累积方式

对记录进行分组，并将每个组缩减为每个组一条记录。

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public <I extends java.io.Serializable> GearsBuilder<I> accumulateBy​(
	gears.operations.ExtractorOperation<T> extractor, 
	gears.operations.AccumulateByOperation<T,​I> accumulator)

public <I extends java.io.Serializable> GearsBuilder<I> accumulateBy​(
	gears.operations.ValueInitializerOperation<I> valueInitializer, 
	gears.operations.ExtractorOperation<T> extractor, 
	gears.operations.AccumulateByOperation<T,​I> accumulator)

Iterates through the records in the pipe, groups them based on the provided extractor, and then reduces each group to a single record per group with the accumulator function.

The initial value of the accumulator is null unless you provide a value initializer operation as a parameter.

Parameters

Type parameters:

Name Description
I The template type of the returned builder

Function parameters:

Name Type Description
accumulator AccumulateByOperation<T,​I> A function with logic to update the accumulator value with each record
extractor ExtractorOperation Extracts a specific value from each record
valueInitializer ValueInitializerOperation Whenever the accumulated value is null, use this function to initialize it

Returns

Returns a GearsBuilder object with a new template type.

Examples

Both of the following examples count the number of unique values.

Without the valueInitializer parameter:

GearsBuilder.CreateGearsBuilder(reader).
   	accumulateBy(r->{
   		return r.getStringVal();
   	},(k, a, r)->{
   		Integer ret = null;
   		if(a == null) {
   			ret = 0;
   		}else {
   			ret = (Integer)a;
   		}
   		return ret + 1;
});

With the valueInitializer parameter:

GearsBuilder.CreateGearsBuilder(reader).
    	accumulateBy(()->{
   		return 0;
   	},r->{
   		return r.getStringVal();
   	},(k, a, r)->{
   		return a + 1;
   	});
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