groovy 的inject()方法使用简例

inject()方法有强大的累计功能，以闭包作为参数处理一个数据结构并返回一个值。inject()方法的第一个参数是第二个参数（闭包）的立即返回值，无需引入额外的变量就能实现累积计算，例如统计图书馆所有丛书库存，每套丛书有不同套数，每套丛书有不同册数和库存数量，业务模型如下

public class Library {
private List<BookSeries> bookSeriesList;

// getter and setter
}

public class BookSeries {
private int suitNum;
private List<Book> books;
// getter and setter

}

public class Book {
private String name;
private int store;
private double price;

// getter and setter
}

因了inject()的强大累积计算功能，我写了如下测试：

class BookSeriesTest extends Specification {

def "验证丛书总数计算正确"() {
given: "图书馆丛书列表"
def library = new Library(
bookSeriesList: [
new BookSeries(suitNum: 1, books: [
new Book(name: "水浒传（上）", store: 1, price: 100.00),
new Book(name: "水浒传（中）", store: 2, price: 120.00),
new Book(name: "水浒传（下）", store: 3, price: 150.00),
]),

new BookSeries(suitNum: 2, books: [
new Book(name: "三国演义（上）", store: 4, price: 100.00),
new Book(name: "三国演义（中）", store: 5, price: 120.00),
new Book(name: "三国演义（下）", store: 6, price: 150.00),
]),

new BookSeries(suitNum: 3, books: [
new Book(name: "西游记（上）", store: 7, price: 100.00),
new Book(name: "西游记（中）", store: 8, price: 120.00),
new Book(name: "西游记（下）", store: 9, price: 150.00),
]),

new BookSeries(suitNum: 4, books: [
new Book(name: "红楼梦（上）", store: 10, price: 100.00),
new Book(name: "红楼梦（中）", store: 11, price: 120.00),
new Book(name: "红楼梦（下）", store:
4000
12, price: 150.00),
]),

new BookSeries(suitNum: 0, books: [
new Book(name: "大秦帝国（上）", store: 10, price: 100.00),
new Book(name: "大秦帝国（中）", store: 10, price: 120.00),
new Book(name: "大秦帝国（下）", store: 10, price: 150.00),
]),
]
)
when:"统计图书馆丛书总库存"
def totalStore = library.bookSeriesList.inject(0) { libraryTotalStore, bookSeries ->
libraryTotalStore + (bookSeries.suitNum > 0 ? bookSeries.books.store.sum() * bookSeries.suitNum : 0)
}

then:"验证库存统计正确"
totalStore == (1 + 2 + 3) * 1 + (4 + 5 + 6) * 2 + (7 + 8 + 9) * 3 + (10 + 11 + 12) * 4
println totalStore
}

最后，你能想象，如果用java写when部分的代码吗？可能需要写多重循环，还要临时变量保存累积库存，何必如此麻烦，groovy一行代码搞定（因篇幅限制，我折行处理）。groovy简洁的语法，高度描述性，构建对象也非常简单直观，非常适合写测试。groovy更多实用方法，后续持续coding。

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