使用隐式类覆盖方法 [英] Using implicit class to override method

问题描述

我的目的是改变String 中== 方法的行为以调用equalsIgnoreCase.

My intention is to change the behaviour of the == method in String to call equalsIgnoreCase.

此代码

implicit class LowerCase(s: String) {
      override def ==(that: LowerCase) = that.equalsIgnoreCase(this)
}

导致此错误

error: type mismatch;
 found   : MyClass.LowerCase
 required: String
      override def ==(that: String) = that.equalsIgnoreCase(this)

推荐答案

如果类中已经有这样的方法，scala 编译器不会搜索隐式转换.

In case there is already such method in class scala compiler would not search for implicit conversion.

注意你的实现是无效的，应该是:

Note that your implementation is invalid, it should be:

implicit class LowerCase(val s: String) {
      def ==(that: LowerCase) = that.s.equalsIgnoreCase(this.s)
}

请注意，它仍然没有用.

Note that it's still useless.

如果您想将其用作 Map 的键，您应该手动指定类型:

In case you want to use it as Map's key you should specify type manually:

implicit class LowerCase(val s: String) {
      // Use `equals`, not `==`
      override def equals(that: Any) = that match {
        case t: LowerCase => t.s.equalsIgnoreCase(this.s)
        case _ => false
      }

  override def toString() = s
}

scala> Set[LowerCase]("A", "a", "b")
res0: scala.collection.immutable.Set[LowerCase] = Set(A, b)

如果您想将此方法与这样的变量一起使用 "a" == "A" 您应该使用另一个方法名称:

If you want to use this method with variables like this "a" == "A" you should use another method name:

implicit class LowerCase(val s: String) extends AnyVal {
      def ===(that: String) = s.equalsIgnoreCase(that)
}

scala> "a" === "A"
res0: Boolean = true

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