同事写了段代码，差点没看懂，java1.8特性List转Map & Function.identity()

需求

将一个用户List转成以userName为Key，user本身为value的Map。

常规写法

public static Map<String,SysUser> userList2Map(List<SysUser> userList) {
        
        if (userList == null || userList.size() == 0){
            return new HashMap<>();
        }
        
        Map<String,SysUser> userMap = new HashMap<>(userList.size());

        for (SysUser user:userList){
            userMap.put(user.getUserName(),user);
        }

        return userMap;
    }

刚学Stream流后的写法

public static Map<String,SysUser> List2Map(List<SysUser> userList) {

        return userList.stream().distinct().collect(Collectors.toMap(new Function<SysUser, String>() {
            @Override
            public String apply(SysUser user) {
                return user.getUserName();
            }
        }, new Function<SysUser, SysUser>() {
            @Override
            public SysUser apply(SysUser user) {
                return user;
            }
        }));
    }

又学了拉姆达表达式后的写法

public static Map<String,SysUser> List2Map(List<SysUser> userList) {

        return userList.stream()
                .distinct()
                .collect(Collectors.toMap(user1 -> user1.getUserName(), user -> user));
    }

还学了方法引用后的写法

public static Map<String,SysUser> List2Map(List<SysUser> userList) {

        return userList.stream()
                .distinct()
                .collect(Collectors.toMap(SysUser::getUserName, user -> user));
    }

又看了Function.identity()后的写法

public static Map<String,SysUser> List2Map(List<SysUser> userList) {

        return userList.stream()
                .distinct()
                .collect(Collectors.toMap(SysUser::getUserName,Function.identity()));
    }

高端的食材往往只需要最朴素的烹饪法方式！

如果只是简单的需求，还是最原始的写法清晰明了！

后面的写法只是为了方便学习stream流的使用，让自己能看懂其他卷王写的“优秀”代码

Function.identity()

是Function类一个静态方法，作用就是返回参数自己，上面的例子中，传入user，返回的也是user本身，所以可以直接使用Function.identity()

/**
     * Returns a function that always returns its input argument.
     *
     * @param <T> the type of the input and output objects to the function
     * @return a function that always returns its input argument
     */
    static <T> Function<T, T> identity() {
        return t -> t;
    }