关于收集（供应商，累加器，合并器） [英] About collect (supplier, accumulator, combiner)

问题描述

我不明白以下方法的第三个参数的效用：

 < R> R collect（供应商< R>供应商，
 BiConsumer< R，？super T>累加器，
 BiConsumer< R，R>组合器）
   
 
 
 java.util.function.Supplier-java.util.function.BiConsumer-java.util.function.BiConsumer-rel =noreferrertitle =javaDoc> javaDoc ：
 
 
 
 
这产生的结果相当于：
 
 
 
 
 
  R result = supplier.get（）; 
 for（T element：this stream）
 accumulator.accept（result，element）; 
返回结果; 
  
 
 
因为你可以看到参数 combiner 未使用。例如，以下内容将字符串累积到ArrayList中：
  List< String> asList = stringStream.collect（ArrayList :: new，ArrayList :: add，
 ArrayList :: addAll）; 
  
但我期望这样：
 列表与LT;字符串> asList = stringStream.collect（ArrayList :: new，ArrayList :: add）; 
  
 
 
解决方案
 组合器 Stream 是并行时使用c $ c>，因为在这种情况下，有几个线程将 Stream 的元素收集到最终输出的子列表 ArrayList ，这些子列表必须合并以生成最终的 ArrayList 。 
 
I do not understand the utility of the third parameter of the following method:
<R> R collect(Supplier<R> supplier,
              BiConsumer<R,? super T> accumulator,
              BiConsumer<R,R> combiner)
from javaDoc:

  

    
This produces a result equivalent to:
  


 R result = supplier.get();
 for (T element : this stream)
     accumulator.accept(result, element);
 return result;

as you can see the parameter combiner is not used. For example, the following will accumulate strings into an ArrayList:
 List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
                                            ArrayList::addAll);
but I expected this:
List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add );

解决方案
The combiner is used when your Stream is parallel, since in that case several threads collect elements of the Stream into sub-lists of the final output ArrayList, and these sub-lists have to be combined to produce the final ArrayList.

                        
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