java理论基础Stream reduce实现集合元素归约

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

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

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

Integer total3 = employees.stream()
        .reduce(0,(totalAge,emp) -> totalAge + emp.getAge(),Integer::sum); //注意这里reduce方法有三个参数
System.out.println(total); //346

Integer total2 = employees
        .parallelStream()
        .map(Employee::getAge)
        .reduce(0,Integer::sum,Integer::sum);  //注意这里reduce方法有三个参数
System.out.println(total); //346