Scala: comparing fresh objects
compiler-construction, compiler-warnings, scala
Solution
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.
Problem
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 fresh 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 } } ```