Java8中怎么利用Stream实现函数式接口
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函数式接口
什么是函数式接口?简单来说就是只有一个抽象函数的接口。为了使得函数式接口的定义更加规范,java8 提供了@FunctionalInterface 注解告诉编译器在编译器去检查函数式接口的合法性,以便在编译器在编译出错时给出提示。为了更加规范定义函数接口,给出如下函数式接口定义规则:
有且仅有一个抽象函数
必须要有@FunctionalInterface 注解
可以有默认方法
可以看出函数式接口的编写定义非常简单,不知道大家有没有注意到,其实我们经常会用到函数式接口,如Runnable 接口,它就是一个函数式接口:
@FunctionalInterface public interface Runnable { /** * When an object implementing interfaceRunnable
is used * to create a thread, starting the thread causes the object's *run
method to be called in that separately executing * thread. ** The general contract of the method
run
is that it may * take any action whatsoever. * * @see java.lang.Thread#run() */ public abstract void run(); }
过去我们会使用匿名内部类来实现线程的执行体:
new Thread(new Runnable() { @Override public void run() { System.out.println("Hello FunctionalInterface"); } }).start();
现在我们使用Lambda 表达式,这里函数式接口的使用没有体现函数式编程思想,这里输出字符到标准输出流中,产生了副作用,起到了简化代码的作用,当然还有装B。
new Thread(()->{ System.out.println("Hello FunctionalInterface"); }).start();
Java8 util.function 包下自带了43个函数式接口,大体分为以下几类:
Consumer 消费接口
Function 功能接口
Operator 操作接口
Predicate 断言接口
Supplier 生产接口
其他接口都是在此基础上变形定制化罢了。
函数式接口详细介绍
这里只介绍最基础的函数式接口,至于它的变体只要明白了基础自然就能够明白。前篇:玩转Java8中的 Stream 之从零认识 Stream
Consumer
消费者接口,就是用来消费数据的。
@FunctionalInterface public interface Consumer{ /** * 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 andThen(Consumer super T> after) { Objects.requireNonNull(after); return (T t) -> { accept(t); after.accept(t); }; } }
Consumer 接口中有accept 抽象方法,accept接受一个变量,也就是说你在使用这个函数式接口的时候,给你提供了数据,你只要接收使用就可以了;andThen 是一个默认方法,接受一个Consumer 类型,当你对一个数据使用一次还不够爽的时候,你还能再使用一次,当然你其实可以爽无数次,只要一直使用andThan方法。
Function
何为Function呢?比如电视机,给你带来精神上的愉悦,但是它需要用电啊,电视它把电转换成了你荷尔蒙,这就是Function,简单电说,Function 提供一种转换功能。
@FunctionalInterface public interface Function{ /** * 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 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 Function 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 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 Function 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 the type of the input and output objects to the function * @return a function that always returns its input argument */ static Function identity() { return t -> t; } }
Function 接口 最主要的就是apply 函数,apply 接受T类型数据并返回R类型数据,就是将T类型的数据转换成R类型的数据,它还提供了compose、andThen、identity 三个默认方法,compose 接受一个Function,andThen也同样接受一个Function,这里的andThen 与Consumer 的andThen 类似,在apply之后在apply一遍,compose 则与之相反,在apply之前先apply(这两个apply具体处理内容一般是不同的),identity 起到了类似海关的作用,外国人想要运货进来,总得交点税吧,然后货物才能安全进入中国市场,当然了想不想收税还是你说了算的:。
Operator
可以简单理解成算术中的各种运算操作,当然不仅仅是运算这么简单,因为它只定义了运算这个定义,但至于运算成什么样你说了算。由于没有最基础的Operator,这里将通过 BinaryOperator、IntBinaryOperator来理解Operator 函数式接口,先从简单的IntBinaryOperator开始。
IntBinaryOperator
从名字可以知道,这是一个二元操作,并且是Int 类型的二元操作,那么这个接口可以做什么呢,除了加减乘除,还可以可以实现平方(两个相同int 数操作起来不就是平方吗),还是先看看它的定义吧:
@FunctionalInterface public interface IntBinaryOperator { /** * Applies this operator to the given operands. * * @param left the first operand * @param right the second operand * @return the operator result */ int applyAsInt(int left, int right); }
IntBinaryOperator 接口内只有一个applyAsInt 方法,其接收两个int 类型的参数,并返回一个int 类型的结果,其实这个跟Function 接口的apply 有点像,但是这里限定了,只能是int类型。
BinaryOperator
BinaryOperator 二元操作,看起来它和IntBinaryOperator 是父子关系,实际上这两者没有半点关系,但他们在功能上还是有相似之处的:
@FunctionalInterface public interface BinaryOperatorextends BiFunction { /** * Returns a {@link BinaryOperator} which returns the lesser of two elements * according to the specified {@code Comparator}. * * @param the type of the input arguments of the comparator * @param comparator a {@code Comparator} for comparing the two values * @return a {@code BinaryOperator} which returns the lesser of its operands, * according to the supplied {@code Comparator} * @throws NullPointerException if the argument is null */ public static BinaryOperator minBy(Comparator super T> comparator) { Objects.requireNonNull(comparator); return (a, b) -> comparator.compare(a, b) <= 0 ? a : b; } /** * Returns a {@link BinaryOperator} which returns the greater of two elements * according to the specified {@code Comparator}. * * @param the type of the input arguments of the comparator * @param comparator a {@code Comparator} for comparing the two values * @return a {@code BinaryOperator} which returns the greater of its operands, * according to the supplied {@code Comparator} * @throws NullPointerException if the argument is null */ public static BinaryOperator maxBy(Comparator super T> comparator) { Objects.requireNonNull(comparator); return (a, b) -> comparator.compare(a, b) >= 0 ? a : b; } }
BinaryOperator 是 BiFunction 生的,而IntBinaryOperator 是从石头里蹦出来的,BinaryOperator 自身定义了minBy、maxBy默认方法,并且参数都是Comparator,就是根据传入的比较器的比较规则找出最小最大的数据。
Predicate
断言、判断,对输入的数据根据某种标准进行评判,最终返回boolean值:
@FunctionalInterface public interface Predicate{ /** * 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
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 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
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 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 Predicate isEqual(Object targetRef) { return (null == targetRef) ? Objects::isNull : object -> targetRef.equals(object); } }
Predicate的test 接收T类型的数据,返回 boolean 类型,即对数据进行某种规则的评判,如果符合则返回true,否则返回false;Predicate接口还提供了 and、negate、or,与 取反 或等,isEqual 判断两个参数是否相等等默认函数。
Supplier
生产、提供数据:
@FunctionalInterface public interface Supplier{ /** * Gets a result. * * @return a result */ T get(); }
非常easy,get方法返回一个T类数据,可以提供重复的数据,或者随机种子都可以,就这么简单。
函数式接口实战
Consumer
Consumer 用的太多了,不想说太多,如下:
public class Main { public static void main(String[] args) { Stream.of(1,2,3,4,5,6) .forEach(integer -> System.out.println(integer)); //输出1,2,3,4,5,6 } }
这里使用标准输出,还是产生了副作用,但是这种程度是可以允许的
Function
1.转换,将字符串转成长度
public class Main { public static void main(String[] args) { Stream.of("hello","FunctionalInterface") .map(e->e.length()) .forEach(System.out::println); } }
2.运算
public class FunctionTest { public static void main(String[] args) { public static void main(String[] args) { Functionsquare = integer -> integer * integer; //定义平方运算 List list = new ArrayList<>(); list.add(1); list.add(2); list.add(3); list.add(4); list.stream() .map(square.andThen(square)) //四次方 .forEach(System.out::println); System.out.println("------"); list.stream() .map(square.compose(e -> e - 1)) //减一再平方 .forEach(System.out::println); System.out.println("------"); list.stream().map(square.andThen(square.compose(e->e/2))) //先平方然后除2再平方 .forEach(System.out::println); } }
结果如图:
Operator
1.BinaryOperator
这里实现找最大值:
public class BinaryOperatorTest { public static void main(String[] args) { Stream.of(2,4,5,6,7,1) .reduce(BinaryOperator.maxBy(Comparator.comparingInt(Integer::intValue))).ifPresent(System.out::println); } }
Comparator 后期会讲到
2.IntOperator
这里实现累加功能:
public class BinaryOperatorTest { public static void main(String[] args) { IntBinaryOperator intBinaryOperator = (e1, e2)->e1+e2; //定义求和二元操作 IntStream.of(2,4,5,6,7,1) .reduce(intBinaryOperator).ifPresent(System.out::println); } }
Predicate
筛选出大于0最小的两个数
public class Main { public static void main(String[] args) { IntStream.of(200,45,89,10,-200,78,94) .filter(e->e>0) //过滤小于0的数 .sorted() //自然顺序排序 .limit(2) //取前两个 .forEach(System.out::println); } }
Supplier
这里一直生产2这个数字,为了能停下来,使用limit
public class Main { public static void main(String[] args) { Stream.generate(()->2) .limit(10) .forEach(System.out::println); } }
如图:
上述就是小编为大家分享的Java8中怎么利用Stream实现函数式接口了,如果刚好有类似的疑惑,不妨参照上述分析进行理解。如果想知道更多相关知识,欢迎关注创新互联行业资讯频道。
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