快学scala第十七章习题答案

Scala17定义一个不可变类Pair[T,S]，带一个swap方法，返回组件交换过packageobjectMainextendsApp{valp=newPair(97->'a')vala=p.swap}classPair[T,S](valp:(T,S)){defswap={(p._2,}}packageobjectMainextendsApp{valp=newPair(97->'a')p.p=98->}classPair[T](varp:(T,T)){defswap={(p._2,}}packageobjectMainextendsApp p= println(p.swap( - 'a' }classPair[T, swap[T,S](p:(T, S))= (p._2,p._1)}}在17.3节中，如果我们想把Pair[]的第一个组件替换成Student，为什么不需要给receFirst方法定一个下界？ packageobjectMainextendsApp{valp1=newvalp2=newvals1=newStudentvalpair=newPair(p1,pair.receFirst(s1)//返回值类型为 }classPair[T](valfirst:T,valsecond:T)def }classclassStudentextendsRichIntComparable[Int]而不是IntInt。而接口Comparable位于java.lang包。实现 Comparable[Int]比实现Comparable[RichInt]通用得多。packageimportscala.runtime.RichIntobjectMainextendsApp{vala=newRichInt(1)valb=newRichInt(2)valc=3//println(amaxb)//typemismatch;found:scala.runtime.RichIntrequired:Intprintln(amaxc)}编写一个泛型方法middle，返回任何I packageobjectMainextendsAppdefmiddle[T](it:I ble[T]):T={deflist=it.toListlist(list.size/}valstr=}World'smiddleis查看I 解答：foldLeft,foldRight,groupBy等方法使用了类型参数A。因为在函数Pair[T]定义一个等效的方法？defreceFirst[R>:T](newFirst:R){first=newFirst) 回答：首先，newFirstfirstfirstTnewFirst是RTRPair[T]，covarianttypeToccursincontravariantpositionintypeTofvaluecovarianttypeToccursincontravariantpositionintypeTofvaluepackageobjectMainextendsApp}//covarianttypeToccursincontravariantpositionintypeTofvalue//covarianttypeToccursincontravariantpositionintypeTofvalue//classPair[+T](valfirst:T,valsecond:T){//ERRORclassPair[T](varfirst:T,varsecond:T){//typemismatch;found:newFirst.type(withunderlyingtypeR)required://defrepalceFirst[R>:T](newFirst:R){first=newFirst}}Pair[+T]中定义defreceFirst(newFirst:定义一个Pair[Double]的子类NastyDoublePair，重写receFirst方法，newFirstNastyDoublePairPair[Any]调用NastyDoublePairPair[Any]调用rece("o")时错：typemismatch。packageobjectMainextendsApp}classPair[+T](valfirst:T,valsecond:T)//covarianttypeToccursincontravariantpositionintypeTofvalue//defreceFirst(newFirst:T)={null}//def ceFirst[R>:T](newFirst:R)=newPair[R](newFirst,}classNastyDoublePair[Double](overridevalfirst:Double,overridevalsecond:Double)extendsPair[Double](first,second){//typemismatch;found:Double(inclassNastyDoublePair)required://overridedefreceFirst(newFirst:Double)=newPair(math.sqrt(newFirst),}Pair[S,T]，swappackageobjectMainextendsApp{vala=newPair(2,4)valb=newPair("Hi",}classPair[S,T](privatevarfirst:S,privatevarsecon