为什么ArrayList比java中的HashSet更快？

Question

I used to thing that HashSet is a pretty fast data structure implementation because it uses hashes (and is implemented via HashMap in its turn). I was solving some problems and decided to check performance issue, so here it is:

You are given an array with numbers - [11, 3, 11, 11, 3, 2, 0, -2, 2] You are supposed to write a function that returns the number that appears "odd" number of times.

Here is my solution:

public class OddNumInArray {

public static List<Integer> oddNumList(int [] ar){
    Collection <Integer> l = new ArrayList<>();
    for (int n: ar) {
        if (l.contains(n)) {
            l.remove(n);
        }
        else {
            l.add(n);
        }
    }
    return (List) l;
}

public static Set<Integer> oddNumHSet(int [] ar){
    Set <Integer> l = new HashSet<>();
    for (int n: ar) {
        if (l.contains(n)) {
            l.remove(n);
        }
        else {
            l.add(n);
        }
    }
    return l;
}

public static void main(String [ ]arg) {
    int [] a1 = new int [10000000];
    for (int i=0; i<10; i++){
        a1[i]=(new Random()).nextInt(5);
    }
    long cur= System.nanoTime();
    System.out.println(oddNumList(a1));
    long c1 = System.nanoTime()-cur;
    System.out.println("TIME CONSUMED:" +c1);
    cur= System.nanoTime();
    System.out.println(oddNumHSet(a1));
    long c2 = System.nanoTime()-cur;
    System.out.println("TIME CONSUMED:" + c2);
    System.out.println("c1/c2*100: "+ (new Double(c1)/new Double(c2)*100));
}

}

And here is an output:

[1, 0]
TIME CONSUMED:101804000
[0, 1]
TIME CONSUMED:183261000
c1/c2*100: 55.55137208680516

So, why is implementation with ArrayList is quicker than one with HashSet by almost 2 times? Thank you.

Answer 1

ArrayList doesn't have code to check for duplicates . So, it just adds elements as and how you try to add them. A HashSet on the other hand is meant to have only unique elements, so it makes a check to prevent insertion of duplicate elements.