GroupReducer (Lucene 9.0.0 grouping API)

java.lang.Object
- org.apache.lucene.search.grouping.GroupReducer<T,C>

Type Parameters:

T - the type of the value used for grouping

C - the type of Collector used to reduce each group
```
public abstract class GroupReducer<T,C extends Collector>
extends Object
```
Concrete implementations of this class define what to collect for individual groups during the second-pass of a grouping search.
Each group is assigned a Collector returned by newCollector(), and LeafCollector.collect(int) is called for each document that is in a group

See Also:

SecondPassGroupingCollector

Constructor Summary

Constructors
Constructor Description

GroupReducer()

Method Summary

All Methods Instance Methods Abstract Methods Concrete Methods
Modifier and Type	Method	Description
`void`	`collect(T value, int doc)`	Collect a given document into a given group
`C`	`getCollector(T value)`	Get the Collector for a given group
`abstract boolean`	`needsScores()`	Whether or not this reducer requires collected documents to be scored
`protected abstract C`	`newCollector()`	Creates a new Collector for each group
`void`	`setGroups(Collection<SearchGroup<T>> groups)`	Define which groups should be reduced.
`void`	`setNextReader(LeafReaderContext ctx)`	Called when the parent `SecondPassGroupingCollector` moves to a new segment
`void`	`setScorer(Scorable scorer)`	Set the Scorer on all group collectors

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

- Constructor Detail
  - GroupReducer
```
public GroupReducer()
```
- Method Detail
  - setGroups
```
public void setGroups(Collection<SearchGroup<T>> groups)
```
    Define which groups should be reduced.
    Called by SecondPassGroupingCollector
  - needsScores
```
public abstract boolean needsScores()
```
    Whether or not this reducer requires collected documents to be scored
  - newCollector
```
protected abstract C newCollector()
```
    Creates a new Collector for each group
  - getCollector
```
public final C getCollector(T value)
```
    Get the Collector for a given group
  - collect
```
public final void collect(T value,
                          int doc)
                   throws IOException
```
    Collect a given document into a given group
    
    Throws:
    
    IOException - on error
  - setScorer
```
public final void setScorer(Scorable scorer)
                     throws IOException
```
    Set the Scorer on all group collectors
    
    Throws:
    
    IOException
  - setNextReader
```
public final void setNextReader(LeafReaderContext ctx)
                         throws IOException
```
    Called when the parent SecondPassGroupingCollector moves to a new segment
    
    Throws:
    
    IOException