Distinct intersecting values of two arrays

I have written two different methods that gives the same output. The output is a list of distinct intersecting values of two arrays. I am amazed with the different amount of time taken to solve this problem. It would be something interesting to investigate. Here are the suggested solutions:

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Set;

public class ArraysIntersection {

 private void findIntersectionMethod1(int[] a, int[] b){
  long start = System.nanoTime();
  int indexA = 0;
  int indexB = 0;
  ArrayList unique = new ArrayList();
  while (indexA < a.length && indexB < b.length) {
   if (a[indexA] == b[indexB]) {
    if (!unique.contains(a[indexA])) {
     unique.add(a[indexA]);
    }
    indexA++;
    indexB++;
   } else if (a[indexA] < b[indexB]) {
    indexA++;
   } else {
    indexB++;
   }
  }
  long elapsed = (System.nanoTime() - start);
  System.out.println("Elapsed time (findIntersectionMethod1): " + elapsed + "ns");
  System.out.println("Intersection of elements: " + unique);
 }

 private void findIntersectionMethod2(int[] arr1, int[] arr2){
  long start = System.nanoTime();
  ArrayList unique = new ArrayList();
  for (int i = 0; i < arr1.length; i++) {
   for (int j = 0; j < arr2.length; j++) {
    if (arr1[i] == arr2[j]) {
     if (!unique.contains(arr1[i])) {
      unique.add(arr1[i]);
     }
    }
   }
  }
  long elapsed = (System.nanoTime() - start);
  System.out.println("Elapsed time (findIntersectionMethod2): " + elapsed + "ns");
  System.out.println("Intersection of elements: " + unique);
 }

 private void findIntersectionMethod3(int[] a, int[] b){
  long start = System.nanoTime();
  Set unique = new HashSet();
  int indexA = 0;
  int indexB = 0;
  while (indexA < a.length && indexB < b.length) {
   if (a[indexA] == b[indexB]) {
    unique.add(a[indexA]);
    indexA++;
    indexB++;
   } else if (a[indexA] < b[indexB]) {
    indexA++;
   } else {
    indexB++;
   }
  }

  long elapsed = (System.nanoTime() - start);
  System.out.println("Elapsed time (findIntersectionMethod3): " + elapsed + "ns");
  System.out.println("Intersection of elements: " + unique);
 }

 public static void main(String[] args) {

  int a[]={1, 1, 2, 4, 4, 7, 8, 8, 9, 14, 14, 20, 20, 20};
  int b[]={1, 1, 1, 2, 2, 8, 8, 20};

  ArraysIntersection arrIntersect = new ArraysIntersection();
  arrIntersect.findIntersectionMethod1(a, b); 
  arrIntersect.findIntersectionMethod2(a, b); 
  arrIntersect.findIntersectionMethod3(a, b); 
 }
}

The output of these methods are as below (I'm telling you, you'll be amazed looking at the time taken for each method):

Elapsed time (findIntersectionMethod1): 455000ns
Intersection of elements: [1, 2, 8, 20]
Elapsed time (findIntersectionMethod2): 15000ns
Intersection of elements: [1, 2, 8, 20]
Elapsed time (findIntersectionMethod3): 65000ns
Intersection of elements: [2, 8, 1, 20]

I'm sure there are much better solutions to solve this interesting problem that I have not thought of.