org.apache.flink.api.java.functions

Interface KeySelector<IN,KEY>

Type Parameters:

IN - Type of objects to extract the key from.

KEY - Type of key.

All Superinterfaces:

Function, Serializable

All Known Implementing Classes:

GroupedProcessingTimeWindowExample.FirstFieldKeyExtractor, KeySelectorUtil.ArrayKeySelector, KeySelectorUtil.ComparableKeySelector, KeySelectorUtil.OneKeySelector, KeySelectorWithType

@Public
public interface KeySelector<IN,KEY>
extends Function, Serializable

The KeySelector allows to use arbitrary objects for operations such as reduce, reduceGroup, join, coGoup, etc. The extractor takes an object and returns the key for that object.

Method Summary

Modifier and Type	Method and Description
KEY	getKey(IN value) User-defined function that extracts the key from an arbitrary object.

Method Detail

getKey

KEY getKey(IN value)
    throws Exception

User-defined function that extracts the key from an arbitrary object. For example for a class:

        public class Word {
                String word;
                int count;
        }
 

The key extractor could return the word as a key to group all Word objects by the String they contain. The code would look like this

        public String getKey(Word w) {
                return w.word;
        }
 

Parameters:

value - The object to get the key from.

Returns:

The extracted key.

Throws:

Exception - Throwing an exception will cause the execution of the respective task to fail, and trigger recovery or cancellation of the program.