Scala到Java（函数式编程） [英] Scala to Java (functional programming)

问题描述

我被要求将一些Scala代码翻译成Java课程。但是，分配的要求是Java 8和外部库，例如功能性Java 和完全懒惰，不允许。 Scala中的行是：

  charges.groupBy（_。cc）.values.map（_。reduce（_ combine _ ））。toList 
  

我已经可以写 groupBy 和值但 .map 和 _。reduce 仍然逃避我。我查看了这两个库的源代码以及Scala源代码，试图找到一些可以帮助我将它们放在一起的东西，但是我一直未能取得任何进展。

GroupBy实现如下：

  public Map< CreditCard，列表与LT;电荷>> groupBy（List< Charge> list）
 {
 Map< CreditCard，List< Charge>> map = new TreeMap< CreditCard，List< Charge>>（）; 
（费用c：列表）
 {
列表<费用> group = map.get（c.cc）; 
 if（group == null）
 {
 group = new ArrayList（）; 
 map.put（c.cc，group）; 
} 
 group.add（c）; 
} 
返回地图; 
 
  

解决方案

您可以使用 Google Guava 。它不需要java8。你会特别对调用 FluentIterable 的类感兴趣。这里有一些方法可以帮助你：
$ b

• index（Function keyFunction） - 用于为每个值生成密钥的函数


• transform（函数函数） - 将{函数}应用于这个流畅迭代的每个元素
  

和还有更多。

I have been asked to 'translate' some Scala code to Java for a course. However, the requirements of the assignment are that Java 8 and external libraries, such as Functional Java and Totally Lazy, are not allowed. The line in Scala is:

charges.groupBy(_.cc).values.map(_.reduce(_ combine _)).toList

I have been able to write groupBy and values but .map and _.reduce still elude me. I have looked at the source code of those two libraries as well as the Scala source to try and find something to help me with putting these together but I have not been able to make any headway.

GroupBy is implemented as follows:

public Map<CreditCard, List<Charge>> groupBy(List<Charge> list)
{
    Map<CreditCard, List<Charge>> map = new TreeMap<CreditCard, List<Charge>>();
    for(Charge c: list)
    {
        List<Charge> group = map.get(c.cc);
        if(group == null)
        {
            group = new ArrayList();
            map.put(c.cc, group);
        }
        group.add(c);
    }
    return map;
}

解决方案

you can use Google Guava for this. it doesn't require java8. you would especially be interested in class which call FluentIterable. here's some methods that could help you:

• index(Function keyFunction) - the function used to produce the key for each value
• transform(Function function) - applies {@code function} to each element of this fluent iterable

and there are a lot more.

这篇关于Scala到Java（函数式编程）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！