Function.java

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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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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
 */
/*
 * 函数式接口：Function<T, R>
 *
 * 参数：T
 * 返回：R
 * 示例：将字符串s转换为整数
 *       Function<String, Integer> f = s->Integer.parseInt(s);
 */
@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 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
     */
	// 标识转换
    static <T> java.util.function.Function<T, T> identity() {
        return 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(java.util.function.Function)
     */
	// f1.compose(f2)：先执行f2，再执行f1
    default <V> java.util.function.Function<V, R> compose(java.util.function.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(java.util.function.Function)
     */
	// f1.andThen(f2)：先执行f1，再执行f2
    default <V> java.util.function.Function<T, V> andThen(java.util.function.Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }
}