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$4 extends com.google.common.collect.ForwardingSet<E>
  minor version: 0
  major version: 59
  flags: flags: (0x0020) ACC_SUPER
  this_class: com.google.common.collect.Maps$4
  super_class: com.google.common.collect.ForwardingSet
{
  private final java.util.Set val$set;
    descriptor: Ljava/util/Set;
    flags: (0x1012) ACC_PRIVATE, ACC_FINAL, ACC_SYNTHETIC

  void <init>(java.util.Set);
    descriptor: (Ljava/util/Set;)V
    flags: (0x0000) 
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // com.google.common.collect.Maps$4 this
         0: .line 1109
            aload 0 /* this */
            aload 1
            putfield com.google.common.collect.Maps$4.val$set:Ljava/util/Set;
            aload 0 /* this */
            invokespecial com.google.common.collect.ForwardingSet.<init>:()V
            return
        end local 0 // com.google.common.collect.Maps$4 this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Maps$4;
    MethodParameters:
         Name  Flags
      val$set  final

  protected java.util.Set<E> delegate();
    descriptor: ()Ljava/util/Set;
    flags: (0x0004) ACC_PROTECTED
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.Maps$4 this
         0: .line 1112
            aload 0 /* this */
            getfield com.google.common.collect.Maps$4.val$set:Ljava/util/Set;
            areturn
        end local 0 // com.google.common.collect.Maps$4 this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Maps$4;
    Signature: ()Ljava/util/Set<TE;>;

  public boolean add();
    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$4 this
        start local 1 // java.lang.Object element
         0: .line 1117
            new java.lang.UnsupportedOperationException
            dup
            invokespecial java.lang.UnsupportedOperationException.<init>:()V
            athrow
        end local 1 // java.lang.Object element
        end local 0 // com.google.common.collect.Maps$4 this
      LocalVariableTable:
        Start  End  Slot     Name  Signature
            0    1     0     this  Lcom/google/common/collect/Maps$4;
            0    1     1  element  TE;
    Signature: (TE;)Z
    MethodParameters:
         Name  Flags
      element  

  public boolean addAll(java.util.Collection<? extends E>);
    descriptor: (Ljava/util/Collection;)Z
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // com.google.common.collect.Maps$4 this
        start local 1 // java.util.Collection es
         0: .line 1122
            new java.lang.UnsupportedOperationException
            dup
            invokespecial java.lang.UnsupportedOperationException.<init>:()V
            athrow
        end local 1 // java.util.Collection es
        end local 0 // com.google.common.collect.Maps$4 this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Maps$4;
            0    1     1    es  Ljava/util/Collection<+TE;>;
    Signature: (Ljava/util/Collection<+TE;>;)Z
    MethodParameters:
      Name  Flags
      es    

  protected java.lang.Object delegate();
    descriptor: ()Ljava/lang/Object;
    flags: (0x1044) ACC_PROTECTED, ACC_BRIDGE, ACC_SYNTHETIC
    Code:
      stack=1, locals=1, args_size=1
         0: .line 1
            aload 0
            invokevirtual com.google.common.collect.Maps$4.delegate:()Ljava/util/Set;
            areturn
      LocalVariableTable:
        Start  End  Slot  Name  Signature

  protected java.util.Collection delegate();
    descriptor: ()Ljava/util/Collection;
    flags: (0x1044) ACC_PROTECTED, ACC_BRIDGE, ACC_SYNTHETIC
    Code:
      stack=1, locals=1, args_size=1
         0: .line 1
            aload 0
            invokevirtual com.google.common.collect.Maps$4.delegate:()Ljava/util/Set;
            areturn
      LocalVariableTable:
        Start  End  Slot  Name  Signature
}
Signature: Lcom/google/common/collect/ForwardingSet<TE;>;
SourceFile: "Maps.java"
EnclosingMethod: com.google.common.collect.Maps.removeOnlySet:(Ljava/util/Set;)Ljava/util/Set;
NestHost: com.google.common.collect.Maps
InnerClasses:
  com.google.common.collect.Maps$4