Point类的Java hashCode

Question

I have a simple custom Point class as follows and I would like to know if my hashCode implemention could be improved or if this is the best it's going to get.

public class Point 
{
    private final int x, y;

    public Point(int x, int y)
    {
        this.x = x;
        this.y = y;
    }

    public int getX() 
    {
        return x;
    }

    public int getY()
    {
        return y;
    }

    @Override
    public boolean equals(Object other) 
    {
        if (this == other)
          return true;

        if (!(other instanceof Point))
          return false;

        Point otherPoint = (Point) other;
        return otherPoint.x == x && otherPoint.y == y;
    }


    @Override
    public int hashCode()
    {
        return (Integer.toString(x) + "," + Integer.toString(y)).hashCode();
    }

}

Answer 1

Please do not use Strings. There's a lot of theory behind this and several implementations (division method, multiplication one, etc...). If you have about a hour you can watch this MIT-Class

This being said, here is what Netbeans 7.1 suggests:

@Override
public int hashCode() {
    int hash = 7;
    hash = 71 * hash + this.x;
    hash = 71 * hash + this.y;
    return hash;
}

October 2015 Edit

I started using IntelliJ a while back, I live happier now. This is what its automatic hashCode generation produces. It's a little less verbose. Note the use of prime numbers as well.

@Override
public int hashCode() {
    int result = x;
    result = 31 * result + y;
    return result;
}

Answer 2

The manual multiplication of values of all significant member fields as suggested by Gevorg is probably the most efficient and has a good value distribution. However, if you favour readability, there are nice alternatives available either in Java 7...

import java.util.Objects;

...

@Override
public int hashCode() {
    return Objects.hash(x, y);
}

... or in the Guava library:

import com.google.common.base.Objects;

....

@Override
public int hashCode() {
    return Objects.hashCode(x, y);
}

Both of these varags methods simply delegate to Arrays.hashCode(Object[] a) , so there is a slight impact on performance because of the autoboxing of ints and creating an array of object references, but it should be far less significant than using reflection.

And the readability is just great, since you simply see, which fields are used for the hashcode computation and all the multiply and add syntax is just hidden under the hood of Arrays.hashCode(Object[] a) :

public static int hashCode(Object a[]) {
    if (a == null)
        return 0;

    int result = 1;

    for (Object element : a)
        result = 31 * result + (element == null ? 0 : element.hashCode());

    return result;
}

Answer 3

I would recommend using a simpler and more performant method without strings, perhaps Josh Bloch's method from this answer , in your case just:

return 37 * x + y;

EDIT: nybbler is correct. What is actually recommended is:

int result = 373; // Constant can vary, but should be prime
result = 37 * result + x;
result = 37 * result + y;

Answer 4

A really nice way to hash a 2D point into a single integer is to use a number spiral!

http://ulamspiral.com/images/IntegerSpiral.gif

@Override
public int hashCode() {
    int ax = Math.abs(x);
    int ay = Math.abs(y);
    if (ax>ay && x>0) return 4*x*x-3*x+y+1;
    if (ax>ay && x<=0) return 4*x*x-x-y+1;
    if (ax<=ay && y>0) return 4*y*y-y-x+1;
    return 4*y*y-3*y+x+1;
}

While this method requires a few more calculations, there will be no unpredictable collisions. It also has the nice property that points closer to the origin in general will have smaller hash value. (Still can overflow with x or y > sqrt(MAX_VALUE) however)

Answer 5

I used to write my own hash and equals functions then I found this : )

import org.apache.commons.lang.builder.HashCodeBuilder;
import org.apache.commons.lang.builder.EqualsBuilder;

@Override
public boolean equals(Object obj) {
   return EqualsBuilder.reflectionEquals(this, obj);
 }
@Override
public int hashCode() {
   return HashCodeBuilder.reflectionHashCode(this);
 }

of course keep in mind the following:

Because reflection involves types that are dynamically resolved, certain Java virtual machine optimizations can not be performed. Consequently, reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code which are called frequently in performance-sensitive applications. SRC

Answer 6

From the JDK's Point class (inherited from Point2d):

public int hashCode() {
    long bits = java.lang.Double.doubleToLongBits(getX());
    bits ^= java.lang.Double.doubleToLongBits(getY()) * 31;
    return (((int) bits) ^ ((int) (bits >> 32)));
}

That looks slightly better than your implementation.

Answer 7

You can have a look into existing Point type classes implementations:

/**
343      * Returns the hashcode for this <code>Point2D</code>.
344      * @return a hash code for this <code>Point2D</code>.
345      */
346     public int hashCode() {
347     long bits = java.lang.Double.doubleToLongBits(getX());
348     bits ^= java.lang.Double.doubleToLongBits(getY()) * 31;
349     return (((int) bits) ^ ((int) (bits >> 32)));
350     }

from: http://kickjava.com/src/java/awt/geom/Point2D.java.htm#ixzz1lMCZCCZw

Simple guide for hashCode implementation can be found here

Answer 8

By default, Eclipse will use a hashCode() function for your Point class similar to:

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + getOuterType().hashCode();
    result = prime * result + x;
    result = prime * result + y;
    return result;
}

At the very least, incorporating a prime number into your hashCode algorithm will help with it's "uniqueness".