Java中Function的使用及說明

更新時(shí)間：2023年05月31日 09:33:38 作者：華妃

這篇文章主要介紹了Java中Function的使用及說明，具有很好的參考價(jià)值，希望對大家有所幫助。如有錯(cuò)誤或未考慮完全的地方，望不吝賜教

Java Function的使用

一、方法介紹

表示接受一個(gè)參數(shù)并產(chǎn)生結(jié)果的函數(shù)。

參數(shù)類型

T - 函數(shù)輸入的類型
R - 函數(shù)的結(jié)果類型

方法介紹

R apply(T t)

將此函數(shù)應(yīng)用于給定的參數(shù)。

default Function<V, R> compose(Function<? super V, ? extends T> before)

返回一個(gè)組合函數(shù)，首先將before函數(shù)應(yīng)用于其輸入，然后將此函數(shù)應(yīng)用于結(jié)果。如果任一函數(shù)的評估引發(fā)異常，則將其轉(zhuǎn)發(fā)給組合函數(shù)的調(diào)用者。

default Function<T, V> andThen(Function<? super R, ? extends V> after)

返回一個(gè)組合函數(shù)，首先將此函數(shù)應(yīng)用于其輸入，然后將after函數(shù)應(yīng)用于結(jié)果。如果任一函數(shù)的評估引發(fā)異常，則將其轉(zhuǎn)發(fā)給組合函數(shù)的調(diào)用者。

static Function<T, T> identity()

返回一個(gè)總是返回其輸入?yún)?shù)的函數(shù)。

源碼

@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;
    }
}

二、demo

public class Test {
    public static void main(String[] args) throws Exception {
        Function<Integer, Integer> add = p -> p + 10;
        Integer result = add.apply(10);
        // 這里會(huì)輸出 20，因?yàn)檫@個(gè)函數(shù)定義的操作時(shí)把參數(shù)加上 10 后返回
        System.out.println(result);
        Function<Integer, Integer> multiplyTen = a -> a * 10;
        Function<Integer, Integer> addTen = a -> a + 10;
        // 先增加 10，然后再乘 10，輸出結(jié)果 110
        Function<Integer, Integer> addTenThenMultiplyTen = multiplyTen.compose(addTen);
        System.out.println(addTenThenMultiplyTen.apply(1));
        // 先乘 10，然后再加 10，輸出結(jié)果 20
        Function<Integer, Integer> multiplyTenAddTenThen = multiplyTen.andThen(addTen);
        System.out.println(multiplyTenAddTenThen.apply(1));
    }
}

結(jié)果

Java內(nèi)置函數(shù) Function函數(shù)

Java內(nèi)置Function參數(shù)，類包是在 java.base 模塊下 java.util.function 包中，其方法主要用于對一個(gè)請求參數(shù)的處理，并返回一個(gè)結(jié)果。

Function源碼

package java.util.function;
import java.util.Objects;
/**
 * 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;
    }
}

Function主要方法

apply方法

當(dāng)前方法就是我們使用匿名函數(shù)時(shí)需要重寫的方法，其中請求參數(shù)和返回參數(shù)都需要在我們生成Function對象的時(shí)候傳進(jìn)去，而apply方法也是這個(gè)類最核心的方法。

    /**
     * Applies this function to the given argument.
     *
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);

創(chuàng)建簡單的Function對象

Function是現(xiàn)有43個(gè)函數(shù)中基礎(chǔ)的函數(shù)之一，簡單的方法可以省略方法體{}，和if的寫法一樣，但是復(fù)雜寫法不可以省略{}。

/**
	 * ([參數(shù)列表]) ->{
 	 * 	代碼體;
 	 * }
 	 * 或
	 * ([參數(shù)列表]) ->代碼體
	 *
	 */
    public static void main(String[] args) {
        //Function<T, R> 傳入一個(gè)參數(shù)，并返回一個(gè)參數(shù)，兩個(gè)參數(shù)類型需要自己傳 可以對數(shù)據(jù)進(jìn)行處理
        Function<String,Integer> stringIntegerAddFunction=(str)->Integer.parseInt(str)+1;
        System.out.println(stringIntegerAddFunction.apply("100"));
        Function<String,Integer> stringIntegerAddFunction2=(str)->{
            Integer integer=Integer.parseInt(str);
            return integer+1;
        };
        System.out.println(stringIntegerAddFunction2.apply("100"));
    }

兩個(gè)寫法雖然不一致，但是執(zhí)行的結(jié)果是一樣的。

簡單Funciton對象的返回結(jié)果

compose方法、andThen方法和identity方法

使用匿名函數(shù)時(shí)，匿名函數(shù)接口的類中只允許存在一個(gè)方法，而之所有這三方法，是因?yàn)榻涌谥锌梢酝ㄟ^關(guān)鍵字default定義默認(rèn)方法，實(shí)現(xiàn)類如果不想要默認(rèn)方法的實(shí)現(xiàn)邏輯可以根據(jù)需求重新定義，通過關(guān)鍵字static定義靜態(tài)方法，實(shí)現(xiàn)類如果不想要靜態(tài)方法的實(shí)現(xiàn)邏輯可以根據(jù)需求重新定義。

compose方法

有的時(shí)候，我們需要將兩個(gè)或多個(gè)方法進(jìn)行組合使用，這個(gè)時(shí)候就需要compose方法，compose會(huì)通過從右到左的順序執(zhí)行我們拼接的方法。

/**
	 * ([參數(shù)列表]) ->{
 	 * 	代碼體;
 	 * }
 	 * 或
	 * ([參數(shù)列表]) ->代碼體
	 *
	 */
 public static void main(String[] args) {
        //Function<T, R> 傳入一個(gè)參數(shù)，并返回一個(gè)參數(shù)，兩個(gè)參數(shù)類型需要自己傳 可以對數(shù)據(jù)進(jìn)行處理
        Function<String,Integer> stringIntegerAddFunction=(str)->Integer.parseInt(str)+1;
        Function<Integer,String> integerStringFunction=integer -> String.valueOf(integer*2);
        System.out.println(stringIntegerAddFunction.compose(integerStringFunction).apply(100));
    }

可以通過結(jié)果看出，compose拼接會(huì)先執(zhí)行被拼接的函數(shù)(integerStringFunction)，再回去調(diào)用我們拼接的函數(shù)(stringIntegerAddFunction),為方便查看，我們請求的apply方法的類型已經(jīng)發(fā)送了改變。

使用compose方法返回結(jié)果

andThen方法

andThen方法則剛好相反，這個(gè)函數(shù)會(huì)把拼接的函數(shù)從左到右執(zhí)行。

/**
	 * ([參數(shù)列表]) ->{
 	 * 	代碼體;
 	 * }
 	 * 或
	 * ([參數(shù)列表]) ->代碼體
	 *
	 */
    public static void main(String[] args) {
        //Function<T, R> 傳入一個(gè)參數(shù)，并返回一個(gè)參數(shù)，兩個(gè)參數(shù)類型需要自己傳 可以對數(shù)據(jù)進(jìn)行處理
        Function<String,Integer> stringIntegerAddFunction=(str)->Integer.parseInt(str)+1;
        Function<Integer,String> integerStringFunction=integer -> String.valueOf(integer*2);
        System.out.println(stringIntegerAddFunction.andThen(integerStringFunction).apply("100"));
    }

根據(jù)結(jié)果可以看出，andThen拼接會(huì)先執(zhí)行被拼接的函數(shù)(stringIntegerAddFunction)，再回去調(diào)用我們拼接的函數(shù)(integerStringFunction),為方便查看，我們請求的apply方法的類型已經(jīng)發(fā)送了改變。

andThen方法返回結(jié)果