Mapper output.collect（）？

Question

映射器調用了這兩次。

out.collect(new Text("Car"),new Text("Subaru"); 
out.collect(new Text("Cr-v"),new Text("Honda"); 

reduce()也會被叫兩次嗎？

Answer 1

我假設你正在談論OutputCollector.collect(K,V)?

reduce()被調用一次，對每個[鍵，（值列表）。爲了解釋，讓我們說你叫：

out.collect(new Text("Car"),new Text("Subaru"); 
out.collect(new Text("Car"),new Text("Honda"); 
out.collect(new Text("Car"),new Text("Ford"); 
out.collect(new Text("Truck"),new Text("Dodge"); 
out.collect(new Text("Truck"),new Text("Chevy"); 

然後reduce()將兩次與對

reduce(Car, <Subaru, Honda, Ford>) 
reduce(Truck, <Dodge, Chevy>) 

所以在你的榜樣，是的，功能reduce()會被調用兩次被調用。我希望有所幫助。