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Java8新特性04 内置功能接口

分类: IT文章 • 2024-03-14 15:13:32

内置功能接口借鉴了Google Guava库的实践。Java8的内置功能接口在java.util.function包下，主要有四大核心的功能性接口：谓词（predicate）、函数（function）、生产者（supplier）、消费者（consumer）

一. 谓词（Predicate)

这里是谓词的源码：

  1 /*
  2  * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
  3  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  4  *
  5  *
  6  *
  7  *
  8  *
  9  *
 10  *
 11  *
 12  *
 13  *
 14  *
 15  *
 16  *
 17  *
 18  *
 19  *
 20  *
 21  *
 22  *
 23  *
 24  */
 25 package java.util.function;
 26 
 27 import java.util.Objects;
 28 
 29 /**
 30  * Represents a predicate (boolean-valued function) of one argument.
 31  *
 32  * <p>This is a <a href="package-summary.html">functional interface</a>
 33  * whose functional method is {@link #test(Object)}.
 34  *
 35  * @param <T> the type of the input to the predicate
 36  *
 37  * @since 1.8
 38  */
 39 @FunctionalInterface
 40 public interface Predicate<T> {
 41 
 42     /**
 43      * Evaluates this predicate on the given argument.
 44      *
 45      * @param t the input argument
 46      * @return {@code true} if the input argument matches the predicate,
 47      * otherwise {@code false}
 48      */
 49     boolean test(T t);
 50 
 51     /**
 52      * Returns a composed predicate that represents a short-circuiting logical
 53      * AND of this predicate and another.  When evaluating the composed
 54      * predicate, if this predicate is {@code false}, then the {@code other}
 55      * predicate is not evaluated.
 56      *
 57      * <p>Any exceptions thrown during evaluation of either predicate are relayed
 58      * to the caller; if evaluation of this predicate throws an exception, the
 59      * {@code other} predicate will not be evaluated.
 60      *
 61      * @param other a predicate that will be logically-ANDed with this
 62      *              predicate
 63      * @return a composed predicate that represents the short-circuiting logical
 64      * AND of this predicate and the {@code other} predicate
 65      * @throws NullPointerException if other is null
 66      */
 67     default Predicate<T> and(Predicate<? super T> other) {
 68         Objects.requireNonNull(other);
 69         return (t) -> test(t) && other.test(t);
 70     }
 71 
 72     /**
 73      * Returns a predicate that represents the logical negation of this
 74      * predicate.
 75      *
 76      * @return a predicate that represents the logical negation of this
 77      * predicate
 78      */
 79     default Predicate<T> negate() {
 80         return (t) -> !test(t);
 81     }
 82 
 83     /**
 84      * Returns a composed predicate that represents a short-circuiting logical
 85      * OR of this predicate and another.  When evaluating the composed
 86      * predicate, if this predicate is {@code true}, then the {@code other}
 87      * predicate is not evaluated.
 88      *
 89      * <p>Any exceptions thrown during evaluation of either predicate are relayed
 90      * to the caller; if evaluation of this predicate throws an exception, the
 91      * {@code other} predicate will not be evaluated.
 92      *
 93      * @param other a predicate that will be logically-ORed with this
 94      *              predicate
 95      * @return a composed predicate that represents the short-circuiting logical
 96      * OR of this predicate and the {@code other} predicate
 97      * @throws NullPointerException if other is null
 98      */
 99     default Predicate<T> or(Predicate<? super T> other) {
100         Objects.requireNonNull(other);
101         return (t) -> test(t) || other.test(t);
102     }
103 
104     /**
105      * Returns a predicate that tests if two arguments are equal according
106      * to {@link Objects#equals(Object, Object)}.
107      *
108      * @param <T> the type of arguments to the predicate
109      * @param targetRef the object reference with which to compare for equality,
110      *               which may be {@code null}
111      * @return a predicate that tests if two arguments are equal according
112      * to {@link Objects#equals(Object, Object)}
113      */
114     static <T> Predicate<T> isEqual(Object targetRef) {
115         return (null == targetRef)
116                 ? Objects::isNull
117                 : object -> targetRef.equals(object);
118     }
119 }
View Code

说明：谓词是单参数的返回布尔值的函数式接口，输入一个函数，返回true或者false。同时该接口也包含多个默认方法或静态方法使用谓词转换成复杂的逻辑表达式（与、或、非等）

举例：

 1 package com.test.a;
 2 
 3 import java.util.function.Predicate;
 4 
 5 public class Test {
 6     public static void main(String args[]) {
 7         
 8        String str="hello";
 9        
10        //1.测试抽象方法
11        Predicate<String> predicate1=(String s)->s.length()>6;
12        boolean result1=predicate1.test(str);
13        System.out.println(result1);//false
14        
15        //2.使用 default Predicate<T> and(Predicate<? super T> other)
16        Predicate<String> predicate2=(String s)->s.length()==5;
17        boolean result2=predicate1.and(predicate2).test(str);
18        System.out.println(result2);//false
19        
20        //3.使用 default Predicate<T> or(Predicate<? super T> other) 
21        boolean result3=predicate1.or(predicate2).test(str);
22        System.out.println(result3);//true
23        
24        //4.使用default Predicate<T> negate()
25        boolean result4=predicate1.negate().test(str);
26        System.out.println(result4);//true
27        
28        //5.使用static <T> Predicate<T> isEqual(Object targetRef)
29        Predicate<String> predicate5=Predicate.isEqual("hello2");
30        boolean result5=predicate5.test(str);
31        System.out.println(result5);//false
32  
33     }
34 }
View Code

二. 函数（Function）

这是函数的源码：

  1 /*
  2  * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
  3  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  4  *
  5  *
  6  *
  7  *
  8  *
  9  *
 10  *
 11  *
 12  *
 13  *
 14  *
 15  *
 16  *
 17  *
 18  *
 19  *
 20  *
 21  *
 22  *
 23  *
 24  */
 25 package java.util.function;
 26 
 27 import java.util.Objects;
 28 
 29 /**
 30  * Represents a function that accepts one argument and produces a result.
 31  *
 32  * <p>This is a <a href="package-summary.html">functional interface</a>
 33  * whose functional method is {@link #apply(Object)}.
 34  *
 35  * @param <T> the type of the input to the function
 36  * @param <R> the type of the result of the function
 37  *
 38  * @since 1.8
 39  */
 40 @FunctionalInterface
 41 public interface Function<T, R> {
 42 
 43     /**
 44      * Applies this function to the given argument.
 45      *
 46      * @param t the function argument
 47      * @return the function result
 48      */
 49     R apply(T t);
 50 
 51     /**
 52      * Returns a composed function that first applies the {@code before}
 53      * function to its input, and then applies this function to the result.
 54      * If evaluation of either function throws an exception, it is relayed to
 55      * the caller of the composed function.
 56      *
 57      * @param <V> the type of input to the {@code before} function, and to the
 58      *           composed function
 59      * @param before the function to apply before this function is applied
 60      * @return a composed function that first applies the {@code before}
 61      * function and then applies this function
 62      * @throws NullPointerException if before is null
 63      *
 64      * @see #andThen(Function)
 65      */
 66     default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
 67         Objects.requireNonNull(before);
 68         return (V v) -> apply(before.apply(v));
 69     }
 70 
 71     /**
 72      * Returns a composed function that first applies this function to
 73      * its input, and then applies the {@code after} function to the result.
 74      * If evaluation of either function throws an exception, it is relayed to
 75      * the caller of the composed function.
 76      *
 77      * @param <V> the type of output of the {@code after} function, and of the
 78      *           composed function
 79      * @param after the function to apply after this function is applied
 80      * @return a composed function that first applies this function and then
 81      * applies the {@code after} function
 82      * @throws NullPointerException if after is null
 83      *
 84      * @see #compose(Function)
 85      */
 86     default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
 87         Objects.requireNonNull(after);
 88         return (T t) -> after.apply(apply(t));
 89     }
 90 
 91     /**
 92      * Returns a function that always returns its input argument.
 93      *
 94      * @param <T> the type of the input and output objects to the function
 95      * @return a function that always returns its input argument
 96      */
 97     static <T> Function<T, T> identity() {
 98         return t -> t;
 99     }
100 }
View Code

说明：Function接受一个参数，并产生一个结果。默认方法可以将多个函数串在一起（compose，andThen)

default <V> Function<V, R> compose(Function<? super V, ? extends T> before)返回一个先执行before函数对象apply方法，再执行当前函数对象apply方法的函数对象；

default <V> Function<T, V> andThen(Function<? super R, ? extends V> after)返回一个先执行当前函数对象apply方法，再执行after函数对象apply方法的函数对象；

举例：

 1 package com.test.a;
 2 
 3 import java.util.function.Function;
 4 
 5 public class Test {
 6     public static void main(String args[]) {
 7   
 8         String str="hello";
 9         Function<String, Integer> function=(String)->2;
10         Integer result=function.apply(str);
11         System.out.println(result);
12         
13         Function<Integer, Integer> function2=(Integer i )->i*2;
14         Function<Integer, Integer> function3=(Integer j )->j*j;
15         int result2=function2.compose(function3).apply(2);//先算function3的结果，再算function2的结果
16         System.out.println(result2);
17         int result3=function2.andThen(function3).apply(2);//先算function2的结果，再算function3的结果
18         System.out.println(result3);
19         
20     
21     }
22 }
View Code 
1 2
2 8
3 16
View Code

 三. 生产者（Supplier)

这是生产者的源码：

 1 /*
 2  * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 3  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 4  *
 5  *
 6  *
 7  *
 8  *
 9  *
10  *
11  *
12  *
13  *
14  *
15  *
16  *
17  *
18  *
19  *
20  *
21  *
22  *
23  *
24  */
25 package java.util.function;
26 
27 /**
28  * Represents a supplier of results.
29  *
30  * <p>There is no requirement that a new or distinct result be returned each
31  * time the supplier is invoked.
32  *
33  * <p>This is a <a href="package-summary.html">functional interface</a>
34  * whose functional method is {@link #get()}.
35  *
36  * @param <T> the type of results supplied by this supplier
37  *
38  * @since 1.8
39  */
40 @FunctionalInterface
41 public interface Supplier<T> {
42 
43     /**
44      * Gets a result.
45      *
46      * @return a result
47      */
48     T get();
49 }
View Code

说明：Suppliers产生一个给定的泛型类型的结果。与Function不同的是Suppliers不接受输入参数。

举例：

 1 import java.util.function.Supplier;
 2 
 3 public class Test {
 4     public static void main(String args[]) {
 5   
 6         Supplier<String> supplier=()->"hello";
 7         System.out.println(supplier.get());
 8     
 9     }
10 }
11 
12 
13 hello
14 
15 note:上面的String可以改成具体的对象，比如person对象，从而调用对象的set和get方法等操作
View Code

四. 消费者（Consumer）

这是消费者的源码：

 1 /*
 2  * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 3  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 4  *
 5  *
 6  *
 7  *
 8  *
 9  *
10  *
11  *
12  *
13  *
14  *
15  *
16  *
17  *
18  *
19  *
20  *
21  *
22  *
23  *
24  */
25 package java.util.function;
26 
27 import java.util.Objects;
28 
29 /**
30  * Represents an operation that accepts a single input argument and returns no
31  * result. Unlike most other functional interfaces, {@code Consumer} is expected
32  * to operate via side-effects.
33  *
34  * <p>This is a <a href="package-summary.html">functional interface</a>
35  * whose functional method is {@link #accept(Object)}.
36  *
37  * @param <T> the type of the input to the operation
38  *
39  * @since 1.8
40  */
41 @FunctionalInterface
42 public interface Consumer<T> {
43 
44     /**
45      * Performs this operation on the given argument.
46      *
47      * @param t the input argument
48      */
49     void accept(T t);
50 
51     /**
52      * Returns a composed {@code Consumer} that performs, in sequence, this
53      * operation followed by the {@code after} operation. If performing either
54      * operation throws an exception, it is relayed to the caller of the
55      * composed operation.  If performing this operation throws an exception,
56      * the {@code after} operation will not be performed.
57      *
58      * @param after the operation to perform after this operation
59      * @return a composed {@code Consumer} that performs in sequence this
60      * operation followed by the {@code after} operation
61      * @throws NullPointerException if {@code after} is null
62      */
63     default Consumer<T> andThen(Consumer<? super T> after) {
64         Objects.requireNonNull(after);
65         return (T t) -> { accept(t); after.accept(t); };
66     }
67 }
View Code

说明：Consumers代表一个单一的输入参数上执行操作。Consumer的操作可能会更改输入参数的内部状态。

举例：

 1 package com.test.a;
 2 
 3 import java.util.function.Consumer;
 4 
 5 public class Test {
 6     public static void main(String args[]) {
 7 
 8         Consumer<Person> consumer = (Person person) -> person.setAge(23);
 9         Person person = new Person();
10         person.setAge(25);
11         consumer.accept(person);
12         System.out.println(person.getAge());
13     }
14 }
15 
16 
17 23
View Code

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