[Java] Java 函数式编程

曾经终败给现在 2024-02-21 10:38 60阅读 0赞

> 前言： Java函数式编程，是一种强大的编程范式，能够让你的代码更加简洁，优雅。Java 8 引入了函数式编程的支持，其中Lambda表达式和函数式接口是函数式编程的两个重要概念。在本篇文章中，我们将会详细介绍Java函数式编程以及常用的函数式接口。

--------------------

#### 文章目录 ####

*   *  Lambda表达式
     *  函数式接口
     *   *  Function 函数映射
         *   *  源码
         *  Consumer 消费型接口
         *   *  源码
         *  Predicate 断言型接口
         *   *  源码
         *  Supplier 供给型接口
         *   *  源码
     *  方法引用
     *  其他函数式接口
     *   *  Bi类型接口
         *  操作基本数据类型的接口
     *  总结

### Lambda表达式 ###

Lambda表达式是一种匿名函数，可以理解为一段可以传递的代码。在 Java 中，Lambda 表达式可以替代只有一个抽象方法的接口。下面是一个Lambda表达式的例子:

() -> System.out.println("Hello World")

其中，左侧括号内是Lambda表达式的参数列表（如果没有参数，则为空），箭头“->”将 Lambda 表达式的参数列表和表达式主体分隔开，右侧则是Lambda表达式的主体（也就是Lambda表达式要执行的代码块）。Lambda表达式是使用编写函数式接口的简便方法。

### 函数式接口 ###

函数式接口是指仅包含一个抽象方法的接口。在 Java 中，函数式接口可以使用Lambda表达式来实现，从而实现函数式编程。Java提供了一些常用的函数式接口，如Function、Consumer、Predicate、Supplier等。

按照下面的格式定义，你也能写出函数式接口：

@FunctionalInterface
     修饰符 interface 接口名称 {
        
        返回值类型 方法名称(可选参数信息);
        // 其他非抽象方法内容
     }

虽然`@FunctionalInterface`注解不是必须的，但是自定义函数式接口最好还是都加上，一是养成良好的编程习惯，二是防止他人修改，一看到这个注解就知道是函数式接口，避免他人往接口内添加抽象方法造成不必要的麻烦。

@FunctionalInterface
    public interface MyFunction {
        
        void print(String s);
    }

上面我自定义的一个函数式接口，那么这个接口的作用是什么呢？**就是输出一串字符串，属于消费型接口，是模仿Consumer接口写的**，只不过这个没有使用泛型，而是将参数具体类型化了，不知道Consumer没关系，下面会介绍到，其实java8中提供了很多常用的函数式接口，Consumer就是其中之一，一般情况下都不需要自己定义，直接使用就好了。那么怎么使用这个自定义的函数式接口呢？我们可以用函数式接口作为参数，调用时传递Lambda表达式。如果一个方法的参数是Lambda，那么这个参数的类型一定是函数式接口。例如：

public class MyFunctionTest {
        
        public static void main(String[] args) {
        
            String text = "试试自定义函数好使不";
            printString(text, System.out::print);
        }
    
        private static void printString(String text, MyFunction myFunction) {
        
            myFunction.print(text);
        }
    }

执行以后就会输出“试试自定义函数好使不”这句话，如果某天需求变了，我不想输出这句话了，想输出别的，那么直接替换text就好了。**函数式编程是没有副作用的，最大的好处就是函数的内部是无状态的，既输入确定输出就确定**。函数式编程还有更多好玩的套路，这就需要靠大家自己探索了。

#### Function 函数映射 ####

**抽象方法：** R apply(T t)，传入一个参数，返回想要的结果。

public interface Function<T, R> {
 
 R apply(T t);
 }

Function接口接受一个参数并返回结果。我们可以使用andThen方法来将多个函数串联起来，进行组合操作。示例代码：

Function<Integer, String> intToString = Object::toString;
 Function<String, String> quote = s -> "'" + s + "'";
 Function<Integer, String> quoteIntToString = intToString.andThen(quote);
 
 String result = quoteIntToString.apply(123);
 System.out.println(result);

**默认方法：**

*  `compose(Function before)`，先执行compose方法参数before中的apply方法，然后将执行结果传递给调用compose函数中的apply方法在执行。  
    **使用方式：**

Function<Integer, Integer> function1 = e -> e * 2;
 Function<Integer, Integer> function2 = e -> e * e;
 
 Integer apply2 = function1.compose(function2).apply(3);
 System.out.println(apply2);

还是举一个乘法的例子，compose方法执行流程是先执行function2的表达式也就是3*3=9，然后在将执行结果传给function1的表达式也就是9*2=18，所以最终的结果是18。

*  `andThen(Function after)`，先执行调用andThen函数的apply方法，然后在将执行结果传递给andThen方法after参数中的apply方法在执行。它和compose方法整好是相反的执行顺序。  
    **使用方式：**

Function<Integer, Integer> function1 = e -> e * 2;
 Function<Integer, Integer> function2 = e -> e * e;
 
 Integer apply3 = function1.andThen(function2).apply(3);
 System.out.println(apply3);

这里我们和compose方法使用一个例子，所以是一模一样的例子，由于方法的不同，执行顺序也就不相同，那么结果是大大不同的。andThen方法是先执行function1表达式，也就是3*2=6，然后在执行function2表达式也就是6*6=36。结果就是36。

\*\*静态方法：\*\*identity()，获取一个输入参数和返回结果相同的Function实例。

**使用方式：**

Function<Integer, Integer> identity = Function.identity();
 Integer apply = identity.apply(3);
 System.out.println(apply);

平常没有遇到过使用这个方法的场景，总之这个方法的作用就是输入什么返回结果就是什么。

##### 源码 #####

::: details

/*
 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation. Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
 package java.util.function;
 
 import java.util.Objects;
 
 /**
 * Represents a function that accepts one argument and produces a result.
 *
 * 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;
 }
 }

:::

#### Consumer 消费型接口 ####

**抽象方法：** void accept(T t)，接收一个参数进行消费，但无需返回结果。

public interface Consumer<T> {
 
 void accept(T t);
 }

Consumer接口接受一个参数，但没有返回值。我们可以使用andThen方法来将多个Consumer组合起来，进行链式操作。示例代码：

List<String> list = Arrays.asList("a", "b", "c");
 Consumer<String> print = System.out::print;
 Consumer<String> println = System.out::println;
 
 list.forEach(print.andThen(println));

##### 源码 #####

::: details

@FunctionalInterface
 public interface Consumer<T> {
 
 
 /**
 * Performs this operation on the given argument.
 *
 * @param t the input argument
 */
 void accept(T t);
 
 /**
 * Returns a composed {@code Consumer} that performs, in sequence, this
 * operation followed by the {@code after} operation. If performing either
 * operation throws an exception, it is relayed to the caller of the
 * composed operation. If performing this operation throws an exception,
 * the {@code after} operation will not be performed.
 *
 * @param after the operation to perform after this operation
 * @return a composed {@code Consumer} that performs in sequence this
 * operation followed by the {@code after} operation
 * @throws NullPointerException if {@code after} is null
 */
 default Consumer<T> andThen(Consumer<? super T> after) {
 
 Objects.requireNonNull(after);
 return (T t) -> {
 accept(t); after.accept(t); };
 }
 }

:::

#### Predicate 断言型接口 ####

**抽象方法：** boolean test(T t),传入一个参数，返回一个布尔值。

public interface Predicate<T> {
 
 boolean test(T t);
 }

Predicate接口接受一个参数，返回一个布尔值。我们可以使用and、or、negate方法将多个Predicate组合起来，进行复合逻辑的判断。示例代码：

List<String> list = Arrays.asList("cat", "dog", "bird", "lion", "tiger");
 Predicate<String> startsWithC = s -> s.startsWith("c");
 Predicate<String> endsWithR = s -> s.endsWith("r");
 Predicate<String> containsO = s -> s.contains("o");
 
 list.stream()
 .filter(startsWithC.and(endsWithR).or(containsO))
 .forEach(System.out::println);

##### 源码 #####

::: details

/*
 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation. Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
 package java.util.function;
 
 import java.util.Objects;
 
 /**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * 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.
 *
 * 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.
 *
 * 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);
 }
 }

:::

#### Supplier 供给型接口 ####

\*\*抽象方法：\*\*T get()，无参数，有返回值。

public interface Supplier<T> {
 
 T get();
 }

Supplier接口不接受任何参数，返回一个结果。我们可以使用get方法来获取结果。示例代码：

Supplier<String> helloSupplier = () -> "Hello";
 System.out.println(helloSupplier.get() + " world");

##### 源码 #####

::: details

/*
 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation. Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
 package java.util.function;
 
 /**
 * Represents a supplier of results.
 *
 * There is no requirement that a new or distinct result be returned each
 * time the supplier is invoked.
 *
 * This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #get()}.
 *
 * @param <T> the type of results supplied by this supplier
 *
 * @since 1.8
 */
 @FunctionalInterface
 public interface Supplier<T> {
 
 
 /**
 * Gets a result.
 *
 * @return a result
 */
 T get();
 }

:::

### 方法引用 ###

方法引用是一种更简洁的Lambda表达式，可以通过方法名称来引用已经存在的方法。方法引用通过 `::` 操作符将方法名与对象或类名分隔开来表示。

下面是一些方法引用的例子：

Function<String, Integer> strToInt = Integer::parseInt;
 Supplier<Date> newDate = Date::new;
 Consumer<String> print = System.out::print;

### 其他函数式接口 ###

#### Bi类型接口 ####

`BiConsumer、BiFunction、BiPrediate` 是 `Consumer、Function、Predicate`的扩展，可以传入多个参数，没有 BiSupplier 是因为 Supplier 没有入参。

#### 操作基本数据类型的接口 ####

`IntConsumer、IntFunction、IntPredicate、IntSupplier、LongConsumer、LongFunction、LongPredicate、LongSupplier、DoubleConsumer、DoubleFunction、DoublePredicate、DoubleSupplier`。其实常用的函数式接口就那四大接口Consumer、Function、Prediate、Supplier，其他的函数式接口就不一一列举了，有兴趣的可以去java.util.function这个包下详细的看。

### 总结 ###

在本篇文章中，我们介绍了Java函数式编程以及常用的函数式接口。Lambda表达式和函数式接口是函数式编程的两个重要概念，可以让代码更加简洁和灵活。Java提供了一些常用的函数式接口，如Function、Consumer、Predicate、Supplier等，可以通过方法引用更加简洁地实现函数式编程。使用函数式编程，可以让你的代码更加优雅，简洁。