BiFunction.java

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 * accompanied this code).
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package java.util.function;

import java.util.Objects;

/**
 * Represents a function that accepts two arguments and produces a result.
 * This is the two-arity specialization of {@link Function}.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object, Object)}.
 *
 * @param <T> the type of the first argument to the function
 * @param <U> the type of the second argument to the function
 * @param <R> the type of the result of the function
 *
 * @see Function
 * @since 1.8
 */
/*
 * 函数式接口：BiFunction<T, U, R>
 *
 * 参数：T, U
 * 返回：R
 */
@FunctionalInterface
public interface BiFunction<T, U, R> {
    
    /**
     * Applies this function to the given arguments.
     *
     * @param t the first function argument
     * @param u the second function argument
     *
     * @return the function result
     */
    R apply(T t, U u);
    
    /**
     * 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
     */
    // f1.andThen(f2)：先执行f1，再执行f2
    default <V> java.util.function.BiFunction<T, U, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t, U u) -> after.apply(apply(t, u));
    }
}