Synthetic Function "##" in scala

Question

I recently saw some code like this code:

val x: Any = "a"
val y = x.## // y: Int = 97

Well, ok the output is just the ASCI value of 'a', but lets have a look at

List(1,2).## // Int = 985731359
List(1,2).toString.## // Int = 1063384434

My IDE tells about '##' that it is a 'Synthetic Function'. So what is ## doing and what is a Synthetic Function?

Answer 1

It's basically an alias of hashCode, with a couple of notable exceptions that make it somewhat safer:

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.

(source: https://www.scala-lang.org/api/current/scala/Any.html###:Int)

Examples:

normal value

scala> val x: Any = "a"
x: Any = a

scala> x.hashCode
res2: Int = 97

scala> x.##
res3: Int = 97

null value

scala> null.hashCode
java.lang.NullPointerException
  ... 33 elided

scala> null.##
res5: Int = 0

---

A synthetic field, instead, is a field generated by the compiler to work around the underlying JVM limitations, especially when dealing with inner anonymous classes, a concept extraneous to the JVM.

Here's a good explanation of what it means in details: http://javapapers.com/core-java/java-synthetic-class-method-field/