What code is generated for an equals/hashCode method of a case class?

问题

I have some Java code which I'm translating to Scala.

The code consists of some immutable classes which would fit the purpose of a case class in Scala.

But I don't want to introduce bugs, therefore I want to be sure that the code being generated for equals and hashCode is/behaves equivalent to the current implementation.

I already looked in "Programming in Scala" but it only says

Third, the compiler adds “natural” implementations of methods toString, hashCode, and equals to your class.

回答1:

Scala has a compiler option -Xprint:typer, which you can use to get the "post-typing source code that it uses internally".

scala -Xprint:typer -e 'case class Foo(a: String, b: Int)'

Here you see something like:

override def hashCode(): Int = ScalaRunTime.this._hashCode(Foo.this);
override def toString(): String = ScalaRunTime.this._toString(Foo.this);
override def equals(x$1: Any): Boolean = Foo.this.eq(x$1).||(x$1 match {
  case (a: String,b: Int)this.Foo((a$1 @ _), (b$1 @ _)) if a$1.==(a).&&(b$1.==(b)) => x$1.asInstanceOf[this.Foo].canEqual(Foo.this)
  case _ => false
});

But, this doesn't tell you how hashCode is generated. Here's the source for that:

def _hashCode(x: Product): Int = {
  var code = x.productPrefix.hashCode()
  val arr =  x.productArity
  var i = 0
  while (i < arr) {
    val elem = x.productElement(i)
    code = code * 41 + (if (elem == null) 0 else elem.hashCode())
    i += 1
  }
  code
}

And, in this example, the first case of the equals pattern matching would just be:

case that: Foo => this.a == that.a && this.b == that.b

来源：https://stackoverflow.com/questions/4526706/what-code-is-generated-for-an-equals-hashcode-method-of-a-case-class

标签

java

class

scala

equals

hashcode