What is the difference between `##` and `hashCode`?

Question

What is the difference between methods ## and hashCode?

They seem to be outputting the same values no matter which class or hashCode

Answer 1

Just want to add to the answers of other posters that although the ## method strives to keep the contract between equality and hash codes, it is apparently not good enough in some cases, like when you are comparing doubles and longs (scala 2.10.2):

> import java.lang._
import java.lang._

> val lng = Integer.MAX_VALUE.toLong + 1
lng: Long = 2147483648

> val dbl = Integer.MAX_VALUE.toDouble + 1
dbl: Double = 2.147483648E9

> lng == dbl
res65: Boolean = true

> lng.## == dbl.##
res66: Boolean = false

> (lng.##, lng.hashCode)
res67: (Int, Int) = (-2147483647,-2147483648)

> (dbl.##, dbl.hashCode)
res68: (Int, Int) = (-2147483648,1105199104)

Answer 2

## was introduced because hashCode is not consistent with the == operator in Scala. If a == b then a.## == b.## regardless of the type of a and b (if custom hashCode implementations are correct). The same is not true for hashCode as can be seen in the examples given by other posters.

Answer 3

"Subclasses" of AnyVal do not behave properly from a hashing perspective:

scala> 1.0.hashCode
res14: Int = 1072693248

Of course this is boxed to a call to:

scala> new java.lang.Double(1.0).hashCode
res16: Int = 1072693248

We might prefer it to be:

scala> new java.lang.Double(1.0).##
res17: Int = 1

scala> 1.0.##
res15: Int = 1

We should expect this given that the int 1 is also the double 1. Of course this issue does not arise in Java. Without it, we'd have this problem:

Set(1.0) contains 1 //compiles but is false

Luckily:

scala> Set(1.0) contains 1
res21: Boolean = true

Answer 4

In addition to what everyone else said, I'd like to say that ## is null-safe, because null.## returns 0 whereas null.hashCode throws NullPointerException.

From scaladoc:

Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.