java8 - >五种求和方式

    /**
     * 五种求和方式
     * @Author: Mr.li
     * @Date: 2020/9/8
     */
    @Test
    public void demo3(){
        List<Person> personlist = new ArrayList<>();
        Person person = new Person();
        for(int i=0;i<10;i++){
            person.setId(i);
            person.setName("你好=="+i);
            personlist.add(person);
        }
        Long firstSum = personlist.stream()
                .collect(Collectors.summarizingLong(Person::getId)).getSum();
        Long secondSum = personlist.stream()
                .collect(Collectors.summingLong(Person::getId));
        int thirdSum = personlist.stream()
                .mapToInt(Person::getId).sum();
        Integer forthSum = personlist.stream()
                .collect(Collectors.reducing(0,Person::getId,(i,j)->i+j));

        IntSummaryStatistics stats = personlist.stream().mapToInt(Person::getId).summaryStatistics();
        Long fiveSum = stats.getSum();
        System.out.println(firstSum + ":" + secondSum + ":" + thirdSum + ":" + forthSum + ":" + fiveSum);

    }

redult：