http://lucene.apache.org/lucene-parent/lucene-grouping: Lucene Grouping Module (The Apache Software Foundation) 
public abstract class org.apache.lucene.search.grouping.GroupReducer<T, C extends org.apache.lucene.search.Collector>
  minor version: 0
  major version: 59
  flags: flags: (0x0421) ACC_PUBLIC, ACC_SUPER, ACC_ABSTRACT
  this_class: org.apache.lucene.search.grouping.GroupReducer
  super_class: java.lang.Object
{
  private final java.util.Map<T, org.apache.lucene.search.grouping.GroupReducer$GroupCollector<C>> groups;
    descriptor: Ljava/util/Map;
    flags: (0x0012) ACC_PRIVATE, ACC_FINAL
    Signature: Ljava/util/Map<TT;Lorg/apache/lucene/search/grouping/GroupReducer$GroupCollector<TC;>;>;

  public void <init>();
    descriptor: ()V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=3, locals=1, args_size=1
        start local 0 // org.apache.lucene.search.grouping.GroupReducer this
         0: .line 43
            aload 0 /* this */
            invokespecial java.lang.Object.<init>:()V
         1: .line 45
            aload 0 /* this */
            new java.util.HashMap
            dup
            invokespecial java.util.HashMap.<init>:()V
            putfield org.apache.lucene.search.grouping.GroupReducer.groups:Ljava/util/Map;
         2: .line 43
            return
        end local 0 // org.apache.lucene.search.grouping.GroupReducer this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    3     0  this  Lorg/apache/lucene/search/grouping/GroupReducer<TT;TC;>;

  public void setGroups(java.util.Collection<org.apache.lucene.search.grouping.SearchGroup<T>>);
    descriptor: (Ljava/util/Collection;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=5, locals=4, args_size=2
        start local 0 // org.apache.lucene.search.grouping.GroupReducer this
        start local 1 // java.util.Collection groups
         0: .line 53
            aload 1 /* groups */
            invokeinterface java.util.Collection.iterator:()Ljava/util/Iterator;
            astore 3
            goto 3
      StackMap locals: org.apache.lucene.search.grouping.GroupReducer java.util.Collection top java.util.Iterator
      StackMap stack:
         1: aload 3
            invokeinterface java.util.Iterator.next:()Ljava/lang/Object;
            checkcast org.apache.lucene.search.grouping.SearchGroup
            astore 2 /* group */
        start local 2 // org.apache.lucene.search.grouping.SearchGroup group
         2: .line 54
            aload 0 /* this */
            getfield org.apache.lucene.search.grouping.GroupReducer.groups:Ljava/util/Map;
            aload 2 /* group */
            getfield org.apache.lucene.search.grouping.SearchGroup.groupValue:Ljava/lang/Object;
            new org.apache.lucene.search.grouping.GroupReducer$GroupCollector
            dup
            aload 0 /* this */
            invokevirtual org.apache.lucene.search.grouping.GroupReducer.newCollector:()Lorg/apache/lucene/search/Collector;
            invokespecial org.apache.lucene.search.grouping.GroupReducer$GroupCollector.<init>:(Lorg/apache/lucene/search/Collector;)V
            invokeinterface java.util.Map.put:(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
            pop
        end local 2 // org.apache.lucene.search.grouping.SearchGroup group
         3: .line 53
      StackMap locals:
      StackMap stack:
            aload 3
            invokeinterface java.util.Iterator.hasNext:()Z
            ifne 1
         4: .line 56
            return
        end local 1 // java.util.Collection groups
        end local 0 // org.apache.lucene.search.grouping.GroupReducer this
      LocalVariableTable:
        Start  End  Slot    Name  Signature
            0    5     0    this  Lorg/apache/lucene/search/grouping/GroupReducer<TT;TC;>;
            0    5     1  groups  Ljava/util/Collection<Lorg/apache/lucene/search/grouping/SearchGroup<TT;>;>;
            2    3     2   group  Lorg/apache/lucene/search/grouping/SearchGroup<TT;>;
    Signature: (Ljava/util/Collection<Lorg/apache/lucene/search/grouping/SearchGroup<TT;>;>;)V
    MethodParameters:
        Name  Flags
      groups  

  public abstract boolean needsScores();
    descriptor: ()Z
    flags: (0x0401) ACC_PUBLIC, ACC_ABSTRACT

  protected abstract C newCollector();
    descriptor: ()Lorg/apache/lucene/search/Collector;
    flags: (0x0404) ACC_PROTECTED, ACC_ABSTRACT
    Signature: ()TC;

  public final C getCollector();
    descriptor: (Ljava/lang/Object;)Lorg/apache/lucene/search/Collector;
    flags: (0x0011) ACC_PUBLIC, ACC_FINAL
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // org.apache.lucene.search.grouping.GroupReducer this
        start local 1 // java.lang.Object value
         0: .line 72
            aload 0 /* this */
            getfield org.apache.lucene.search.grouping.GroupReducer.groups:Ljava/util/Map;
            aload 1 /* value */
            invokeinterface java.util.Map.get:(Ljava/lang/Object;)Ljava/lang/Object;
            checkcast org.apache.lucene.search.grouping.GroupReducer$GroupCollector
            getfield org.apache.lucene.search.grouping.GroupReducer$GroupCollector.collector:Lorg/apache/lucene/search/Collector;
            areturn
        end local 1 // java.lang.Object value
        end local 0 // org.apache.lucene.search.grouping.GroupReducer this
      LocalVariableTable:
        Start  End  Slot   Name  Signature
            0    1     0   this  Lorg/apache/lucene/search/grouping/GroupReducer<TT;TC;>;
            0    1     1  value  TT;
    Signature: (TT;)TC;
    MethodParameters:
       Name  Flags
      value  

  public final void collect(T, );
    descriptor: (Ljava/lang/Object;I)V
    flags: (0x0011) ACC_PUBLIC, ACC_FINAL
    Code:
      stack=2, locals=4, args_size=3
        start local 0 // org.apache.lucene.search.grouping.GroupReducer this
        start local 1 // java.lang.Object value
        start local 2 // int doc
         0: .line 80
            aload 0 /* this */
            getfield org.apache.lucene.search.grouping.GroupReducer.groups:Ljava/util/Map;
            aload 1 /* value */
            invokeinterface java.util.Map.get:(Ljava/lang/Object;)Ljava/lang/Object;
            checkcast org.apache.lucene.search.grouping.GroupReducer$GroupCollector
            astore 3 /* collector */
        start local 3 // org.apache.lucene.search.grouping.GroupReducer$GroupCollector collector
         1: .line 81
            aload 3 /* collector */
            getfield org.apache.lucene.search.grouping.GroupReducer$GroupCollector.leafCollector:Lorg/apache/lucene/search/LeafCollector;
            iload 2 /* doc */
            invokeinterface org.apache.lucene.search.LeafCollector.collect:(I)V
         2: .line 82
            return
        end local 3 // org.apache.lucene.search.grouping.GroupReducer$GroupCollector collector
        end local 2 // int doc
        end local 1 // java.lang.Object value
        end local 0 // org.apache.lucene.search.grouping.GroupReducer this
      LocalVariableTable:
        Start  End  Slot       Name  Signature
            0    3     0       this  Lorg/apache/lucene/search/grouping/GroupReducer<TT;TC;>;
            0    3     1      value  TT;
            0    3     2        doc  I
            1    3     3  collector  Lorg/apache/lucene/search/grouping/GroupReducer$GroupCollector<TC;>;
    Exceptions:
      throws java.io.IOException
    Signature: (TT;I)V
    MethodParameters:
       Name  Flags
      value  
      doc    

  public final void setScorer(org.apache.lucene.search.Scorable);
    descriptor: (Lorg/apache/lucene/search/Scorable;)V
    flags: (0x0011) ACC_PUBLIC, ACC_FINAL
    Code:
      stack=2, locals=4, args_size=2
        start local 0 // org.apache.lucene.search.grouping.GroupReducer this
        start local 1 // org.apache.lucene.search.Scorable scorer
         0: .line 88
            aload 0 /* this */
            getfield org.apache.lucene.search.grouping.GroupReducer.groups:Ljava/util/Map;
            invokeinterface java.util.Map.values:()Ljava/util/Collection;
            invokeinterface java.util.Collection.iterator:()Ljava/util/Iterator;
            astore 3
            goto 3
      StackMap locals: org.apache.lucene.search.grouping.GroupReducer org.apache.lucene.search.Scorable top java.util.Iterator
      StackMap stack:
         1: aload 3
            invokeinterface java.util.Iterator.next:()Ljava/lang/Object;
            checkcast org.apache.lucene.search.grouping.GroupReducer$GroupCollector
            astore 2 /* collector */
        start local 2 // org.apache.lucene.search.grouping.GroupReducer$GroupCollector collector
         2: .line 89
            aload 2 /* collector */
            getfield org.apache.lucene.search.grouping.GroupReducer$GroupCollector.leafCollector:Lorg/apache/lucene/search/LeafCollector;
            aload 1 /* scorer */
            invokeinterface org.apache.lucene.search.LeafCollector.setScorer:(Lorg/apache/lucene/search/Scorable;)V
        end local 2 // org.apache.lucene.search.grouping.GroupReducer$GroupCollector collector
         3: .line 88
      StackMap locals:
      StackMap stack:
            aload 3
            invokeinterface java.util.Iterator.hasNext:()Z
            ifne 1
         4: .line 91
            return
        end local 1 // org.apache.lucene.search.Scorable scorer
        end local 0 // org.apache.lucene.search.grouping.GroupReducer this
      LocalVariableTable:
        Start  End  Slot       Name  Signature
            0    5     0       this  Lorg/apache/lucene/search/grouping/GroupReducer<TT;TC;>;
            0    5     1     scorer  Lorg/apache/lucene/search/Scorable;
            2    3     2  collector  Lorg/apache/lucene/search/grouping/GroupReducer$GroupCollector<TC;>;
    Exceptions:
      throws java.io.IOException
    MethodParameters:
        Name  Flags
      scorer  

  public final void setNextReader(org.apache.lucene.index.LeafReaderContext);
    descriptor: (Lorg/apache/lucene/index/LeafReaderContext;)V
    flags: (0x0011) ACC_PUBLIC, ACC_FINAL
    Code:
      stack=3, locals=4, args_size=2
        start local 0 // org.apache.lucene.search.grouping.GroupReducer this
        start local 1 // org.apache.lucene.index.LeafReaderContext ctx
         0: .line 97
            aload 0 /* this */
            getfield org.apache.lucene.search.grouping.GroupReducer.groups:Ljava/util/Map;
            invokeinterface java.util.Map.values:()Ljava/util/Collection;
            invokeinterface java.util.Collection.iterator:()Ljava/util/Iterator;
            astore 3
            goto 3
      StackMap locals: org.apache.lucene.search.grouping.GroupReducer org.apache.lucene.index.LeafReaderContext top java.util.Iterator
      StackMap stack:
         1: aload 3
            invokeinterface java.util.Iterator.next:()Ljava/lang/Object;
            checkcast org.apache.lucene.search.grouping.GroupReducer$GroupCollector
            astore 2 /* collector */
        start local 2 // org.apache.lucene.search.grouping.GroupReducer$GroupCollector collector
         2: .line 98
            aload 2 /* collector */
            aload 2 /* collector */
            getfield org.apache.lucene.search.grouping.GroupReducer$GroupCollector.collector:Lorg/apache/lucene/search/Collector;
            aload 1 /* ctx */
            invokeinterface org.apache.lucene.search.Collector.getLeafCollector:(Lorg/apache/lucene/index/LeafReaderContext;)Lorg/apache/lucene/search/LeafCollector;
            putfield org.apache.lucene.search.grouping.GroupReducer$GroupCollector.leafCollector:Lorg/apache/lucene/search/LeafCollector;
        end local 2 // org.apache.lucene.search.grouping.GroupReducer$GroupCollector collector
         3: .line 97
      StackMap locals:
      StackMap stack:
            aload 3
            invokeinterface java.util.Iterator.hasNext:()Z
            ifne 1
         4: .line 100
            return
        end local 1 // org.apache.lucene.index.LeafReaderContext ctx
        end local 0 // org.apache.lucene.search.grouping.GroupReducer this
      LocalVariableTable:
        Start  End  Slot       Name  Signature
            0    5     0       this  Lorg/apache/lucene/search/grouping/GroupReducer<TT;TC;>;
            0    5     1        ctx  Lorg/apache/lucene/index/LeafReaderContext;
            2    3     2  collector  Lorg/apache/lucene/search/grouping/GroupReducer$GroupCollector<TC;>;
    Exceptions:
      throws java.io.IOException
    MethodParameters:
      Name  Flags
      ctx   
}
Signature: <T:Ljava/lang/Object;C::Lorg/apache/lucene/search/Collector;>Ljava/lang/Object;
SourceFile: "GroupReducer.java"
NestMembers:
  org.apache.lucene.search.grouping.GroupReducer$GroupCollector
InnerClasses:
  private final GroupCollector = org.apache.lucene.search.grouping.GroupReducer$GroupCollector of org.apache.lucene.search.grouping.GroupReducer