Scala：比较新鲜对象 [英] Scala: comparing fresh objects

问题描述

我正在浏览scala测试，我不明白为什么在比较两个新鲜对象时编译器会产生警告。

I was browsing scala tests and I don't understand why the compiler produces a warning when you compare "two fresh objects".

这是测试的输出：
http：/ /lampsvn.epfl.ch/trac/scala/browser/scala/trunk/test/files/neg/checksensible.check

示例：

checksensible.scala:12: warning: comparing a fresh object using `!=' will always yield true
println(new Exception() != new Exception())
                        ^

如果我写了一个实现 == 方法也将产生此警告：

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

编辑：感谢oxbow_lakes，我必须重写equals方法，而不是==

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
    }
}

推荐答案

请注意，您切勿覆盖 == 方法（您应覆盖等于方法）。我认为，通过新鲜对象，Scala意味着没有定义的等于方法的新对象 。

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.

如果不覆盖等于，则 == 比较是参考比较（即使用 eq ），因此：

If you do not override equals, the == comparison is a reference comparison (i.e. using eq) and hence:

new F() == new F()

始终为假。

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