java8 function接口浅析（个人见解，如有错误，还请指正）

先引入一篇介绍的文章：JDK8 函数式接口 -干货使用Function 详细讲解快速上手
案例转载自大佬文章

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JDK8的function接口，里面包含四个方法apply 、andThen 、compose、identity

1. apply方法
   案例

public class FunctionOneExample{
  public static void main(String args[]){
    Function<User, String> funcUserToString= (User e)-> {return e.getName();};
    List<User> UserList= 
     Arrays.asList(new User("A-0", 13), 
      new User("A-1", 25),
      new User("B-2", 10),
      new User("C-3", 15),
      new User("D-4", 29));
    List<String> userNameList=convertUserListToNamesList(UserList, funcUserToString);
    userNameList.forEach(System.out::println);
 }
 public static List<String> convertUserListToNamesList(List<User> UserList, Function<User, String> funcUserToString){
   List<String> userNameList=new ArrayList<String>(); 
   for(User user:UserList){
     userNameList.add(funcUserToString.apply(user));
   }
   return userNameList;
  }
}
输出结果：
A-0
A-1
B-2
C-3
D-4

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Function 的意思是将User类映射成String类型。
函数“funcUserToString”的内容是返回User的name。
funcUserToString.apply(user)的意思调用这个函数，得到函数返回的映射值。

2. andThen()方法
   案例

public class FunctionAndThenOneExample{
  public static void main(String args[]){
    Function<User, String> funcUserToString= (User e)-> {return e.getName();};
    List<User> UserList=
     Arrays.asList(new User("Tike-one", 13), 
      new User("Hello-world", 25),
      new User("Earth-one", 10),
      new User("Nanshou-one", 15),
      new User("Dogkeep-one", 29));
    Function<String,String> initialFunction= (String s)->s.substring(0,1);
    List<String> userNameListInitials=convertUserListToNamesList(UserList, funcUserToString.andThen(initialFunction));
    userNameListInitials.forEach(str->{System.out.print(" "+str);});
 }
  public static List<String> convertUserListToNamesList(List<User> UserList, Function<User, String> funcUserToString){
   List<String> userNameList=new ArrayList<String>(); 
   for(User user:UserList){
     userNameList.add(funcUserToString.apply(user));
   }
   return userNameList;
  }
}

输出结果：
T H E N D
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UserList, funcUserToString.andThen(initialFunction)：在函数funcUserToString.apply(user)的时候，先执行main方法里的funcUserToString，获取到了用户名，然后调用initialFunction函数截取第一个字符。
文档定义如下：

也就是说，after函数会在函数调用后再调用。

3. Compose()方法
   案例

public class FunctionComposeExample{
  public static void main(String args[]){
    Function<User, String> funcUserToString= (User e)-> {return e.getName();};
    Function<User, User> funcuserFirstName= 
     (User e)-> {int index= e.getName().indexOf("-");
                 String firstName=e.getName().substring(0,index);
                 e.setName(firstName);
                 return e;};
    List<User> UserList=
      Arrays.asList(new User("Tike one", 13), 
       new User("Hello-world", 25),
       new User("Earth-one", 10),
       new User("Nanshou-one", 15),
       new User("Dogkeep-one", 29));
    List<String> userFirstNameList= convertUserListToNamesList(UserList,funcUserToString.compose(funcuserFirstName));
    userFirstNameList.forEach(str->{System.out.print(" "+str);});
  }
 public static List<String> convertUserListToNamesList(List<User> UserList, Function<User, String> funcUserToString){
   List<String> userNameList=new ArrayList<String>(); 
   for(User user:UserList){
     userNameList.add(funcUserToString.apply(user));
   }
   return userNameList;
  }
}
结果
Tike Hello Earth Nanshou Dogkeep
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funcUserToString.compose(funcuserFirstName)：和andThen方法相反，compose方法会先执行参数里的函数funcuserFirstName，然后将得到的结果返回给funcUserToString。
文档定义如下：

先执行函数before，得到的结果返回给调用函数。

4. identity()方法
   案例

public class FunctionTRIdentityExample{
  public static void main(String args[]){
    List UserList=
     Arrays.asList(new User("Tike-one", 13), 
      new User("Hello-world", 25),
      new User("Earth-one", 10),
      new User("Nanshou-one", 15),
      new User("Dogkeep-one", 29));
    List userNameListInitials=applyIdentityTouserList(UserList, Function.identity());
    userNameListInitials.forEach(System.out::println);
 }
  public static List applyIdentityTouserList(List UserList, Function funcuserTouser){
   List userNameList=new ArrayList(); 
   for(User user:UserList){
     userNameList.add(funcuserTouser.apply(user));
   }
   return userNameList;
  }
}
结果
User Name:Tike-one Age:13
User Name:Hello-world Age:25
User Name:Earth-one Age:10
User Name:Nanshou-one Age:15
User Name:Dogkeep-one Age:29
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Function.identity()：调用该静态方法，返回类名以及成员变量和值。
文档说明：