HashSet 没有意识到两个对象是相同的

Question

我看过其他几次在 Whosebug 上提出这个问题，但 none 其他用例似乎解决了我的问题。 HashSet does not seem to realize that two objects are the same.

基本上，这是我的class。

private static class Inconsistency
    {
        int a;
        int b;
        boolean isConsistency;


        //Default constructor. To be used when we don't have an inconsistency
        public Inconsistency()
        {
            this.a = -1;
            this.b = -1;
            this.isConsistency = false;
        }

        public Inconsistency(int a, int b, boolean isConsistency)
        {
            this.a = a;
            this.b = b;
            this.isConsistency = isConsistency;
        }

        @Override
        public String toString()
        {
            if (this.isConsistency) 
            {
                return "(" + this.a + ", " + this.b + ")";
            } 
            else 
            {
                return "No inconsistency";
            }

        }
        @Override
        public int hashCode() {
            final int prime = 31;
            int result = 1;
            result = prime * result + a;
            result = prime * result + b;
            result = prime * result + (isConsistency ? 1231 : 1237);
            return result;
        }

        @Override
        public boolean equals(Object other)
        {

            if (this == other)
            {
                return true;
            }
            if (other == null)
            {
                return false;
            }

            if (this.getClass() != other.getClass()) 
            { 
                return false; 
            }

            Inconsistency otherInconsistency = (Inconsistency) other;
            return ((this.a == otherInconsistency.a) && (this.b == otherInconsistency.b) && (this.isConsistency == otherInconsistency.isConsistency))
                || ((this.a == otherInconsistency.b) && (this.b == otherInconsistency.a) && (this.isConsistency == otherInconsistency.isConsistency));
        }
    }

我正在尝试将 class 的对象存储在哈希图中。

按照我定义 equals 方法的方式，一个不一致 A (10, 20, true) 应该等于另一个不一致 B (20, 10, true)，当我测试我的 equals 方法时，这个工作正常。但是，当我尝试将 A 和 B 都插入 HashSet 时，它们都被错误地添加了。我知道我应该操纵我的哈希码函数，但我不确定如何去做。

这是一个展示错误行为的驱动程序

    Inconsistency A = new Inconsistency(10,20, true);
    Inconsistency B = new Inconsistency(20,10, true);

    System.out.println(A.equals(B)); // prints true as expected


    HashSet<Inconsistency> test = new HashSet<>();
    test.add(A);
    test.add(B);

    System.out.println(test); // prints [(10, 20), (20, 10)]. The two objects are equal but are both added to hashset

所以问题很明确，我如何确保两个相等的对象 A 和 B 都不会添加到我的 HashSet 中？

Answer 1

您对 equals 的定义意味着两个 Inconsistency 元素颠倒的对象是 .equals，但您对 hashCode 的定义不是 return 如果 a 和 b 的顺序不同，则相同的哈希码，如果 HashSet 或其他基于哈希的集合是为了正常工作。

解决此问题的最简单方法是做一些可交换的事情——无论 a 和 b 的顺序如何，结果都相同。例如：

result = prime * result + a + b;

而不是

result = prime * result + a;
result = prime * result + b;

HashSet 没有意识到两个对象是相同的

HashSet does not realize that two objects are the same

java

hashset