JAVA8学习——深刻浅出函数式接口FunctionInterface（学习过程）

函数式接口

函数式接口详解：FunctionInterface接口

话很少说，先打开源码，查阅一番。寻得FunctionInterface接口

package java.util.function;

import java.util.Objects;

/**
 * Represents a function that accepts one argument and produces a result.
 *
 * <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.
     *
     * @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.
     *
     * @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
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @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.
     *
     * @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.
     *
     * @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;
    }
}

函数式接口代码测试：FunctionTest

public class FunctionTest {
    public static void main(String[] args) {
        FunctionTest test = new FunctionTest();
        // 传递行为，而不是传递值
        System.out.println(test.comput(1, value -> 2 * value));

        System.out.println(test.comput(2, value -> 5 + value));

        System.out.println(test.comput(3,Integer::intValue));

        System.out.println(test.convert(4,value -> value + "helloworld"));
    }

    public int comput(int a, Function<Integer, Integer> function) {
        //apply ,传递的是行为
        int result = function.apply(a);
        return result;
    }

    public String convert(int a, Function<Integer, String> function) {
        return function.apply(a);
    }

    //    对于以前只传递值的写法,几种行为就要定义几种写法。 如今可使用上面的方式去 传递行为
    public int method1(int a) {
        return a + 1;
    }

    public int method2(int a) {
        return a * 5;
    }

    public int method3(int a) {
        return a * a;
    }
}

高阶函数：若是一个函数接收一个函数做为参数，或者返回一个函数做为返回值，那么该函数就叫作高阶函数。函数式编程语言js等语言里面都支持大量的高阶函数，JAVA从1.8开始也开始支持高阶函数。

FunctionInterface接口中提供的方法详解

1. apply
2. compose （function组合）
3. andThen
4. identity

代码测试上述方法

package java.util.function;

import java.util.Objects;

/**
 * Represents a function that accepts one argument and produces a result.
 *
 * <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.
     *
     * @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.
     *
     * @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
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @throws NullPointerException if before is null
     
     * 先应用beforefunction，再应用实例的function
     实际上：将两个function组合在一块儿了。先执行before方法，而后将处理的结果传递给当前对象的apply方法。实现了两个function的串联。既然实现了两个function的串联，就能实现多个函数的串联。
     * @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.
     *
     * @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
     
     和before函数相反，先应用this function，而后再使用after方法。 实现两个方法的串联。
     
     * @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.
     *
     * @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;
    }
}

• compose：
  先应用beforefunction，再应用实例的function
  实际上：将两个function组合在一块儿了。先执行before方法，而后将处理的结果传递给当前对象的apply方法。实现了两个function的串联。既然实现了两个function的串联，就能实现多个函数的串联。
• andThen:
  和before函数相反，先应用this function，而后再使用after方法。 实现两个方法的串联。
• identity：
  传入什么，返回什么。

这些方法都是颇有价值的。

/**
 *  compose , andThen 方法的使用
 */
public class FunctionTest2 {

    public static void main(String[] args) {
        FunctionTest2 test2 = new FunctionTest2();
        int compute = test2.compute(2, v -> v * 3, v -> v * v);//12
        int compute2 = test2.compute2(2, v -> v * 3, v -> v * v);//36
        System.out.println(compute);
        System.out.println(compute2);
    }

    public int compute(int a, Function<Integer, Integer> function1, Function<Integer, Integer> function2) {
        return function1.compose(function2).apply(a);
    }
    public int compute2(int a, Function<Integer, Integer> function1, Function<Integer, Integer> function2) {
        return function1.andThen(function2).apply(a);
    }
}

BiFunction类详解

package java.util.function;

import java.util.Objects;

/**
 * Represents a function that accepts two arguments and produces a result.
 * This is the two-arity specialization of {@link Function}.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object, Object)}.
 *
 * @param <T> the type of the first argument to the function
 * @param <U> the type of the second argument to the function
 * @param <R> the type of the result of the function
 *
 * @see Function
 * @since 1.8
 */
@FunctionalInterface
public interface BiFunction<T, U, R> {

    /**
     * Applies this function to the given arguments.
     *
     * @param t the first function argument
     * @param u the second function argument
     * @return the function result
     */
    R apply(T t, U u);

    /**
     * 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.
     *
     * @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
     */
    default <V> BiFunction<T, U, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t, U u) -> after.apply(apply(t, u));
    }
}

BiFunction类，双向接口类，提供了两个输入参数，一个输出参数

//BiFunction类的使用。 有两个输入参数
    public int compute3(int a, int b, BiFunction<Integer, Integer, Integer> biFunction) {
        return biFunction.apply(a, b);
    }

    public int compute4(int a, int b, BiFunction<Integer, Integer, Integer> biFunction,Function<Integer,Integer> function) {
        return biFunction.andThen(function).apply(a,b);
    }
    
    //使用BiFunction来完成
    System.out.println(test2.compute3(1,2,(value1,value2)-> value1 + value2));
    System.out.println(test2.compute3(1,2,(value1,value2)-> value1 - value2));
    System.out.println(test2.compute3(1, 2, (value1, value2) -> value1 * value2));
    System.out.println(test2.compute3(1, 2, (value1, value2) -> value1 / value2));
        
    //使用BiFunction中的andThen.
    System.out.println(test2.compute4(2,3,(value1,value2)->value1+value2,value->value*value));

为何BiFunction类中没有Compose方法呢？

倒推一下：由于若是有Compose方法，会先执行参数的Function。不管参数是Function仍是BiFunction，返回值也都是一个值，而后就没办法再去执行BiFuntion.

---

使用lambda表达式解决简单的开发问题：

定义一个简单的Person类，而后使用lambda表达式解决一些问题
public class Person {
    private String username;
    private int age;
}

package com.dawa.jdk8;

import java.util.Arrays;
import java.util.List;
import java.util.function.BiFunction;
import java.util.stream.Collectors;

public class PersonTest {
    public static void main(String[] args) {

        Person person1 = new Person("zhangsan", 20);
        Person person2 = new Person("lisi", 30);
        Person person3 = new Person("wangwu", 40);

        List<Person> list = Arrays.asList(person1,person2,person3);

        PersonTest test = new PersonTest();
        //测试第一个方法。
        List<Person> list1 = test.getPersonByUsername("zhangsan", list);
        list1.forEach(person -> System.out.println(person.getUsername()));
        //测试第二种方法
        List<Person> personByAge = test.getPersonByAge(20, list);
        personByAge.forEach(person -> System.out.println(person.getAge()));
        //测试第三方法
        List<Person> peopleList = test.getPersonByArg(20, list,
                (age, personList) -> personList.stream().filter(p
                erson -> person.getAge() > age).collect(Collectors.toList()));
        peopleList.forEach(person -> System.out.println(person.getUsername()));

    }

    //使用lambda表达式定义一个 处理的方法
    //filter 方法，参数是一个Predicate 谓语
    //stream 提供了一个将流转换成集合的方法 collect(Collectors.toList())
    public List<Person> getPersonByUsername(String username, List<Person> personList) {
        return personList.stream().
        filter(person -> 
        person.getUsername().equals("zhangsan")).collect(Collectors.toList());
    }

    //第二种方式,先直接使用lambda表达式将BiFunction定义好，而后直接将方法的两个参数传入到BiFunction.
    public List<Person> getPersonByAge(int age, List<Person> personList) {
        BiFunction<Integer, List<Person>, List<Person>> biFunction = (ageArg, Persons) -> {
          return   Persons.stream().filter(person -> person.getAge() > ageArg).collect(Collectors.toList());
        };
        return biFunction.apply(age,personList);
    }
    
    //第三种方式，动做也让他本身传递。 函数式接口的好处。
    public List<Person> getPersonByArg(int age, List<Person> personList, BiFunction<Integer, List<Person>, List<Person>> biFunction) {
        return biFunction.apply(age, personList);
    }
}

真谛：函数式接口，传递的是行为，而不是数据。

Predicate 类详解（谓词）

package java.util.function;

import java.util.Objects;

/**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #test(Object)}.
 *
 * @param <T> the type of the input to the predicate
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Predicate<T> {

    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * AND of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code false}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ANDed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * AND of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    /**
     * Returns a predicate that represents the logical negation of this
     * predicate.
     *
     * @return a predicate that represents the logical negation of this
     * predicate
     */
    default Predicate<T> negate() {
        return (t) -> !test(t);
    }

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * OR of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code true}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ORed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * OR of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> or(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

    /**
     * Returns a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}.
     *
     * @param <T> the type of arguments to the predicate
     * @param targetRef the object reference with which to compare for equality,
     *               which may be {@code null}
     * @return a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}
     */
    static <T> Predicate<T> isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }
}

默认调用：boolean test(T t)给定一个输入参数，判断是否知足条件。知足则返回true，不知足返回false。

测试案例：

package com.dawa.jdk8;

import java.util.function.Predicate;

public class PreDicateTest {
    public static void main(String[] args) {
        Predicate<String> predicate = p -> p.length() > 5;

        System.out.println(predicate.test("hello"));

    }
}

这个接口会在流stream里面获得大量的运用。上述案例在 stream的 filter（）方法参数中使用。

到如今为止，Function包下的接口已经基础了两个了。
但是只是讲了几个特别重要的接口，其余的接口是没时间一个一个讲的。

这个时候我去看了一下源码，发现JAVA8底层源码，大量的使用函数接口来进行实现。

道阻且长。~ 加油。 2019年12月29日22:18:25。明天就要上班了，今晚早点休息。