本节主要介绍了Kotlin标准库中提供的对于集合的扩展，都是非常实用强大的功能

我们通过为Shop类编写各种功能的扩展来练习这部分功能，测试数据在对应测试目录下的TestShop.kt

13. n13Introduction

获取Shop中Customer的集合

fun Shop.getSetOfCustomers(): Set
    
      {    return customers.toSet()}复制代码
    

14. n14FilterMap

Kotlin为集合扩展了filter，map,forEach等等方法，这些在Java中称为Steram Api，功能都是相似的，但是无疑Kotlin用起来更方便

本练习要求我们获取所有Customser的City，并放到一个Set中，还有获取来自某一个City的所有Customer的List

fun Shop.getCitiesCustomersAreFrom(): Set
    
      {    return customers.map { it.city }.toSet()}复制代码
    

fun Shop.getCustomersFrom(city: City): List
    
      {    // Return a list of the customers who live in the given city    return customers.filter { it.city==city }}复制代码
    

15. n15AllAnyAndOtherPredicates

检查Customer是否来自某一City

fun Customer.isFrom(city: City): Boolean {    // Return true if the customer is from the given city    return this.city == city}复制代码

检查多个Customer是否来自同一City

fun Shop.checkAllCustomersAreFrom(city: City): Boolean {    // Return true if all customers are from the given city    return customers.all { it.city == city }}复制代码

检查是否有Customer来自指定City

fun Shop.hasCustomerFrom(city: City): Boolean {    // Return true if there is at least one customer from the given city    return customers.any { it.city == city }}复制代码

统计来自某一City的Customer的数量

fun Shop.countCustomersFrom(city: City): Int {    // Return the number of customers from the given city    return customers.count { it.city == city }}复制代码

查找来自指定City的第一个Customer，没有找到的话返回null

fun Shop.findFirstCustomerFrom(city: City): Customer? {    // Return the first customer who lives in the given city, or null if there is none    return customers.firstOrNull { it.city == city }}复制代码

16. n16FlatMap

获取Customer订购的Product集合

val Customer.orderedProducts: Set
    
      get() {    // Return all products this customer has ordered    return orders.flatMap { it.products }.toSet()}复制代码
    

获取Shop卖出的Product集合

val Shop.allOrderedProducts: Set
    
      get() {    // Return all products that were ordered by at least one customer    return customers.flatMap { it.orderedProducts }.toSet()}复制代码
    

17. n17MaxMin.kt

查找Order最多的Customer

fun Shop.getCustomerWithMaximumNumberOfOrders(): Customer? {    // Return a customer whose order count is the highest among all customers    return customers.maxBy { it.orders.size }}复制代码

查找已购买价格最高的Product

fun Customer.getMostExpensiveOrderedProduct(): Product? {    // Return the most expensive product which has been ordered    return orderedProducts.maxBy { it.price }}复制代码

18. n18Sort

通过Customer的Order数量排序

fun Shop.getCustomersSortedByNumberOfOrders(): List
    
      {    // Return a list of customers, sorted by the ascending number of orders they made    return customers.sortedBy { it.orders.size }}复制代码
    

sortedBy为从小到大，sortedByDescending为从大到小

19. n19Sum

统计已下订单总价,需要注意的是，一个Customer可能多次购买了同一Product

fun Customer.getTotalOrderPrice(): Double {    // Return the sum of prices of all products that a customer has ordered.    // Note: a customer may order the same product for several times.    return orders.sumByDouble { it.products.sumByDouble { it.price } }}复制代码

也可以

fun Customer.getTotalOrderPrice(): Double {    // Return the sum of prices of all products that a customer has ordered.    // Note: a customer may order the same product for several times.    return orders.flatMap { it.products }.sumByDouble { it.price }}复制代码

20. n20GroupBy

分组

fun Shop.groupCustomersByCity(): Map
    
     > {    // Return a map of the customers living in each city    return customers.groupBy { it.city }}复制代码
    

21. n21Partition

获取未交付Order数量多于已交付Order的Customer集合

fun Shop.getCustomersWithMoreUndeliveredOrdersThanDelivered(): Set
    
      {    // Return customers who have more undelivered orders than delivered    return customers.filter {        val (delivered,undelivered) = it.orders.partition { it.isDelivered }        undelivered.size>delivered.size    }.toSet()}复制代码
    

22. n22Fold

获取所有Customer都购买了的Product

fun Shop.getSetOfProductsOrderedByEveryCustomer(): Set
    
      {    // Return the set of products ordered by every customer    return customers.fold(allOrderedProducts, {        orderedByAll, customer ->        orderedByAll.intersect(customer.orders.flatMap { it.products }.toSet())    })}复制代码
    

flod是一个类似reduce的函数，该函数可以把前一次计算的返回值当成下一次计算的参数，flod可以设置初始值，intersect方法的意思是求交集

23. n23CompoundTasks

获取订购了某Product的Customer集合

fun Customer.getMostExpensiveDeliveredProduct(): Product? {    // Return the most expensive product among all delivered products    // (use the Order.isDelivered flag)    return orders.filter { it.isDelivered }.flatMap { it.products }.maxBy { it.price }}复制代码

获取某Product被购买的次数

fun Shop.getNumberOfTimesProductWasOrdered(product: Product): Int {    // Return the number of times the given product was ordered.    // Note: a customer may order the same product for several times.    return customers.flatMap { it.orders.flatMap { it.products } }.count { it==product }}复制代码

24. n24ExtensionsOnCollections

用Kotlin重写_24_JavaCode中的doSomethingStrangeWithCollection方法

fun doSomethingStrangeWithCollection(collection: Collection
    
     ): Collection
     
      ? {    val groupsByLength = Maps.newHashMap
      
       >()    for (s in collection) {        var strings: MutableList
       
        ? = groupsByLength[s.length] as MutableList
        
         ?        println(s)        if (strings == null) {            strings = Lists.newArrayList()            groupsByLength.put(s.length, strings)        }        strings!!.add(s)    }    var maximumSizeOfGroup = 0    for (group in groupsByLength.values) {        if (group.size > maximumSizeOfGroup) {            maximumSizeOfGroup = group.size        }    }    for (group in groupsByLength.values) {        if (group.size == maximumSizeOfGroup) {            return group        }    }    return null}复制代码
        
       
      
     
    

第二节完，好多东西一知半解