奇怪的HashSet包含()行为

Question

java中的HashSet让我很困惑,当使用contains()时它会查找hashcode()和equals()结果吗？但在这种情况下,它表现不正常.如果你把这种代码放在大型项目中,有时会出现问题.问题是为什么最后一个语句打印FALSE？什么包含()在引擎盖下做什么？

class R
{
    int count;
    public R(int count)
    {
        this.count = count;
    }
    public String toString()
    {
        return "R(count attribution:" + count + ")";
    }
    public boolean equals(Object obj)
    {
        if (obj instanceof R)
        {
            R r = (R)obj;
            if (r.count == this.count)
            {
                return true;
            }
        }
        return false;
    }
    public int hashCode()
    {
        return this.count;
    }
}
public class TestHashSet2
{
    public static void main(String[] args) 
    {
        HashSet hs = new HashSet();
        hs.add(new R(5));
        hs.add(new R(-3));
        hs.add(new R(9));
        hs.add(new R(-2));

        System.out.println(hs);

        //change first element
        Iterator it = hs.iterator();
        R first = (R)it.next();
        first.count = -3;
        System.out.println(hs);
        //remove
        hs.remove(new R(-3));
        System.out.println(hs);

        R r1 = new R(-3);
        System.out.println(r1.hashCode());
        Iterator i = hs.iterator();
        R r2 = (R)i.next();
        System.out.println(r2.hashCode());   //same hashcode -3
        System.out.println(r1.equals(r2));   //equals true

        System.out.println("hs contains object which count=-3 ?" + hs.contains(new R(-3)));  //false
    }
}

Answer 1

通过在将对象插入其中后更改对象的值HashSet,您将破坏数据结构的完整性.在那之后,你不能依靠它来完成它的工作.

使用可变对象作为集合的任何映射或值的键通常是个坏主意.幸运的是,最常用于此目的的类(String,Integer)是不可变的.