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 
class org.eclipse.collections.impl.block.factory.Functions$MapKeyFunction<K> implements org.eclipse.collections.api.block.function.Function<java.util.Map$Entry<K, ?>, K>
  minor version: 0
  major version: 59
  flags: flags: (0x0020) ACC_SUPER
  this_class: org.eclipse.collections.impl.block.factory.Functions$MapKeyFunction
  super_class: java.lang.Object
{
  private static final long serialVersionUID;
    descriptor: J
    flags: (0x001a) ACC_PRIVATE, ACC_STATIC, ACC_FINAL
    ConstantValue: 1

  private void <init>();
    descriptor: ()V
    flags: (0x0002) ACC_PRIVATE
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // org.eclipse.collections.impl.block.factory.Functions$MapKeyFunction this
         0: .line 803
            aload 0 /* this */
            invokespecial java.lang.Object.<init>:()V
            return
        end local 0 // org.eclipse.collections.impl.block.factory.Functions$MapKeyFunction this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lorg/eclipse/collections/impl/block/factory/Functions$MapKeyFunction<TK;>;

  public K valueOf(java.util.Map$Entry<K, ?>);
    descriptor: (Ljava/util/Map$Entry;)Ljava/lang/Object;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=2, args_size=2
        start local 0 // org.eclipse.collections.impl.block.factory.Functions$MapKeyFunction this
        start local 1 // java.util.Map$Entry entry
         0: .line 810
            aload 1 /* entry */
            invokeinterface java.util.Map$Entry.getKey:()Ljava/lang/Object;
            areturn
        end local 1 // java.util.Map$Entry entry
        end local 0 // org.eclipse.collections.impl.block.factory.Functions$MapKeyFunction this
      LocalVariableTable:
        Start  End  Slot   Name  Signature
            0    1     0   this  Lorg/eclipse/collections/impl/block/factory/Functions$MapKeyFunction<TK;>;
            0    1     1  entry  Ljava/util/Map$Entry<TK;*>;
    Signature: (Ljava/util/Map$Entry<TK;*>;)TK;
    MethodParameters:
       Name  Flags
      entry  

  public java.lang.Object valueOf(java.lang.Object);
    descriptor: (Ljava/lang/Object;)Ljava/lang/Object;
    flags: (0x1041) ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
    Code:
      stack=2, locals=2, args_size=2
         0: .line 1
            aload 0
            aload 1
            checkcast java.util.Map$Entry
            invokevirtual org.eclipse.collections.impl.block.factory.Functions$MapKeyFunction.valueOf:(Ljava/util/Map$Entry;)Ljava/lang/Object;
            areturn
      LocalVariableTable:
        Start  End  Slot  Name  Signature
}
Signature: <K:Ljava/lang/Object;>Ljava/lang/Object;Lorg/eclipse/collections/api/block/function/Function<Ljava/util/Map$Entry<TK;*>;TK;>;
SourceFile: "Functions.java"
NestHost: org.eclipse.collections.impl.block.factory.Functions
InnerClasses:
  public abstract Entry = java.util.Map$Entry of java.util.Map
  private MapKeyFunction = org.eclipse.collections.impl.block.factory.Functions$MapKeyFunction of org.eclipse.collections.impl.block.factory.Functions