scala 中隐式转换 implicit的应用

下面是一个Rational的类

class Rational(n: Int, d: Int) {  require(d != 0)    private val g = gcd(n.abs, d.abs)       val numer: Int = n / g  val denom: Int = d / g    def this(n: Int) = this(n, 1)    override def toString = numer + "/" + denom  def add(that: Rational): Rational =     new Rational(        numer * that.denom + that.numer * denom,        denom * that.denom    )    def lessThan(that: Rational) =    this.numer * that.denom < that.numer * this.denom    def max(that: Rational) =    if (this.lessThan(that)) that else this      private def gcd(a: Int, b: Int): Int =    if (b == 0) a else gcd(b, a%b)      def + (that: Rational): Rational =     new Rational(        numer * that.denom + that.numer * denom,        denom * that.denom    )    def + (i: Int): Rational =    new Rational(numer + i * denom, denom)    def * (that: Rational): Rational =    new Rational(        numer * that.numer, denom * that.denom        )    def * (i: Int): Rational =    new Rational(numer * i, denom)    def - (i: Int):Rational =    new Rational(numer - i * denom, denom)    def - (that: Rational): Rational =    new Rational(        numer * that.denom - that.numer * denom,        denom * that.denom        )    def / (that: Rational):Rational =    new Rational(numer * that.denom, denom * that.numer)    def / (i: Int): Rational =    new Rational(numer, denom * i)}

下面简单的使用一下Rational类

object Test extends App{     val x = new Rational(1,2)   val y = new Rational(2,3) <pre name="code" class="plain">  <span style="color:#009900;">println(y * 2)</span>

}

可以正常输出结果：4/3。

我们把输出的改为：

object Test extends App{     val x = new Rational(1,2)   val y = new Rational(2,3)   println(2 * y)}

会报如下，错误：

overloaded method value * with alternatives: (x: Double)Double <and> (x: Float)Float <and> (x: Long)Long <and> (x: Int)Int <and> (x: Char)Int <and> (x: Short)Int <and> (x: Byte)Int cannot be applied to (mytest.Rational)

原因是：

The problem here is that 2 * r is equivalent to 2.*(r), so it is a method
call on the number 2, which is an integer. But the Int class contains no
multiplication method that takes a Rational argument—it couldn’t because
class Rational is not a standard class in the Scala library.
However, there is another way to solve this problem in Scala: You can
create an implicit conversion that automatically converts integers to rational
numbers when needed. Try adding this line in the interpreter:

这个时候就需要隐式转换

object Test extends App{     val x = new Rational(1,2)  val y = new Rational(2,3)  implicit def intToRational(x: Int) = new Rational(x) //隐式转换  println(2 * y)

此时，就可以正常输出了。