org.apache.flink.api.common.functions

Interface FoldFunction<O,T>

Type Parameters:

T - Type of the initial input and the returned element

O - Type of the elements that the group/list/stream contains

All Superinterfaces:

Function, Serializable

All Known Implementing Classes:

RichFoldFunction

@Public
public interface FoldFunction<O,T>
extends Function, Serializable

Base interface for Fold functions. Fold functions combine groups of elements to a single value, by applying a binary operation to an initial accumulator element every element from a group elements.

The basic syntax for using a FoldFunction is as follows:

 DataSet<X> input = ...;

 X initialValue = ...;
 DataSet<X> result = input.fold(new MyFoldFunction(), initialValue);
 

Like all functions, the FoldFunction needs to be serializable, as defined in Serializable.

Method Summary

Modifier and Type	Method and Description
T	fold(T accumulator, O value) The core method of FoldFunction, combining two values into one value of the same type.

Method Detail

fold

T fold(T accumulator,
       O value)
throws Exception

The core method of FoldFunction, combining two values into one value of the same type. The fold function is consecutively applied to all values of a group until only a single value remains.

Parameters:

accumulator - The initial value, and accumulator.

value - The value from the group to "fold" into the accumulator.

Returns:

The accumulator that is at the end of the "folding" the group.

Throws:

Exception - This method may throw exceptions. Throwing an exception will cause the operation to fail and may trigger recovery.