java8新特性Map的创建与merge

Map集合的创建与merge操作：

　　　　 Student student1 = new Student("Johnson",UUID.randomUUID());
        Student student2 = new Student("Lily",UUID.randomUUID());
        Student student3 = new Student("KangKang",UUID.randomUUID());
        //创建
        Map<UUID, Student> collect1 = Stream.of(new AbstractMap.SimpleEntry<>(student1.getSid(), student1)).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        Map<UUID, List<Student>> collect2 = Stream.of(new AbstractMap.SimpleEntry<>(student1.getSid(), Lists.newArrayList(student1))).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        Map<UUID, Map<UUID, Student>> collect3 = Maps.newHashMap();

        //合并（添加）
        collect1.merge(student2.getSid(), student2, (oldValue, value) -> value.getSid().compareTo(oldValue.getSid())==0 ? value : oldValue);
        collect2.merge(student2.getSid(), newArrayList(student2), (oldValue, value) -> Stream.concat(oldValue.stream(), value.stream()).collect(toList()));
        collect3.merge(student3.getSid(),collect1,(oldValue,value)->{
            oldValue.putAll(value);
            return oldValue;
        });

static class Student{
        private String name;
        private UUID sid;

        public Student(String name, UUID sid) {
            this.name = name;
            this.sid = sid;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public UUID getSid() {
            return sid;
        }

        public void setSid(UUID sid) {
            this.sid = sid;
        }
    }

merge() 可以这么理解：它将新的值赋值到 key （如果不存在）或更新给定的key 值对应的 value，其源码如下：

   default V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        Objects.requireNonNull(value);
        V oldValue = get(key);
        V newValue = (oldValue == null) ? value :
                   remappingFunction.apply(oldValue, value);
        if(newValue == null) {
            remove(key);
        } else {
            put(key, newValue);
        }
        return newValue;
    }