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.CompactHashMap$KeySetView extends com.google.common.collect.Maps$KeySet<K, V>
  minor version: 0
  major version: 59
  flags: flags: (0x0020) ACC_SUPER
  this_class: com.google.common.collect.CompactHashMap$KeySetView
  super_class: com.google.common.collect.Maps$KeySet
{
  final com.google.common.collect.CompactHashMap this$0;
    descriptor: Lcom/google/common/collect/CompactHashMap;
    flags: (0x1010) ACC_FINAL, ACC_SYNTHETIC

  void <init>(com.google.common.collect.CompactHashMap);
    descriptor: (Lcom/google/common/collect/CompactHashMap;)V
    flags: (0x0000) 
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // com.google.common.collect.CompactHashMap$KeySetView this
         0: .line 539
            aload 0 /* this */
            aload 1
            putfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
         1: .line 540
            aload 0 /* this */
            aload 1
            invokespecial com.google.common.collect.Maps$KeySet.<init>:(Ljava/util/Map;)V
         2: .line 541
            return
        end local 0 // com.google.common.collect.CompactHashMap$KeySetView this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    3     0  this  Lcom/google/common/collect/CompactHashMap$KeySetView;
    MethodParameters:
        Name  Flags
      this$0  final

  public java.lang.Object[] toArray();
    descriptor: ()[Ljava/lang/Object;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=3, locals=1, args_size=1
        start local 0 // com.google.common.collect.CompactHashMap$KeySetView this
         0: .line 545
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            getfield com.google.common.collect.CompactHashMap.keys:[Ljava/lang/Object;
            iconst_0
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            getfield com.google.common.collect.CompactHashMap.size:I
            invokestatic com.google.common.collect.ObjectArrays.copyAsObjectArray:([Ljava/lang/Object;II)[Ljava/lang/Object;
            areturn
        end local 0 // com.google.common.collect.CompactHashMap$KeySetView this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/CompactHashMap$KeySetView;

  public <T> T[] toArray();
    descriptor: ([Ljava/lang/Object;)[Ljava/lang/Object;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=4, locals=2, args_size=2
        start local 0 // com.google.common.collect.CompactHashMap$KeySetView this
        start local 1 // java.lang.Object[] a
         0: .line 550
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            getfield com.google.common.collect.CompactHashMap.keys:[Ljava/lang/Object;
            iconst_0
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            getfield com.google.common.collect.CompactHashMap.size:I
            aload 1 /* a */
            invokestatic com.google.common.collect.ObjectArrays.toArrayImpl:([Ljava/lang/Object;II[Ljava/lang/Object;)[Ljava/lang/Object;
            areturn
        end local 1 // java.lang.Object[] a
        end local 0 // com.google.common.collect.CompactHashMap$KeySetView this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/CompactHashMap$KeySetView;
            0    1     1     a  [Ljava/lang/Object;
    Signature: <T:Ljava/lang/Object;>([TT;)[TT;
    MethodParameters:
      Name  Flags
      a     

  public boolean remove(java.lang.Object);
    descriptor: (Ljava/lang/Object;)Z
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=2
        start local 0 // com.google.common.collect.CompactHashMap$KeySetView this
        start local 1 // java.lang.Object o
         0: .line 555
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            aload 1 /* o */
            invokevirtual com.google.common.collect.CompactHashMap.indexOf:(Ljava/lang/Object;)I
            istore 2 /* index */
        start local 2 // int index
         1: .line 556
            iload 2 /* index */
            iconst_m1
            if_icmpne 3
         2: .line 557
            iconst_0
            ireturn
         3: .line 559
      StackMap locals: int
      StackMap stack:
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            iload 2 /* index */
            invokevirtual com.google.common.collect.CompactHashMap.removeEntry:(I)Ljava/lang/Object;
            pop
         4: .line 560
            iconst_1
            ireturn
        end local 2 // int index
        end local 1 // java.lang.Object o
        end local 0 // com.google.common.collect.CompactHashMap$KeySetView this
      LocalVariableTable:
        Start  End  Slot   Name  Signature
            0    5     0   this  Lcom/google/common/collect/CompactHashMap$KeySetView;
            0    5     1      o  Ljava/lang/Object;
            1    5     2  index  I
    RuntimeVisibleTypeAnnotations: 
      METHOD_FORMAL_PARAMETER
        org.checkerframework.checker.nullness.qual.Nullable()
    MethodParameters:
      Name  Flags
      o     

  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.CompactHashMap$KeySetView this
         0: .line 566
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            invokevirtual com.google.common.collect.CompactHashMap.keySetIterator:()Ljava/util/Iterator;
            areturn
        end local 0 // com.google.common.collect.CompactHashMap$KeySetView this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/CompactHashMap$KeySetView;
    Signature: ()Ljava/util/Iterator<TK;>;

  public java.util.Spliterator<K> spliterator();
    descriptor: ()Ljava/util/Spliterator;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=4, locals=1, args_size=1
        start local 0 // com.google.common.collect.CompactHashMap$KeySetView this
         0: .line 571
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            getfield com.google.common.collect.CompactHashMap.keys:[Ljava/lang/Object;
            iconst_0
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            getfield com.google.common.collect.CompactHashMap.size:I
            bipush 17
            invokestatic java.util.Spliterators.spliterator:([Ljava/lang/Object;III)Ljava/util/Spliterator;
            areturn
        end local 0 // com.google.common.collect.CompactHashMap$KeySetView this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/CompactHashMap$KeySetView;
    Signature: ()Ljava/util/Spliterator<TK;>;

  public void forEach(java.util.function.Consumer<? super K>);
    descriptor: (Ljava/util/function/Consumer;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=3, locals=3, args_size=2
        start local 0 // com.google.common.collect.CompactHashMap$KeySetView this
        start local 1 // java.util.function.Consumer action
         0: .line 576
            aload 1 /* action */
            invokestatic com.google.common.base.Preconditions.checkNotNull:(Ljava/lang/Object;)Ljava/lang/Object;
            pop
         1: .line 577
            iconst_0
            istore 2 /* i */
        start local 2 // int i
         2: goto 5
         3: .line 578
      StackMap locals: int
      StackMap stack:
            aload 1 /* action */
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            getfield com.google.common.collect.CompactHashMap.keys:[Ljava/lang/Object;
            iload 2 /* i */
            aaload
            invokeinterface java.util.function.Consumer.accept:(Ljava/lang/Object;)V
         4: .line 577
            iinc 2 /* i */ 1
      StackMap locals:
      StackMap stack:
         5: iload 2 /* i */
            aload 0 /* this */
            getfield com.google.common.collect.CompactHashMap$KeySetView.this$0:Lcom/google/common/collect/CompactHashMap;
            getfield com.google.common.collect.CompactHashMap.size:I
            if_icmplt 3
        end local 2 // int i
         6: .line 580
            return
        end local 1 // java.util.function.Consumer action
        end local 0 // com.google.common.collect.CompactHashMap$KeySetView this
      LocalVariableTable:
        Start  End  Slot    Name  Signature
            0    7     0    this  Lcom/google/common/collect/CompactHashMap$KeySetView;
            0    7     1  action  Ljava/util/function/Consumer<-TK;>;
            2    6     2       i  I
    Signature: (Ljava/util/function/Consumer<-TK;>;)V
    MethodParameters:
        Name  Flags
      action  
}
Signature: Lcom/google/common/collect/Maps$KeySet<TK;TV;>;
SourceFile: "CompactHashMap.java"
NestHost: com.google.common.collect.CompactHashMap
InnerClasses:
  KeySetView = com.google.common.collect.CompactHashMap$KeySetView of com.google.common.collect.CompactHashMap
  KeySet = com.google.common.collect.Maps$KeySet of com.google.common.collect.Maps