https://github.com/eclipse/eclipse-collections/eclipse-collections: Eclipse Collections is a collections framework for Java. It has JDK-compatible List, Set and Map implementations with a rich API and set of utility classes that work with any JDK compatible Collections, Arrays, Maps or Strings. The iteration protocol was inspired by the Smalltalk collection framework.
  • Craig P. Motlin
  • Donald Raab
  • Mohammad A. Rezaei
  • Hiroshi Ito
  • Nikhil J. Nanivadekar 
public class org.eclipse.collections.impl.block.procedure.GroupByUniqueKeyProcedure<T, K> implements org.eclipse.collections.api.block.procedure.Procedure<T>
  minor version: 0
  major version: 59
  flags: flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: org.eclipse.collections.impl.block.procedure.GroupByUniqueKeyProcedure
  super_class: java.lang.Object
{
  private static final long serialVersionUID;
    descriptor: J
    flags: (0x001a) ACC_PRIVATE, ACC_STATIC, ACC_FINAL
    ConstantValue: 1

  private final java.util.Map<K, T> map;
    descriptor: Ljava/util/Map;
    flags: (0x0012) ACC_PRIVATE, ACC_FINAL
    Signature: Ljava/util/Map<TK;TT;>;

  private final org.eclipse.collections.api.block.function.Function<? super T, ? extends K> keyFunction;
    descriptor: Lorg/eclipse/collections/api/block/function/Function;
    flags: (0x0012) ACC_PRIVATE, ACC_FINAL
    Signature: Lorg/eclipse/collections/api/block/function/Function<-TT;+TK;>;

  public void <init>(java.util.Map<K, T>, org.eclipse.collections.api.block.function.Function<? super T, ? extends K>);
    descriptor: (Ljava/util/Map;Lorg/eclipse/collections/api/block/function/Function;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=3
        start local 0 // org.eclipse.collections.impl.block.procedure.GroupByUniqueKeyProcedure this
        start local 1 // java.util.Map newMap
        start local 2 // org.eclipse.collections.api.block.function.Function newKeyFunction
         0: .line 25
            aload 0 /* this */
            invokespecial java.lang.Object.<init>:()V
         1: .line 27
            aload 0 /* this */
            aload 1 /* newMap */
            putfield org.eclipse.collections.impl.block.procedure.GroupByUniqueKeyProcedure.map:Ljava/util/Map;
         2: .line 28
            aload 0 /* this */
            aload 2 /* newKeyFunction */
            putfield org.eclipse.collections.impl.block.procedure.GroupByUniqueKeyProcedure.keyFunction:Lorg/eclipse/collections/api/block/function/Function;
         3: .line 29
            return
        end local 2 // org.eclipse.collections.api.block.function.Function newKeyFunction
        end local 1 // java.util.Map newMap
        end local 0 // org.eclipse.collections.impl.block.procedure.GroupByUniqueKeyProcedure this
      LocalVariableTable:
        Start  End  Slot            Name  Signature
            0    4     0            this  Lorg/eclipse/collections/impl/block/procedure/GroupByUniqueKeyProcedure<TT;TK;>;
            0    4     1          newMap  Ljava/util/Map<TK;TT;>;
            0    4     2  newKeyFunction  Lorg/eclipse/collections/api/block/function/Function<-TT;+TK;>;
    Signature: (Ljava/util/Map<TK;TT;>;Lorg/eclipse/collections/api/block/function/Function<-TT;+TK;>;)V
    MethodParameters:
                Name  Flags
      newMap          
      newKeyFunction  

  public void value();
    descriptor: (Ljava/lang/Object;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=5, locals=3, args_size=2
        start local 0 // org.eclipse.collections.impl.block.procedure.GroupByUniqueKeyProcedure this
        start local 1 // java.lang.Object object
         0: .line 34
            aload 0 /* this */
            getfield org.eclipse.collections.impl.block.procedure.GroupByUniqueKeyProcedure.keyFunction:Lorg/eclipse/collections/api/block/function/Function;
            aload 1 /* object */
            invokeinterface org.eclipse.collections.api.block.function.Function.valueOf:(Ljava/lang/Object;)Ljava/lang/Object;
            astore 2 /* key */
        start local 2 // java.lang.Object key
         1: .line 35
            aload 0 /* this */
            getfield org.eclipse.collections.impl.block.procedure.GroupByUniqueKeyProcedure.map:Ljava/util/Map;
            aload 2 /* key */
            aload 1 /* object */
            invokeinterface java.util.Map.put:(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
            ifnull 3
         2: .line 37
            new java.lang.IllegalStateException
            dup
            new java.lang.StringBuilder
            dup
            ldc "Key "
            invokespecial java.lang.StringBuilder.<init>:(Ljava/lang/String;)V
            aload 2 /* key */
            invokevirtual java.lang.StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
            ldc " already exists in map!"
            invokevirtual java.lang.StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
            invokevirtual java.lang.StringBuilder.toString:()Ljava/lang/String;
            invokespecial java.lang.IllegalStateException.<init>:(Ljava/lang/String;)V
            athrow
         3: .line 39
      StackMap locals: java.lang.Object
      StackMap stack:
            return
        end local 2 // java.lang.Object key
        end local 1 // java.lang.Object object
        end local 0 // org.eclipse.collections.impl.block.procedure.GroupByUniqueKeyProcedure this
      LocalVariableTable:
        Start  End  Slot    Name  Signature
            0    4     0    this  Lorg/eclipse/collections/impl/block/procedure/GroupByUniqueKeyProcedure<TT;TK;>;
            0    4     1  object  TT;
            1    4     2     key  TK;
    Signature: (TT;)V
    MethodParameters:
        Name  Flags
      object  
}
Signature: <T:Ljava/lang/Object;K:Ljava/lang/Object;>Ljava/lang/Object;Lorg/eclipse/collections/api/block/procedure/Procedure<TT;>;
SourceFile: "GroupByUniqueKeyProcedure.java"