100;

public ProcessFactory() {

}

public ProcessFactory(double mean, double stdDeviance, double maxBurst, double minBurst, int numOfProcess) {

this.mean = mean;

this.stdDeviance = stdDeviance;

this.maxBurst = maxBurst;

this.minBurst = minBurst;

this.numOfProcess = numOfProcess;

}

public List batchGenerate() {

List processList = new ArrayList<>();

for (int i = 0; i < numOfProcess; i++) {

processList.add(generate());

}

return processList;

}

public Process generate() {

return new Process(getId(), getArrivalTime(), getPriority(), maxBurst, minBurst);

}

private int getId() {

return id++;

}

private double getArrivalTime() {

arrivalTime += new Random().nextGaussian() * stdDeviance + mean;

return arrivalTime;

}

private int getPriority() {

return priority;

}

}

public class Process {

private int id;

private double arrivalTime;

private int priority;

// setting parameters

private double maxBurst = 60;

private double minBurst = 10;

public Process(int id, double arrivalTime, int priority, double maxBurst, double minBurst) {

this.id = id;

this.arrivalTime = arrivalTime;

this.priority = priority;

this.maxBurst = maxBurst;

this.minBurst = minBurst;

}

public int getId() {

return id;

}

public double getArrivalTime() {

return arrivalTime;

}

public int getPriority() {

return priority;

}

public boolean hasMoreBurst() {

return Math.random() > 0.3;

}

public double getNextBurstTime() {

return (int) (Math.random() * (maxBurst - minBurst) + minBurst);

}

}

public class FIFOQueue implements Queue {

//put your name as the value of the signature.

String signature = "Name";

Node head = new Node(null);

Node pointer = head;

class Node {

Process process;

Node next;

Node(Process p) {

this.process = p;

this.next = null;

}

}

@Override

public void offer(Process p) {

// TODO Implement you code here

pointer.next = new Node(p);

pointer = pointer.next;

}

@Override

public Process poll() {

// TODO Implement you code here

if(isEmpty())

return null;

else{

Process item = head.next.process;

if(head.next == pointer){

head.next = null;

pointer = head;

}else{

head.next = head.next.next;

}

return item;

}

}

@Override

public boolean isEmpty() {

// TODO Implement you code here

return pointer == head;

}

@Override

public String getSignature() {

return signature;

}

}