---
title: seven(times(five()));が35を返すような関数をJavaで実装する
tags: ["Java", "Java SE 8"]
categories: ["Programming", "Java", "Lambda"]
date: 2014-05-28T19:18:01Z
updated: 2014-05-28T19:18:01Z
---

Twitter上でみたのでJavaでやってみた。

> JavaScriptの問題。seven(times(five())); // must return 35となるような関数seven, times, fiveを作れ

Java8だと簡単ですね。(Java7より)

    public class Foo {
        public static void main(String[] args) {
            System.out.println(seven(times(five()))); // 35
        }
    
        public static int seven(IntUnaryOperator operator) {
            return operator.applyAsInt(7);
        }
    
        public static IntUnaryOperator times(ToIntFunction<IntUnaryOperator> right) {
            return left -> right.applyAsInt((x) -> left * x);
        }
    
        public static ToIntFunction<IntUnaryOperator> five() {
            return operator -> operator.applyAsInt(5);
        }
    }


↓のように書いた方が汎用的。

    public class Foo {
        public static void main(String[] args) {
            System.out.println(seven(times(five())));
        }
    
        public static int seven(IntUnaryOperator operator) {
            return leftArgumentWith(7, operator);
        }
    
        public static IntUnaryOperator times(ToIntFunction<IntUnaryOperator> right) {
            return operator((x, y) -> x * y, right);
        }
    
        public static ToIntFunction<IntUnaryOperator> five() {
            return rightArgumentWith(5);
        }
    
        public static int leftArgumentWith(int left, IntUnaryOperator operator) {
            return operator.applyAsInt(left);
        }
    
        public static IntUnaryOperator operator(IntBinaryOperator binaryOperator, ToIntFunction<IntUnaryOperator> right) {
            return left -> right.applyAsInt((x) -> binaryOperator.applyAsInt(left, x));
        }
    
        public static ToIntFunction<IntUnaryOperator> rightArgumentWith(int right) {
            return operator -> operator.applyAsInt(right);
        }
    }