Written and understand the code

import java.util.Scanner;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.Random;

public class DemandPagingSimulation {
private final static int NUM_ALGORITHMS = 7;

public static void main(String[] args) {
Scanner in = new Scanner(System.in);
System.out.print("Size of Virtual Memory (pages)? ");
int numPages = in.nextInt();
System.out.print("Number of frames allocated per process? ");
int numAllocated = in.nextInt();
System.out.print("Reference string length? ");
int numReferences = in.nextInt();
System.out.print("Random type (Uniform, Gaussian or Textbook Example)? ");
char r = in.next().toUpperCase().charAt(0);
in.close();

ArrayList<Integer> ref;
switch (r) {
case 'U':
ref = makeReferenceString(numReferences, numPages);
break;
case 'T':
int[] array = { 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1 };
numReferences = array.length;
ref = new ArrayList<Integer>();
for (int i = 0; i < array.length; i++)
ref.add(array[i]);
break;
default:
ref = makeNormalReferenceString(numReferences, numPages, 5.0);
}

ReplacementAlgorithm[] algos = new ReplacementAlgorithm[NUM_ALGORITHMS];
algos[0] = new TrivialAlgorithm(numAllocated);
algos[1] = new RandomAlgorithm(numAllocated);
algos[2] = new LFUAlgorithm(numAllocated, numPages);
algos[3] = new MFUAlgorithm(numAllocated, numPages);
algos[4] = new LRUAlgorithm(numAllocated);
algos[5] = new FIFOAlgorithm(numAllocated);
algos[6] = new OptimalAlgorithm(numAllocated, ref);

System.out.printf("%5s%3s", "Ref", "");
for (int a = 0; a < NUM_ALGORITHMS; a++) {
System.out.printf("%-22s ", algos[a].getName());
}
System.out.println();

for (int p : ref) {
System.out.printf("%5d ", p);
for (int a = 0; a < NUM_ALGORITHMS; a++) {
int victim = algos[a].processReference(p);
if (victim < 0) {
System.out.printf("%5s ", "HIT");
} else {
System.out.printf("%5d ", victim);
}
System.out.printf("%-15s ", algos[a]);
}
System.out.println();
}

System.out.println();
for (int a = 0; a < NUM_ALGORITHMS; a++) {
System.out.printf("Algorithm %-5s: faults/refs = %d/%d = %.2f%% Hit Rate\n", algos[a].getName(),
algos[a].getFaults(), numReferences,
100.0 * (double) (numReferences - algos[a].getFaults()) / numReferences);
}

}

private static ArrayList<Integer> makeReferenceString(int n, int max) {
ArrayList<Integer> a = new ArrayList<>();

for (int i = 0; i < n; i++)
a.add((int) (Math.random() * max));

return a;
}

private static ArrayList<Integer> makeNormalReferenceString(int n, int max, double stddev) {
ArrayList<Integer> a = new ArrayList<>();
Random rand = new Random();

int reference = 0;
for (int i = 0; i < n; i++) {
a.add(reference);
reference = reference + (int) (rand.nextGaussian() * stddev);
if (reference < 0 || reference > max - 1)
reference = (int) (Math.random() * max);
}

return a;
}
}