用 Java 来实现高阶函数

/**
 * 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> {}

public static void main(String[] args) {

        Function<Function<Integer, Integer>, Function<Integer, Integer>> applyTwice =
                    //f(f(x))
            f -> x -> f.apply(f.apply(x));

        Function<Function<Integer, Integer>, Function<Integer, Integer>> applyThrice =
            f -> {
                return x -> {
                    //f(f(f(x)))
                    return f.apply(f.apply(f.apply(x)));
                };
            };

        System.out.println(applyThrice.apply( x -> x + 1 ).apply(0));

    }