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. 
public class com.google.common.collect.ForwardingMap$StandardKeySet extends com.google.common.collect.Maps$KeySet<K, V>
  minor version: 0
  major version: 59
  flags: flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: com.google.common.collect.ForwardingMap$StandardKeySet
  super_class: com.google.common.collect.Maps$KeySet
{
  final com.google.common.collect.ForwardingMap this$0;
    descriptor: Lcom/google/common/collect/ForwardingMap;
    flags: (0x1010) ACC_FINAL, ACC_SYNTHETIC

  public void <init>(com.google.common.collect.ForwardingMap);
    descriptor: (Lcom/google/common/collect/ForwardingMap;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // com.google.common.collect.ForwardingMap$StandardKeySet this
         0: .line 197
            aload 0 /* this */
            aload 1
            putfield com.google.common.collect.ForwardingMap$StandardKeySet.this$0:Lcom/google/common/collect/ForwardingMap;
         1: .line 198
            aload 0 /* this */
            aload 1
            invokespecial com.google.common.collect.Maps$KeySet.<init>:(Ljava/util/Map;)V
         2: .line 199
            return
        end local 0 // com.google.common.collect.ForwardingMap$StandardKeySet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    3     0  this  Lcom/google/common/collect/ForwardingMap$StandardKeySet;
    MethodParameters:
        Name  Flags
      this$0  final
}
Signature: Lcom/google/common/collect/Maps$KeySet<TK;TV;>;
SourceFile: "ForwardingMap.java"
NestHost: com.google.common.collect.ForwardingMap
InnerClasses:
  protected StandardKeySet = com.google.common.collect.ForwardingMap$StandardKeySet of com.google.common.collect.ForwardingMap
  KeySet = com.google.common.collect.Maps$KeySet of com.google.common.collect.Maps
    RuntimeInvisibleAnnotations: 
      com.google.common.annotations.Beta()