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java理論基礎(chǔ)Stream?reduce實(shí)現(xiàn)集合元素歸約

更新時(shí)間：2022年03月16日 14:52:17 作者：字母哥哥

這篇文章主要為大家介紹了java理論基礎(chǔ)Stream?reduce實(shí)現(xiàn)集合元素歸約示例詳解有需要的朋友可以借鑒參考下，希望能夠有所幫助，祝大家多多進(jìn)步

Integer類型歸約

reduce初始值為0，累加器可以是lambda表達(dá)式，也可以是方法引用。

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
int result = numbers
        .stream()
        .reduce(0, (subtotal, element) -> subtotal + element);
System.out.println(result);  //21

int result = numbers
        .stream()
        .reduce(0, Integer::sum);
System.out.println(result); //21

String類型歸約

不僅可以歸約Integer類型，只要累加器參數(shù)類型能夠匹配，可以對(duì)任何類型的集合進(jìn)行歸約計(jì)算。

List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result = letters
        .stream()
        .reduce("", (partialString, element) -> partialString + element);
System.out.println(result);  //abcde
String result = letters
        .stream()
        .reduce("", String::concat);
System.out.println(result);  //ancde

復(fù)雜對(duì)象歸約

計(jì)算所有的員工的年齡總和。

Employee e1 = new Employee(1,23,"M","Rick","Beethovan");
Employee e2 = new Employee(2,13,"F","Martina","Hengis");
Employee e3 = new Employee(3,43,"M","Ricky","Martin");
Employee e4 = new Employee(4,26,"M","Jon","Lowman");
Employee e5 = new Employee(5,19,"F","Cristine","Maria");
Employee e6 = new Employee(6,15,"M","David","Feezor");
Employee e7 = new Employee(7,68,"F","Melissa","Roy");
Employee e8 = new Employee(8,79,"M","Alex","Gussin");
Employee e9 = new Employee(9,15,"F","Neetu","Singh");
Employee e10 = new Employee(10,45,"M","Naveen","Jain");
List<Employee> employees = Arrays.asList(e1, e2, e3, e4, e5, e6, e7, e8, e9, e10);
Integer total = employees.stream().map(Employee::getAge).reduce(0,Integer::sum);
System.out.println(total); //346

先用map將Stream流中的元素由Employee類型處理為Integer類型（age）。

然后對(duì)Stream流中的Integer類型進(jìn)行歸約

Combiner合并器的使用

除了使用map函數(shù)實(shí)現(xiàn)類型轉(zhuǎn)換后的集合歸約，我們還可以用Combiner合并器來實(shí)現(xiàn)，這里第一次使用到了Combiner合并器。因?yàn)镾tream流中的元素是Employee，累加器的返回值是Integer，所以二者的類型不匹配。這種情況下可以使用Combiner合并器對(duì)累加器的結(jié)果進(jìn)行二次歸約，相當(dāng)于做了類型轉(zhuǎn)換。

Integer total3 = employees.stream()
        .reduce(0,(totalAge,emp) -> totalAge + emp.getAge(),Integer::sum); //注意這里reduce方法有三個(gè)參數(shù)
System.out.println(total); //346

計(jì)算結(jié)果和使用map進(jìn)行數(shù)據(jù)類型轉(zhuǎn)換的方式是一樣的。

并行流數(shù)據(jù)歸約（使用合并器）

對(duì)于大數(shù)據(jù)量的集合元素歸約計(jì)算，更能體現(xiàn)出Stream并行流計(jì)算的威力。

在進(jìn)行并行流計(jì)算的時(shí)候，可能會(huì)將集合元素分成多個(gè)組計(jì)算。為了更快的將分組計(jì)算結(jié)果累加，可以使用合并器。

Integer total2 = employees
        .parallelStream()
        .map(Employee::getAge)
        .reduce(0,Integer::sum,Integer::sum);  //注意這里reduce方法有三個(gè)參數(shù)
System.out.println(total); //346

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