java8新特性 Stream API 终止操作

左手的ㄟ右手 2021-11-29 11:40 196阅读 0赞

Stream的终止操作：终止操作会从流的流水线生成结果，其结果可以是任何不适流的操作，例如：List,Integer,甚至是void

一，查找与匹配

List<Employee> employeeList1 = Arrays.asList(
                new Employee("cdy1",18,1772.22),
                new Employee("cdy2",18,1872.22),
                new Employee("cdy3",18,1672.22),
                new Employee("cdy4",18,1972.22)
        );
    
        List<Employee> employeeList2 = Arrays.asList(
                new Employee("cdy1",18,1772.22),
                new Employee("cdy2",19,1872.22),
                new Employee("cdy3",11,1672.22),
                new Employee("cdy4",18,1972.22)
        );

说明：Predicate 是断言型函数式接口；

1、allMatch(Predicate p) 检查流中所有元素是否都满足条件，返回boolean值

@Test
        public void test1(){
            boolean b1 = employeeList1.stream().allMatch((e) -> {
                if (e.getAge() == 18 || e.getSalary() == 1972.22) {
                    return true;
                }
                return false;
            });
            System.out.println(b1);//truw
            boolean b2 = employeeList2.stream().allMatch((e) -> {
                if (e.getAge() == 18 || e.getSalary() == 1972.22) {
                    return true;
                }
                return false;
            });
            System.out.println(b2); //false
    
        }

2、anyMatch(Predicate p) 检查所有元素中至少有一个元素满足条件

@Test
        public void test2() {
            boolean b = employeeList1.stream().anyMatch((e) -> e.getAge() == 18);
            System.out.println(b);//true
        }

3、noneMatch(Predicate p) 所有元素都不满足条件返回true，否则false

@Test
        public void test3() {
            boolean b = employeeList1.stream().noneMatch((e) -> e.getAge() == 0);
            System.out.println(b);//true
        }

4、findFirst() 返回第一个元素

@Test
        public void test4() {
            Optional<Employee> first = employeeList1.stream()
                                            .sorted((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary()))
                                            .findFirst();
    
            System.out.println(first.get().getSalary());//1672.22
        }

5、findAny() 返回当前流中的任意元素

@Test
        public void test5() {
            Optional<Employee> any = employeeList1.stream().findAny();
            System.out.println(any.get().getSalary());//1772.22
        }

6.reduce(T iden, BinaryOperator b) (规约)可以将流中元素反复结合起来，得到一个值。返回 T

@Test
        public void test8() {
            List<Integer> integers = Arrays.asList(1, 2, 3, 4, 5);
            Integer reduce = integers.stream().reduce(3, (x, y) -> x + y);
            System.out.println(reduce);
    
        }

7.collect(Collector c) 将流转换为其他形式。接收一个 Collector接口的 实现，用于给Stream中元素做汇总的方法

@Test
        public void test9() {
            List<Integer> collect = employeeList2.stream().map(Employee::getAge).collect(Collectors.toList());
            collect.forEach(System.out::println);
    
            Set<String> collect1 = employeeList2.stream().map(Employee::getName).collect(Collectors.toSet());
            collect1.forEach(System.out::print);
    
            employeeList2.stream().collect(Collectors.toCollection(HashSet::new));
    
        }