java - 乘法表

Question

我正在制作乘法表(从 2 到 9)- 例如，这是 10 个随机生成的样本

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

关键是所有的样本必须是不同的和样本

5 * 8 = 

和

8 * 5 = 

认为是一样的

我的想法是制作描述要相乘的数字对的类 Pair，覆盖它的 equals 方法，生成随机数，创建 Pair 并将 Pair 对象添加到 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]);
}

问题是方法 createTable() 返回的一些数组包含等价对例如在

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

有些对 2 * 8 和 8 * 2 不应该存在

错在哪里？

Answer 1

您的hashCode() 方法有误。 hashCode() 必须 为两个相等的对象返回相等的值。根据您的 equals() 方法，5-7 对等于 7-5 对，但它们的 hashCode() 不相同。

要正确实现 hashCode()，您应该始终从最小的数字开始:

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;
}

或者，更简单:

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

或者，更简单(感谢@user2336315):

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