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

缺乏、安全感 2023-09-29 14:54 50阅读 0赞

### 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;
        }