java8 快捷方式

java8 日常使用

1、Collectors.toMap()

toMap(Function keyMapper, Function valueMapper, BinaryOperator mergeFunction, Supplier mapSupplier)

       public static void main(String[] args) {
        Student student = new Student().setId(2).setName("zs").setScore(20);
        Student student2 = new Student().setId(1).setName("lisi").setScore(30);
        List<Student> list = new ArrayList<>();
        list.add(student);
        list.add(student2);
        /**
         Collectors.toMap 方法参数参数1key的处理策略，参数2value的处理策略,参数3当key冲突时候处理的策略,参数4返回何种Map
         注意点 1、key的重复时候处理
               2、转map的出现值的空指针异常
         */
        //转映射map
         Map<String, Integer> map =
        list.stream().collect(Collectors.toMap(Student::getName, Student::getScore));
        list.stream().collect(Collectors.toMap(Student::getName, o->o.getScore()));
        //处理值空指针
        list.stream().collect(Collectors.toMap(Student::getName, o->Optional.ofNullable(o.getScore()).orElse(0)));
        list.stream().collect(Collectors.toMap(Student::getName, o->Optional.ofNullable(o.getScore()).orElse(0),(v1,v2)->v1));
         // 当key没有重复时候
        Map<Integer, Student> map1 =
        list.stream().collect(Collectors.toMap(Student::getId, o -> o));
        // 或者
        list.stream().collect(Collectors.toMap(Student::getId, Function.identity()));

        list.add(student);
        // 当key重复时候
        list.stream().collect(Collectors.toMap(Student::getId, Function.identity(), (v1, v2) -> v1, TreeMap::new));
        list.stream().collect(Collectors.toMap(Student::getId, Function.identity(), (v1, v2) -> v1, LinkedHashMap::new));
        list.stream().collect(Collectors.toMap(Student::getId, Function.identity(), (v1, v2) -> v1, ConcurrentHashMap::new));
        //参数3策略key重复时候进行的合并策略
        list.stream().collect(Collectors.toMap(Student::getName, Function.identity(), Student::getMerge, TreeMap::new));
    }

    @Data
    @Accessors(chain = true)
    static class Student {
        private Integer id;
        private String name;
        private Integer score;
        public static Student getMerge(Student s1, Student s2){
            s1.setScore(s1.getScore()+s2.getScore());
            return s1;
        }
    }