Java之函數式接口@FunctionalInterface詳解
函數式接口的定義
在java8中，滿足下面任意一個條件的接口都是函數式接口：
1、被@FunctionalInterface注釋的接口，滿足@FunctionalInterface注釋的約束。
2、沒有被@FunctionalInterface注釋的接口，但是滿足@FunctionalInterface注釋的約束
@FunctionalInterface注釋的約束：
1、接口有且只能有個一個抽象方法，只有方法定義，沒有方法體
2、在接口中覆寫Object類中的public方法，不算是函數式接口的方法。
比如：
@FunctionalInterface
interface FunctionalInterfaceTest {
String getInfo(String input);
}
函數式接口的實例
- lambda表達式
- 方法的引用
- 已有構造器或方法的引用
public class Main {
public static void main(String[] args)
throws ClassNotFoundException,
IllegalAccessException,
InstantiationException,
NoSuchMethodException,
InvocationTargetException, NoSuchFieldException {
/**
* 1、lambda表達式
* 這種形式最為直觀，lambda表達式，接收一個String類型的參數，返回一個String類型的結果。
* 完全符合函數式接口FunctionInterfaceTest的定義
*/
FunctionalInterfaceTest fiTest1 = str -> str + " copy";
/**
* 2、Main方法當中的functionalInterfaceTestMethod方法接收一個參數，返回一個結果。符合函數式接口
* FunctionInterfaceTest的定義。
* 函數式接口只是定義了個方法的約定（接收一個String類型的參數，返回一個String類型的結果），
* 而對於方法內部進行何種操作則並沒有做任何的限制。在這點上，跟java以前的版本中的實現類與接口之間的
* 關系很類似。不同的是，函數式接口更偏重於計算過程，約束了一個計算過程的輸入和輸出。
*/
FunctionalInterfaceTest fiTest2 = Main::functionalInterfaceTestMethod;
/**
* 3、構造方法引用
* 構造函數的結構：接收輸入參數，然后返回一個對象。這種約束跟函數式接口的約束很像。
* 所以只要“輸入參數類型”與“輸出參數類型”跟FunctionInterfaceTest中的方法約束相同，
* 就可以創建出FunctionInterfaceTest接口的實例，如下，String的構造方法中有
* new String(str)的構造方法，所以可以得到實例。
* 這里存在一個類型推斷的問題，JDK的編譯器已經幫我們自動找到了只有一個參數，且是String類型的構造方法。
* 這就是我們直接String::new，沒有指定使用哪一個構造方法，卻可以創建實例的原因
*/
FunctionalInterfaceTest fiTest3 = String::new;
System.out.println(useFunctionalInterface("Hello World!", fiTest1));
System.out.println(useFunctionalInterface("Hello World!", fiTest2));
System.out.println(useFunctionalInterface("Hello World!", fiTest3));
System.out.println(useFunctionalInterface("Hello World!", str -> str + " created by lambda in the context"));
/**
輸出：
Hello World! copy
Hello World! copy 2 by reference
Hello World!
Hello World! created by lambda in the context
*/
}
public static String functionalInterfaceTestMethod(String str) {
return str + " copy 2 by reference";
}
public static String useFunctionalInterface(String str, FunctionalInterfaceTest fiT) {
return fiT.getInfo(str);
}
}
常用的封裝好的函數式接口
分別為Function<T, R>, Cosumer<T>, Predicate<T>, Supplier<T>
/**
* 常用的函數式接口主要有四種類型，是通過其輸入和輸出的參數來進行區分的。定義了編碼過程中主要的使用場景。
Function<T,R>
接收一個T類型的參數，返回一個R類型的結果
Consumer<T>
接收一個T類型的參數，不返回值
Predicate<T>
接收一個T類型的參數，返回一個boolean類型的結果
Supplier<T>
不接受參數，返回一個T類型的結果
*/
Function<String, String> add_postfix = str -> str + "postfix";
Consumer<String> print_string = System.out::println;
Predicate<Integer> judge_positive = n -> n > 0;
Supplier<String> supplier = () -> "supply";
List<String> list = Arrays.asList("adfsg", "sdafef", "", "s", "231243", "hgjrepjrg");
list.stream()
.map(str -> str + "1")
.filter(str -> str.length() > 2)
.sorted((str1, str2) -> str2.compareTo(str1))
.forEach(System.out::println);
/**
輸出：
sdafef1
hgjrepjrg1
adfsg1
2312431
1234dfgh
*/
此外，對於多參數的情況，Java還封裝了BiFunction<T, U, R>, BiConsumer<T, U>, BiPredicate<T, U>。
// 由於java不能返回多個參數，所以沒有BiSupplier
BiFunction<String, String, String> combine_string = (str1, str2) -> str1 + str2;
BiConsumer<String, String> print_two_string = (str1, str2) -> System.out.println(str1 + str2);
BiPredicate<String, String> str_equal = String::equals;
int bif_result = biFunctionTestMethod("abs", "pdf", (str1, str2) -> str1.length() + str2.length());
biConsumerTestMethod("1234", "dfgh", (str1, str2) -> System.out.println(str1 + str2));
boolean bip_result_1 = biPredictTestMethod("abc", "abc", str_equal),
bip_result_2 = biPredictTestMethod("abc", "def", str_equal);
System.out.println(bif_result);
System.out.println(bip_result_1);
System.out.println(bip_result_2);
/*
輸出：
6
true
false
*/
在此之外，還有compose和andThen方法，其本質就是數學當中的符合函數，唯一的區別：對於函數\(f(x),g(x)\)，compose等價於\(f(g(x))\)，andThen等價於\(g(f(x))\)，就是執行順序不同而已。
// compose 和 andThen
Function<String, String> compose_function = ((Function<String, String>) (str -> str + "abc")).compose((Function<String, String>) (str -> str + str.length()));
System.out.println("Compose function: " + compose_function.apply("Hello World! "));
Function<String, String> andThen_function = ((Function<String, String>) (str -> str + "abc")).andThen((Function<String, String>) (str -> str + str.length()));
System.out.println("AndThen function: " + andThen_function.apply("Hello World! "));
// Bicosumer, cosumer, bifunction 都有類似功能
// BiPredicate, Predicate 的 and, or, negate
System.out.println(str_equal.negate().test("a", "a")); // false
System.out.println(judge_positive.and(n -> n > 2).test(5)); // true
System.out.println(judge_positive.or(n -> n < -1).test(-10)); // true
源碼
Function.java
/**
* 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;
}
}
Consumer.java
/**
* Represents an operation that accepts a single input argument and returns no
* result. Unlike most other functional interfaces, {@code Consumer} is expected
* to operate via side-effects.
*
* <p>This is a <a href="package-summary.html">functional interface</a>
* whose functional method is {@link #accept(Object)}.
*
* @param <T> the type of the input to the operation
*
* @since 1.8
*/
@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.java
/**
* 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);
}
/**
* Returns a predicate that is the negation of the supplied predicate.
* This is accomplished by returning result of the calling
* {@code target.negate()}.
*
* @param <T> the type of arguments to the specified predicate
* @param target predicate to negate
*
* @return a predicate that negates the results of the supplied
* predicate
*
* @throws NullPointerException if target is null
*
* @since 11
*/
@SuppressWarnings("unchecked")
static <T> Predicate<T> not(Predicate<? super T> target) {
Objects.requireNonNull(target);
return (Predicate<T>)target.negate();
}
}
Supplier.java
/**
* Represents a supplier of results.
*
* <p>There is no requirement that a new or distinct result be returned each
* time the supplier is invoked.
*
* <p>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();
}