尽管实现了hashCode()和equals(),HashSet仍会添加重复的条目

Question

我有以下课程:

class Point
{
     double x, y;

     // .... constructor and other functions here

     public boolean equals(Point p)
     {
         if(p==null) return(false);
         return(x==p.x && y==p.y);
     }

    public int hashCode()
    {
        int result=17;

        long c1=Double.doubleToLongBits(x);
        long c2=Double.doubleToLongBits(y);

        int ci1=(int)(c1 ^ (c1 >>> 32));
        int ci2=(int)(c2 ^ (c2 >>> 32));

        result = 31 * result + ci1;
        result = 31 * result + ci2;

        return result;
    }
}

现在,如果我写下面的代码:

    Point x=new Point(11,7);
    Point y=new Point(11,7);

    System.out.println("hash-code of x=" + x.hashCode());
    System.out.println("hash-code of y=" + y.hashCode());
    System.out.println("x.equals(y) = " + x.equals(y));
    System.out.println("x==y = " + (x==y));

    java.util.HashSet<Point> s=new java.util.HashSet<Point>();
    s.add(x);
    System.out.println("Contains "+y.toString()+" = "+s.contains(y));
    s.add(y);
    System.out.println("Set size: "+s.size());
    java.util.Iterator<Point> itr=s.iterator();
    while(itr.hasNext()) System.out.println(itr.next().toString());

我得到以下输出:

hash-code of x=79052753
hash-code of y=79052753
x.equals(y) = true
x==y = false
Contains (11.0,7.0) = false
Set size: 2
(11.0,7.0)
(11.0,7.0)

请帮助我理解为什么contains()返回false(即使在equals()和hashCode()之后返回相同的值)以及如何纠正这一点(即阻止Java添加重复元素).提前致谢.

Answer 1

您没有覆盖Object中的equals方法.

equals方法的签名是:

public boolean equals(Object obj)

并不是

public boolean equals(Point obj)

请不要使用==比较双值.你应该使用Double.equals.

Answer 2

您已更改方法签名Object.equals(Object),因此您未正确覆盖equals.我建议你使用@Override注释来捕获这类错误.你的方法看起来像,

@Override
public boolean equals(Object o)
{
     if (this == o) {
         return true;
     }
     if (o instanceof Point) {
         Point p = (Point) o;
         return(x==p.x && y==p.y);
     }
     return false;
}