Scala: comparing fresh objects
I was browsing scala tests and I don't understand why the compiler produces a warning when you compare "two fresh objects".
This is the test' outputs: http://lampsvn.epfl.ch/trac/scala/browser/scala/trunk/test/files/neg/checksensible.check
Example:
checksensible.scala:12: warning: comparing a fre开发者_运维技巧sh object using `!=' will always yield true
println(new Exception() != new Exception())
^
If I write a class implementing an ==
method it will also produces this warning:
class Foo(val bar: Int) {
def ==(other: Foo) : Boolean = this.bar == other.bar
}
new Foo(1) == new Foo(1)
warning: comparing a fresh object using `==' will always yield false
EDIT: Thanks oxbow_lakes, I must override the equals method, not the ==
class Foo(val bar: Int) {
override def equals(other: Any) : Boolean = other match {
case other: Foo => this.bar == other.bar
case _ => false
}
}
Note that you should never override the ==
method (you should override the equals
method instead). I assume that by a fresh object, Scala means a new object without a defined equals
method.
If you do not override equals
, the ==
comparison is a reference comparison (i.e. using eq
) and hence:
new F() == new F()
will always be false.
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