GroupReduceFunction (Flink : 2.0-SNAPSHOT API)

Type Parameters:

T - Type of the elements that this function processes.

O - The type of the elements returned by the user-defined function.

All Superinterfaces:

Function, Serializable

All Known Implementing Classes:

MergeOperatorStates, OperatorSubtaskStateReducer, RichGroupReduceFunction

Functional Interface:

This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
```
@Public
@FunctionalInterface
public interface GroupReduceFunction<T,O>
extends Function, Serializable
```
The interface for group reduce functions. GroupReduceFunctions process groups of elements. They may aggregate them to a single value, or produce multiple result values for each group. The group may be defined by sharing a common grouping key, or the group may simply be all elements of a data set.
For a reduce functions that works incrementally by combining always two elements, see ReduceFunction.
The basic syntax for using a grouped GroupReduceFunction is as follows:
```
 DataSet<X> input = ...;

 DataSet<X> result = input.groupBy(<key-definition>).reduceGroup(new MyGroupReduceFunction());
 
```
Partial computation can significantly improve the performance of a GroupReduceFunction. This technique is also known as applying a Combiner. Implement the GroupCombineFunction interface to enable partial computations, i.e., a combiner for this GroupReduceFunction.

- Method Summary
  
  All Methods Instance Methods Abstract Methods
  Modifier and Type Method Description
  
  void reduce(Iterable<T> values, Collector<O> out)
  The reduce method.

- Method Detail
  - reduce
```
void reduce(Iterable<T> values,
            Collector<O> out)
     throws Exception
```
    The reduce method. The function receives one call per group of elements.
    
    Parameters:
    
    values - All records that belong to the given input key.
    
    out - The collector to hand results to.
    
    Throws:
    
    Exception - This method may throw exceptions. Throwing an exception will cause the operation to fail and may trigger recovery.