逐步走向响应式编程-常见函数式接口- Function＜T, R＞

Posted 2022-07-18 浦江之猿

前面介绍了函数式接口Predicate和Consumer,本文继续介绍另一函数式接口Function<T, R>，此接口同样也位于 java.util.function包中，它的作用可以看作是消费者和生产者的结合，即消费某种原材料然后生产出某种产品，继续先分析一下源码。

• • 源码解析
  • 案例分析
  • • apply
    • • 构造数据
      • 定义转换逻辑
      • 开始转换
      • 转换结果
    • compose
    • andThen
    • identity
  • 总结

源码解析

package java.util.function;

import java.util.Objects;

/**
 * Represents a function that accepts one argument and produces a result.
 * 此接口中的方法接收一个参数并输出一个结果
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is @link #apply(Object).
 *
 * @param <T> the type of the input to the function
 * @param <R> the type of the result of the function
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Function<T, R> 

    /**
     * Applies this function to the given argument.
     * 此方法对输入的参数进行处理，并得到一个结果
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);

    /**
     * Returns a composed function that first applies the @code before
     * function to its input, and then applies this function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     * 此方法将两个function进行组合，得到一个新的function,组合的过程中如果有一个function出现了异常，异常由调用者来处理。组合逻辑是，将其中一个function以参数的形式传入（before），在compose内部获取before的结果，并传给第二个function组成一个新的function并返回
     * @param <V> the type of input to the @code before function, and to the
     *           composed function
     * @param before the function to apply before this function is applied
     * 这句话好难理解，简单来就说就是将一个function当作一个参数
     * @return a composed function that first applies the @code before
     * function and then applies this function
     * 将before function的结果传给当前function，并返回。
     * @throws NullPointerException if before is null
     *
     * @see #andThen(Function)
     */
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) 
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the @code after function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *此函数和compose函数有点类似，但顺序不同。此函数是先执行调用者，再执行andThen的参数，即将调用者的结果传给参数中的function使用，并返回一个function.
     * @param <V> the type of output of the @code after function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the @code after function
     * @throws NullPointerException if after is null
     *
     * @see #compose(Function)
     */
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) 
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    

    /**
     * Returns a function that always returns its input argument.
     *将apply的参数当着结果返回出来
     * @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;
    

案例分析

通过处理User案例来分析每个方法的使用。提供了两个类User 和 UserEntity：
User

public class User 
    private int age;
    private String name;
    /**
     * @return the age
     */
    public int getAge() 
        return age;
    
    /**
     * @param age
     * @param name
     */
    public User(int age, String name) 
        this.age = age;
        this.name = name;
    
    /**
     * @param age the age to set
     */
    public void setAge(int age) 
        this.age = age;
    
    /**
     * @return the name
     */
    public String getName() 
        return name;
    
    /**
     * @param name the name to set
     */
    public void setName(String name) 
        this.name = name;
    
    

UserEntity

public class UserEntity 
    private int age;
    private String name;
    private int type;
    /**
     * @param age
     * @param name
     * @param type
     */
    public UserEntity(int age, String name, int type) 
        this.age = age;
        this.name = name;
        this.type = type;
    
    /**
     * @return the age
     */
    public int getAge() 
        return age;
    
    /**
     * @param age the age to set
     */
    public void setAge(int age) 
        this.age = age;
    
    /**
     * @return the name
     */
    public String getName() 
        return name;
    
    /**
     * @param name the name to set
     */
    public void setName(String name) 
        this.name = name;
    
    /**
     * @return the type
     */
    public int getType() 
        return type;
    
    /**
     * @param type the type to set
     */
    public void setType(int type) 
        this.type = type;
    
    /* (non-Javadoc)
     * @see java.lang.Object#toString()
     */
    
    @Override
    public String toString() 
        return "UserEntity age=" + age + ", name=" + name + ", type=" + type + "";
    
    

apply

将一组User转换成一组User Entity, 根据User的年龄来定义用户级别（普通用户，vip,svip），其中用户级别是User Entity的一个字段，所以输入参数是List,返回结果是List 。

构造数据

        User user1 = new User(10, "张三");
        User user2 = new User(15, "李四");
        User user3 = new User(16, "王五");
        User user4 = new User(20, "赵六");
        User user5 = new User(25, "田七");
        List<User> users = new ArrayList<>();
        users.add(user1);
        users.add(user2);
        users.add(user3);
        users.add(user4);
        users.add(user5);

定义转换逻辑

 /**
     * Convert users to user entities 
     * age<=15:GENERAL
     * 15<age<=20:VIP
     * 20<age:SVIP
     * @param Function<List<User>,List<UserEntity>>
     */
    static Function<List<User>,List<UserEntity>> multiUsersToEntities(List<User> users)
        Function<List<User>,List<UserEntity>> function = t-> 
            List<UserEntity> userEntities = new ArrayList<>();
            int age;
            String name;
            UserEntity userEntity;
            for(User user:t)
                age = user.getAge();
                name = user.getName(); 
                if( age<=15)
                     userEntity = new UserEntity(age, name, Type.GENERAL.getCode());   
                else if (age > 15 && age <=20)
                    userEntity = new UserEntity(age, name, Type.VIP.getCode());   
                else
                    userEntity = new UserEntity(age, name, Type.SVIP.getCode());   
                
                userEntities.add(userEntity);
            
            return userEntities;
        ;
        return function;
    

开始转换

        Function<List<User>,List<UserEntity>> function = multiUsersToEntities(users);
        List<UserEntity> uEntities = function.apply(users);

转换结果

uEntities.stream().forEach(u->System.out.println(u.toString()));

转换成功后的结果如下

UserEntity age=10, name=张三, type=3
UserEntity age=15, name=李四, type=3
UserEntity age=16, name=王五, type=1
UserEntity age=20, name=赵六, type=1
UserEntity age=25, name=田七, type=2

compose

根据注释中的理解感觉一脸蒙逼，这里简单来说就是将两个函数进行组合，先执行compose里面的函数再执行外面的函数，为了便于理解脱离上面的例子。
定义两个函数如下：

//对输入的值进行两倍处理
Function<Integer,Integer> function2 = t-> t*2;
//对输入的值进行平方处理
Function<Integer,Integer> function3 = t-> t*t;

System.out.println("compose result :"+function2.compose(function3).apply(3));

结果为：compose result :18

andThen

同compose类似，也是将两个函数组合起来使用，但是顺序不一样，这里先执行andThen外面的函数，再执行andThen里面的函数。继续以上面的两个函数为例

System.out.println("andThen result :"+function2.andThen(function3).apply(3));

结果为：andThen result :36

identity

identiy个人没用过，这里举个例子吧，以后深入了解之后继续更新。

Function<Integer,Integer> function4 = Function.identity()；
System.out.println("identity result :"+function4.apply(3));

结果为：identity result :3

总结

内容很简单，在java8的Stream中，Function使用很广泛，后续的博文中会一一介绍。最后希望本文能帮助大家，祝大家在IT之路上少走弯路，一路绿灯不堵车，测试一性通过，bug秒解！
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