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本篇內容主要講解“Java 8中如何使用方法引用”，感興趣的朋友不妨來看看。本文介紹的方法操作簡單快捷，實用性強。下面就讓小編來帶大家學習“Java 8中如何使用方法引用”吧!
在Java8中，使用方法引用非常簡單，如String::isEmpty，但無法使用它否定的方法引用。本文內容即如何解決此問題使得我們能夠更加全面地使用方法引用。
首先看一個使用方法引用的例子：
Stream.of("A", "", "B").filter(String::isEmpty).count()
上面代碼的輸出為1，即空字符串的數目。
如果我們想要獲取非空字符串的數目，就不能直接使用方法引用了。
Stream.of("A", "", "B").filter(s -> !s.isEmpty()).count()
Java8中的Predicate，有predicate.negate()可以轉換為斷言的否定形式，但String::isEmpty卻無法這么做(String::isEmpty.negate()或者!String::isEmpty)。
因為方法引用并不是一個lambda或者函數接口，它能夠被解析為一個或者多個函數接口。如，String::isEmpty至少可以被解析如下：
Predicate<String>
Function
為了解決上述的問題，我們可以通過某種機制顯式地將方法引用轉換為一個函數接口：
public static <T> Predicate<T> as(Predicate<T> predicate) { return predicate;}
通過使用一個靜態方法，接受方法引用參數，返回一個函數接口，即可實現方法引用到函數接口的轉換。接著，我們就可以使用方法引用來實現上面例子中的獲取非空字符串的數目。Stream.of("A", "", "B").filter(as(String::isEmpty).negate()).count();進一步還能使用各種組合的Predicate。
.filter(as(String::isEmpty).negate().and("A"::equals))
由于一個方法引用可能會被解析為多種函數接口，因此如果我們實現很多參數不同的as方法，那么很容易造成混淆。更好的方式則是在方法名中加入函數參數的類型來區分。
import java.util.function.*;public class FunctionCastUtil {public static <T, U> BiConsumer<T, U> asBiConsumer(BiConsumer<T, U> biConsumer) {return biConsumer;}public static <T, U, R> BiFunction<T, U, R> asBiFunction(BiFunction<T, U, R> biFunction) {return biFunction;}public static <T> BinaryOperator<T> asBinaryOperator(BinaryOperator<T> binaryOperator) {return binaryOperator;}public static <T, U> BiPredicate<T, U> asBiPredicate(BiPredicate<T, U> biPredicate) {return biPredicate;}public static BooleanSupplier asBooleanSupplier(BooleanSupplier booleanSupplier) {return booleanSupplier;}public static <T> Consumer<T> asConsumer(Consumer<T> consumer) {return consumer;}public static DoubleBinaryOperator asDoubleBinaryOperator(DoubleBinaryOperator doubleBinaryOperator) {return doubleBinaryOperator;}public static DoubleConsumer asDoubleConsumer(DoubleConsumer doubleConsumer) {return doubleConsumer;}public static <R> DoubleFunction<R> asDoubleFunction(DoubleFunction<R> doubleFunction) {return doubleFunction;}public static DoublePredicate asDoublePredicate(DoublePredicate doublePredicate) {return doublePredicate;}public static DoubleToIntFunction asDoubleToIntFunction(DoubleToIntFunction doubleToIntFunctiontem) {return doubleToIntFunctiontem;}public static DoubleToLongFunction asDoubleToLongFunction(DoubleToLongFunction doubleToLongFunction) {return doubleToLongFunction;}public static DoubleUnaryOperator asDoubleUnaryOperator(DoubleUnaryOperator doubleUnaryOperator) {return doubleUnaryOperator;}public static <T, R> Function<T, R> asFunction(Function<T, R> function) {return function;}public static IntBinaryOperator asIntBinaryOperator(IntBinaryOperator intBinaryOperator) {return intBinaryOperator;}public static IntConsumer asIntConsumer(IntConsumer intConsumer) {return intConsumer;}public static <R> IntFunction<R> asIntFunction(IntFunction<R> intFunction) {return intFunction;}public static IntPredicate asIntPredicate(IntPredicate intPredicate) {return intPredicate;}public static IntSupplier asIntSupplier(IntSupplier intSupplier) {return intSupplier;}public static IntToDoubleFunction asIntToDoubleFunction(IntToDoubleFunction intToDoubleFunction) {return intToDoubleFunction;}public static IntToLongFunction asIntToLongFunction(IntToLongFunction intToLongFunction) {return intToLongFunction;}public static IntUnaryOperator asIntUnaryOperator(IntUnaryOperator intUnaryOperator) {return intUnaryOperator;}public static LongBinaryOperator asLongBinaryOperator(LongBinaryOperator longBinaryOperator) {return longBinaryOperator;}public static LongConsumer asLongConsumer(LongConsumer longConsumer) {return longConsumer;}public static <R> LongFunction<R> asLongFunction(LongFunction<R> longFunction) {return longFunction;}public static LongPredicate asLongPredicate(LongPredicate longPredicate) {return longPredicate;}public static <T> LongSupplier asLongSupplier(LongSupplier longSupplier) {return longSupplier;}public static LongToDoubleFunction asLongToDoubleFunction(LongToDoubleFunction longToDoubleFunction) {return longToDoubleFunction;}public static LongToIntFunction asLongToIntFunction(LongToIntFunction longToIntFunction) {return longToIntFunction;}public static LongUnaryOperator asLongUnaryOperator(LongUnaryOperator longUnaryOperator) {return longUnaryOperator;}public static <T> ObjDoubleConsumer<T> asObjDoubleConsumer(ObjDoubleConsumer<T> objDoubleConsumer) {return objDoubleConsumer;}public static <T> ObjIntConsumer<T> asObjIntConsumer(ObjIntConsumer<T> objIntConsumer) {return objIntConsumer;}public static <T> ObjLongConsumer<T> asObjLongConsumer(ObjLongConsumer<T> objLongConsumer) {return objLongConsumer;}public static <T> Predicate<T> asPredicate(Predicate<T> predicate) {return predicate;}public static <T> Supplier<T> asSupplier(Supplier<T> supplier) {return supplier;}public static <T, U> ToDoubleBiFunction<T, U> asToDoubleBiFunction(ToDoubleBiFunction<T, U> toDoubleBiFunction) {return toDoubleBiFunction;}public static <T> ToDoubleFunction<T> asToDoubleFunction(ToDoubleFunction<T> toDoubleFunction) {return toDoubleFunction;}public static <T, U> ToIntBiFunction<T, U> asToIntBiFunction(ToIntBiFunction<T, U> toIntBiFunction) {return toIntBiFunction;}public static <T> ToIntFunction<T> asToIntFunction(ToIntFunction<T> ioIntFunction) {return ioIntFunction;}public static <T, U> ToLongBiFunction<T, U> asToLongBiFunction(ToLongBiFunction<T, U> toLongBiFunction) {return toLongBiFunction;}public static <T> ToLongFunction<T> asToLongFunction(ToLongFunction<T> toLongFunction) {return toLongFunction;}public static <T> UnaryOperator<T> asUnaryOperator(UnaryOperator<T> unaryOperator) {return unaryOperator;}private FunctionCastUtil() {}}Stream.of("A", "", "B").filter(asPredicate(String::isEmpty).negate()).count()
原文：https://dzone.com/articles/put-your-java-8-method-references-to-work
譯文：https://www.rowkey.me/blog/2017/09/02/java8-method-reference-work/
到此，相信大家對“Java 8中如何使用方法引用”有了更深的了解，不妨來實際操作一番吧！這里是億速云網站，更多相關內容可以進入相關頻道進行查詢，關注我們，繼續學習！
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