https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
class com.google.common.collect.HashBiMap$KeySet$1 extends com.google.common.collect.HashBiMap<K, V>.Itr<K>
  minor version: 0
  major version: 59
  flags: flags: (0x0020) ACC_SUPER
  this_class: com.google.common.collect.HashBiMap$KeySet$1
  super_class: com.google.common.collect.HashBiMap$Itr
{
  final com.google.common.collect.HashBiMap$KeySet this$1;
    descriptor: Lcom/google/common/collect/HashBiMap$KeySet;
    flags: (0x1010) ACC_FINAL, ACC_SYNTHETIC

  void <init>(com.google.common.collect.HashBiMap$KeySet, com.google.common.collect.HashBiMap);
    descriptor: (Lcom/google/common/collect/HashBiMap$KeySet;Lcom/google/common/collect/HashBiMap;)V
    flags: (0x0000) 
    Code:
      stack=2, locals=3, args_size=3
        start local 0 // com.google.common.collect.HashBiMap$KeySet$1 this
         0: .line 456
            aload 0 /* this */
            aload 1
            putfield com.google.common.collect.HashBiMap$KeySet$1.this$1:Lcom/google/common/collect/HashBiMap$KeySet;
            aload 0 /* this */
            aload 2
            invokespecial com.google.common.collect.HashBiMap$Itr.<init>:(Lcom/google/common/collect/HashBiMap;)V
            return
        end local 0 // com.google.common.collect.HashBiMap$KeySet$1 this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/HashBiMap$KeySet$1;
    MethodParameters:
        Name  Flags
      this$0  final
      this$1  final

  K output(com.google.common.collect.HashBiMap$BiEntry<K, V>);
    descriptor: (Lcom/google/common/collect/HashBiMap$BiEntry;)Ljava/lang/Object;
    flags: (0x0000) 
    Code:
      stack=1, locals=2, args_size=2
        start local 0 // com.google.common.collect.HashBiMap$KeySet$1 this
        start local 1 // com.google.common.collect.HashBiMap$BiEntry entry
         0: .line 459
            aload 1 /* entry */
            getfield com.google.common.collect.HashBiMap$BiEntry.key:Ljava/lang/Object;
            areturn
        end local 1 // com.google.common.collect.HashBiMap$BiEntry entry
        end local 0 // com.google.common.collect.HashBiMap$KeySet$1 this
      LocalVariableTable:
        Start  End  Slot   Name  Signature
            0    1     0   this  Lcom/google/common/collect/HashBiMap$KeySet$1;
            0    1     1  entry  Lcom/google/common/collect/HashBiMap$BiEntry<TK;TV;>;
    Signature: (Lcom/google/common/collect/HashBiMap$BiEntry<TK;TV;>;)TK;
    MethodParameters:
       Name  Flags
      entry  
}
Signature: Lcom/google/common/collect/HashBiMap<TK;TV;>.Itr<TK;>;
SourceFile: "HashBiMap.java"
EnclosingMethod: com.google.common.collect.HashBiMap$KeySet.iterator:()Ljava/util/Iterator;
NestHost: com.google.common.collect.HashBiMap
InnerClasses:
  private final BiEntry = com.google.common.collect.HashBiMap$BiEntry of com.google.common.collect.HashBiMap
  abstract Itr = com.google.common.collect.HashBiMap$Itr of com.google.common.collect.HashBiMap
  private final KeySet = com.google.common.collect.HashBiMap$KeySet of com.google.common.collect.HashBiMap
  com.google.common.collect.HashBiMap$KeySet$1