Inconsistent hashcode and equals java

Question

After researching I still can't find the specific solution for my problem. I have an "approximately equals" method that uses an epsilon, while my hashCode method uses the exact values. This breaks the precondition of HashSet when I compare the values.

@Override
public boolean equals(Object o) {
    if (o == this)
        return true;
    if (!(o instanceof EPoint)) {
        return false;
    }
    EPoint ePoint = (EPoint) o;
    return Math.abs(Math.abs(ePoint.lat) - Math.abs(lat)) < EPSILON && Math.abs(Math.abs(ePoint.lon) - Math.abs(lon)) < EPSILON;
}

@Override
public int hashCode() {
    return Objects.hash(lat, lon);
}

I can't find a way to make the hasCode() consistent with my equals method.

Answer 1

Your equals itself breaks the contract even before you get to hashCode because it isn't transitive.

This also immediately leads to the only consistent hashCode implementation being to return a constant, because for any two points there is a (very long) chain of intermediate points so that

1. every two neighbors are equal, therefore

2. every two neighbors must have the same hashCode, therefore

3. beginning and end must have the same hashCode.

Now, this is a consistent implementation, but quite obviously a useless one.

Answer 2

I agree with Kayaman: The way your equals methos is implemented, you can have three EPoints (pointA,pointB,and pointC) with:

pointA.equals(pointB) //true
pointA.equals(pointC) //true
pointB.equals(pointC) //false

And this is not allowed. Creating a method with another name might be a solution.

If, however, you need your "almost Equal" objects to have the same hashcode, you could try a different approach:
Map every EPoint to an EPoint out of a grid. If, e.g. your EPoint's lat and lon where floats, you could map each EPoint to an EPoint with the rounded int-values.
If you need higher precision, you could extend on that and go into first, second,...decimal place).

If you do the equals() and the hashcode() method against the "mapped" Point, this should satisfy all requirements:

@Override
public boolean equals(Object o) {
    if (o == this)
        return true;
    if (!(o instanceof EPoint)) {
        return false;
    }
    EPoint ePoint = (EPoint) o;
    return this.gridLon() == ePoint.gridLon() && ePoint.gridLat() == this.gridLat();
}

@Override
public int hashCode() {
    return Objects.hash(this.gridLon(), this.gridLat());
}