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.Maps$KeySet<K, V> extends com.google.common.collect.Sets$ImprovedAbstractSet<K>
  minor version: 0
  major version: 59
  flags: flags: (0x0020) ACC_SUPER
  this_class: com.google.common.collect.Maps$KeySet
  super_class: com.google.common.collect.Sets$ImprovedAbstractSet
{
  final java.util.Map<K, V> map;
    descriptor: Ljava/util/Map;
    flags: (0x0010) ACC_FINAL
    Signature: Ljava/util/Map<TK;TV;>;
    RuntimeInvisibleAnnotations: 
      com.google.j2objc.annotations.Weak()

  void <init>(java.util.Map<K, V>);
    descriptor: (Ljava/util/Map;)V
    flags: (0x0000) 
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // com.google.common.collect.Maps$KeySet this
        start local 1 // java.util.Map map
         0: .line 3687
            aload 0 /* this */
            invokespecial com.google.common.collect.Sets$ImprovedAbstractSet.<init>:()V
         1: .line 3688
            aload 0 /* this */
            aload 1 /* map */
            invokestatic com.google.common.base.Preconditions.checkNotNull:(Ljava/lang/Object;)Ljava/lang/Object;
            checkcast java.util.Map
            putfield com.google.common.collect.Maps$KeySet.map:Ljava/util/Map;
         2: .line 3689
            return
        end local 1 // java.util.Map map
        end local 0 // com.google.common.collect.Maps$KeySet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    3     0  this  Lcom/google/common/collect/Maps$KeySet<TK;TV;>;
            0    3     1   map  Ljava/util/Map<TK;TV;>;
    Signature: (Ljava/util/Map<TK;TV;>;)V
    MethodParameters:
      Name  Flags
      map   

  java.util.Map<K, V> map();
    descriptor: ()Ljava/util/Map;
    flags: (0x0000) 
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.Maps$KeySet this
         0: .line 3692
            aload 0 /* this */
            getfield com.google.common.collect.Maps$KeySet.map:Ljava/util/Map;
            areturn
        end local 0 // com.google.common.collect.Maps$KeySet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Maps$KeySet<TK;TV;>;
    Signature: ()Ljava/util/Map<TK;TV;>;

  public java.util.Iterator<K> iterator();
    descriptor: ()Ljava/util/Iterator;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.Maps$KeySet this
         0: .line 3697
            aload 0 /* this */
            invokevirtual com.google.common.collect.Maps$KeySet.map:()Ljava/util/Map;
            invokeinterface java.util.Map.entrySet:()Ljava/util/Set;
            invokeinterface java.util.Set.iterator:()Ljava/util/Iterator;
            invokestatic com.google.common.collect.Maps.keyIterator:(Ljava/util/Iterator;)Ljava/util/Iterator;
            areturn
        end local 0 // com.google.common.collect.Maps$KeySet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Maps$KeySet<TK;TV;>;
    Signature: ()Ljava/util/Iterator<TK;>;

  public void forEach(java.util.function.Consumer<? super K>);
    descriptor: (Ljava/util/function/Consumer;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // com.google.common.collect.Maps$KeySet this
        start local 1 // java.util.function.Consumer action
         0: .line 3702
            aload 1 /* action */
            invokestatic com.google.common.base.Preconditions.checkNotNull:(Ljava/lang/Object;)Ljava/lang/Object;
            pop
         1: .line 3704
            aload 0 /* this */
            getfield com.google.common.collect.Maps$KeySet.map:Ljava/util/Map;
            aload 1 /* action */
            invokedynamic accept(Ljava/util/function/Consumer;)Ljava/util/function/BiConsumer;
              Bootstrap: invokestatic java.lang.invoke.LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
                Method arguments:
                  (Ljava/lang/Object;Ljava/lang/Object;)V
                  com/google/common/collect/Maps$KeySet.lambda$0(Ljava/util/function/Consumer;Ljava/lang/Object;Ljava/lang/Object;)V (6)
                  (Ljava/lang/Object;Ljava/lang/Object;)V
            invokeinterface java.util.Map.forEach:(Ljava/util/function/BiConsumer;)V
         2: .line 3705
            return
        end local 1 // java.util.function.Consumer action
        end local 0 // com.google.common.collect.Maps$KeySet this
      LocalVariableTable:
        Start  End  Slot    Name  Signature
            0    3     0    this  Lcom/google/common/collect/Maps$KeySet<TK;TV;>;
            0    3     1  action  Ljava/util/function/Consumer<-TK;>;
    Signature: (Ljava/util/function/Consumer<-TK;>;)V
    MethodParameters:
        Name  Flags
      action  

  public int size();
    descriptor: ()I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.Maps$KeySet this
         0: .line 3709
            aload 0 /* this */
            invokevirtual com.google.common.collect.Maps$KeySet.map:()Ljava/util/Map;
            invokeinterface java.util.Map.size:()I
            ireturn
        end local 0 // com.google.common.collect.Maps$KeySet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Maps$KeySet<TK;TV;>;

  public boolean isEmpty();
    descriptor: ()Z
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.Maps$KeySet this
         0: .line 3714
            aload 0 /* this */
            invokevirtual com.google.common.collect.Maps$KeySet.map:()Ljava/util/Map;
            invokeinterface java.util.Map.isEmpty:()Z
            ireturn
        end local 0 // com.google.common.collect.Maps$KeySet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Maps$KeySet<TK;TV;>;

  public boolean contains(java.lang.Object);
    descriptor: (Ljava/lang/Object;)Z
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // com.google.common.collect.Maps$KeySet this
        start local 1 // java.lang.Object o
         0: .line 3719
            aload 0 /* this */
            invokevirtual com.google.common.collect.Maps$KeySet.map:()Ljava/util/Map;
            aload 1 /* o */
            invokeinterface java.util.Map.containsKey:(Ljava/lang/Object;)Z
            ireturn
        end local 1 // java.lang.Object o
        end local 0 // com.google.common.collect.Maps$KeySet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Maps$KeySet<TK;TV;>;
            0    1     1     o  Ljava/lang/Object;
    MethodParameters:
      Name  Flags
      o     

  public boolean remove(java.lang.Object);
    descriptor: (Ljava/lang/Object;)Z
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // com.google.common.collect.Maps$KeySet this
        start local 1 // java.lang.Object o
         0: .line 3724
            aload 0 /* this */
            aload 1 /* o */
            invokevirtual com.google.common.collect.Maps$KeySet.contains:(Ljava/lang/Object;)Z
            ifeq 3
         1: .line 3725
            aload 0 /* this */
            invokevirtual com.google.common.collect.Maps$KeySet.map:()Ljava/util/Map;
            aload 1 /* o */
            invokeinterface java.util.Map.remove:(Ljava/lang/Object;)Ljava/lang/Object;
            pop
         2: .line 3726
            iconst_1
            ireturn
         3: .line 3728
      StackMap locals:
      StackMap stack:
            iconst_0
            ireturn
        end local 1 // java.lang.Object o
        end local 0 // com.google.common.collect.Maps$KeySet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    4     0  this  Lcom/google/common/collect/Maps$KeySet<TK;TV;>;
            0    4     1     o  Ljava/lang/Object;
    MethodParameters:
      Name  Flags
      o     

  public void clear();
    descriptor: ()V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.Maps$KeySet this
         0: .line 3733
            aload 0 /* this */
            invokevirtual com.google.common.collect.Maps$KeySet.map:()Ljava/util/Map;
            invokeinterface java.util.Map.clear:()V
         1: .line 3734
            return
        end local 0 // com.google.common.collect.Maps$KeySet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    2     0  this  Lcom/google/common/collect/Maps$KeySet<TK;TV;>;

  private static void lambda$0(java.util.function.Consumer, java.lang.Object, java.lang.Object);
    descriptor: (Ljava/util/function/Consumer;Ljava/lang/Object;Ljava/lang/Object;)V
    flags: (0x100a) ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC
    Code:
      stack=2, locals=3, args_size=3
        start local 1 // java.lang.Object k
        start local 2 // java.lang.Object v
         0: .line 3704
            aload 0
            aload 1 /* k */
            invokeinterface java.util.function.Consumer.accept:(Ljava/lang/Object;)V
            return
        end local 2 // java.lang.Object v
        end local 1 // java.lang.Object k
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     1     k  TK;
            0    1     2     v  TV;
}
Signature: <K:Ljava/lang/Object;V:Ljava/lang/Object;>Lcom/google/common/collect/Sets$ImprovedAbstractSet<TK;>;
SourceFile: "Maps.java"
NestHost: com.google.common.collect.Maps
InnerClasses:
  KeySet = com.google.common.collect.Maps$KeySet of com.google.common.collect.Maps
  abstract ImprovedAbstractSet = com.google.common.collect.Sets$ImprovedAbstractSet of com.google.common.collect.Sets
  public final Lookup = java.lang.invoke.MethodHandles$Lookup of java.lang.invoke.MethodHandles
  public abstract Entry = java.util.Map$Entry of java.util.Map