本節主要介紹了Kotlin標準庫中提供的對於集合的擴充套件，都是非常實用強大的功能
我們通過為Shop類編寫各種功能的擴充套件來練習這部分功能，測試資料在對應測試目錄下的TestShop.kt
13. n13Introduction
獲取Shop中Customer的集合
fun Shop.getSetOfCustomers(): Set<Customer> {
return customers.toSet()
}複製程式碼14. n14FilterMap
Kotlin為集合擴充套件了filter，map,forEach等等方法，這些在Java中稱為Steram Api，功能都是相似的，但是無疑Kotlin用起來更方便
本練習要求我們獲取所有Customser的City，並放到一個Set中，還有獲取來自某一個City的所有Customer的List
fun Shop.getCitiesCustomersAreFrom(): Set<City> {
return customers.map { it.city }.toSet()
}複製程式碼fun Shop.getCustomersFrom(city: City): List<Customer> {
// Return a list of the customers who live in the given city
return customers.filter { it.city==city }
}複製程式碼15. n15AllAnyAndOtherPredicates
檢查Customer是否來自某一City
fun Customer.isFrom(city: City): Boolean {
// Return true if the customer is from the given city
return this.city == city
}複製程式碼檢查多個Customer是否來自同一City
fun Shop.checkAllCustomersAreFrom(city: City): Boolean {
// Return true if all customers are from the given city
return customers.all { it.city == city }
}複製程式碼檢查是否有Customer來自指定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 }
}複製程式碼統計來自某一City的Customer的數量
fun Shop.countCustomersFrom(city: City): Int {
// Return the number of customers from the given city
return customers.count { it.city == city }
}複製程式碼查詢來自指定City的第一個Customer，沒有找到的話返回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 }
}複製程式碼16. n16FlatMap
獲取Customer訂購的Product集合
val Customer.orderedProducts: Set<Product> get() {
// Return all products this customer has ordered
return orders.flatMap { it.products }.toSet()
}複製程式碼獲取Shop賣出的Product集合
val Shop.allOrderedProducts: Set<Product> get() {
// Return all products that were ordered by at least one customer
return customers.flatMap { it.orderedProducts }.toSet()
}複製程式碼17. n17MaxMin.kt
查詢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 }
}複製程式碼查詢已購買價格最高的Product
fun Customer.getMostExpensiveOrderedProduct(): Product? {
// Return the most expensive product which has been ordered
return orderedProducts.maxBy { it.price }
}複製程式碼18. n18Sort
通過Customer的Order數量排序
fun Shop.getCustomersSortedByNumberOfOrders(): List<Customer> {
// Return a list of customers, sorted by the ascending number of orders they made
return customers.sortedBy { it.orders.size }
}複製程式碼sortedBy為從小到大，sortedByDescending為從大到小
19. n19Sum
統計已下訂單總價,需要注意的是，一個Customer可能多次購買了同一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 } }
}複製程式碼也可以
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 }
}複製程式碼20. n20GroupBy
分組
fun Shop.groupCustomersByCity(): Map<City, List<Customer>> {
// Return a map of the customers living in each city
return customers.groupBy { it.city }
}複製程式碼21. n21Partition
獲取未交付Order數量多於已交付Order的Customer集合
fun Shop.getCustomersWithMoreUndeliveredOrdersThanDelivered(): Set<Customer> {
// 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()
}複製程式碼22. n22Fold
獲取所有Customer都購買了的Product
fun Shop.getSetOfProductsOrderedByEveryCustomer(): Set<Product> {
// Return the set of products ordered by every customer
return customers.fold(allOrderedProducts, {
orderedByAll, customer ->
orderedByAll.intersect(customer.orders.flatMap { it.products }.toSet())
})
}複製程式碼flod是一個類似reduce的函式，該函式可以把前一次計算的返回值當成下一次計算的引數，flod可以設定初始值，intersect方法的意思是求交集
23. n23CompoundTasks
獲取訂購了某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 }
}複製程式碼獲取某Product被購買的次數
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 }
}複製程式碼24. n24ExtensionsOnCollections
用Kotlin重寫_24_JavaCode中的doSomethingStrangeWithCollection方法
fun doSomethingStrangeWithCollection(collection: Collection<String>): Collection<String>? {
val groupsByLength = Maps.newHashMap<Int, List<String>>()
for (s in collection) {
var strings: MutableList<String>? = groupsByLength[s.length] as MutableList<String>?
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
}複製程式碼第二節完，好多東西一知半解