乘法表

Question

I'm making multiplication table(from 2 to 9) - that is 10 randomly generated samples, for example

2 * 3 = 
4 * 5 = 
... (7 more times)
9 * 5 = 

The point is all samples must be different and samples

5 * 8 = 

and

8 * 5 = 

considered to be the same

My idea is to make class Pair that describes pair of numbers to be multiplied, override it's equals method, generate random numbers, create Pair and add Pair objects to Set.

public class Pair {
    private int first;
    private int second;

    public int getFirst() {
        return first;
    }

    public int getSecond() {
        return second;
    }

    public void setFirst(int first) {
        this.first = first;
    }

    public void setSecond(int second) {
        this.second = second;
    }

    public Pair() {}

    public Pair(int first, int second) {
        this.first = first;
        this.second = second;
    }

    @Override
    public boolean equals(Object o) {
        if (o == null || o.getClass() != this.getClass())
            return false;

        Pair that = (Pair) o;
        return (this.first == that.first && this.second == that.second) ||
                (this.second == that.first && this.first == that.second);
    }

    @Override
    public int hashCode() {
        int result = 17;
        int prime = 31;
        result = result * prime + first;
        result = result * prime + second;
        return result;
    }

    @Override
    public String toString() {
        return first + " * " + second + " =";
    }
}

public static Pair[] createTable(int count) {
        Random random = new Random();
        Set<Pair> set = new HashSet<Pair>();
        while (set.size() < count) {
            int first = random.nextInt(8) + 2;
            int second = random.nextInt(8) + 2;
            set.add(new Pair(first, second));
        }
        return set.toArray(new Pair[count]);
}

The problem is that some arrays that method createTable() returns consist equivalent pairs for example in

[7 * 6 =, 5 * 6 =, 4 * 8 =, 4 * 9 =, 2 * 8 =, 9 * 2 =, 8 * 2 =, 6 * 3 =, 5 * 2 =, 4 * 2 =]

there are pairs 2 * 8 and 8 * 2 not supposed to be there

where is the mistake?

Answer 1

Your hashCode() method is wrong. hashCode() MUST return equal values for two objects that are equal. The pair 5-7 is equal to the pair 7-5 according to your equals() method, but their hashCode() is not the same.

To implement hashCode() correctly, you should always start by the lowest number:

public int hashCode() {
    int result = 17;
    int prime = 31;

    int lowest = first < second ? first : second;
    int highest = first < second ? second : first;

    result = result * prime + lowest;
    result = result * prime + highest;
    return result;
}

or, simpler:

public int hashCode() {
    int lowest = first < second ? first : second;
    int highest = first < second ? second : first;
    return Objects.hash(lowest, highest);
}

or, even simpler (thanks @user2336315):

public int hashCode() {
    return Objects.hash(Math.min(first, second), Math.max(first, second));
}