https://developers.google.com/protocol-buffers/protobuf-java/: Core Protocol Buffers library. Protocol Buffers are a way of encoding structured data in an efficient yet extensible format. 
class com.google.protobuf.Internal$MapAdapter$SetAdapter extends java.util.AbstractSet<java.util.Map$Entry<K, V>>
  minor version: 0
  major version: 59
  flags: flags: (0x0020) ACC_SUPER
  this_class: com.google.protobuf.Internal$MapAdapter$SetAdapter
  super_class: java.util.AbstractSet
{
  private final java.util.Set<java.util.Map$Entry<K, RealValue>> realSet;
    descriptor: Ljava/util/Set;
    flags: (0x0012) ACC_PRIVATE, ACC_FINAL
    Signature: Ljava/util/Set<Ljava/util/Map$Entry<TK;TRealValue;>;>;

  final com.google.protobuf.Internal$MapAdapter this$1;
    descriptor: Lcom/google/protobuf/Internal$MapAdapter;
    flags: (0x1010) ACC_FINAL, ACC_SYNTHETIC

  public void <init>(java.util.Set<java.util.Map$Entry<K, RealValue>>);
    descriptor: (Lcom/google/protobuf/Internal$MapAdapter;Ljava/util/Set;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=3
        start local 0 // com.google.protobuf.Internal$MapAdapter$SetAdapter this
        start local 2 // java.util.Set realSet
         0: .line 480
            aload 0 /* this */
            aload 1
            putfield com.google.protobuf.Internal$MapAdapter$SetAdapter.this$1:Lcom/google/protobuf/Internal$MapAdapter;
            aload 0 /* this */
            invokespecial java.util.AbstractSet.<init>:()V
         1: .line 481
            aload 0 /* this */
            aload 2 /* realSet */
            putfield com.google.protobuf.Internal$MapAdapter$SetAdapter.realSet:Ljava/util/Set;
         2: .line 482
            return
        end local 2 // java.util.Set realSet
        end local 0 // com.google.protobuf.Internal$MapAdapter$SetAdapter this
      LocalVariableTable:
        Start  End  Slot     Name  Signature
            0    3     0     this  Lcom/google/protobuf/Internal$MapAdapter$SetAdapter;
            0    3     2  realSet  Ljava/util/Set<Ljava/util/Map$Entry<TK;TRealValue;>;>;
    Signature: (Ljava/util/Set<Ljava/util/Map$Entry<TK;TRealValue;>;>;)V
    MethodParameters:
         Name  Flags
      this$0   final
      realSet  

  public java.util.Iterator<java.util.Map$Entry<K, V>> iterator();
    descriptor: ()Ljava/util/Iterator;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=4, locals=1, args_size=1
        start local 0 // com.google.protobuf.Internal$MapAdapter$SetAdapter this
         0: .line 486
            new com.google.protobuf.Internal$MapAdapter$IteratorAdapter
            dup
            aload 0 /* this */
            getfield com.google.protobuf.Internal$MapAdapter$SetAdapter.this$1:Lcom/google/protobuf/Internal$MapAdapter;
            aload 0 /* this */
            getfield com.google.protobuf.Internal$MapAdapter$SetAdapter.realSet:Ljava/util/Set;
            invokeinterface java.util.Set.iterator:()Ljava/util/Iterator;
            invokespecial com.google.protobuf.Internal$MapAdapter$IteratorAdapter.<init>:(Lcom/google/protobuf/Internal$MapAdapter;Ljava/util/Iterator;)V
            areturn
        end local 0 // com.google.protobuf.Internal$MapAdapter$SetAdapter this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/protobuf/Internal$MapAdapter$SetAdapter;
    Signature: ()Ljava/util/Iterator<Ljava/util/Map$Entry<TK;TV;>;>;

  public int size();
    descriptor: ()I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.protobuf.Internal$MapAdapter$SetAdapter this
         0: .line 491
            aload 0 /* this */
            getfield com.google.protobuf.Internal$MapAdapter$SetAdapter.realSet:Ljava/util/Set;
            invokeinterface java.util.Set.size:()I
            ireturn
        end local 0 // com.google.protobuf.Internal$MapAdapter$SetAdapter this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/protobuf/Internal$MapAdapter$SetAdapter;
}
Signature: Ljava/util/AbstractSet<Ljava/util/Map$Entry<TK;TV;>;>;
SourceFile: "Internal.java"
NestHost: com.google.protobuf.Internal
InnerClasses:
  public MapAdapter = com.google.protobuf.Internal$MapAdapter of com.google.protobuf.Internal
  private IteratorAdapter = com.google.protobuf.Internal$MapAdapter$IteratorAdapter of com.google.protobuf.Internal$MapAdapter
  private SetAdapter = com.google.protobuf.Internal$MapAdapter$SetAdapter of com.google.protobuf.Internal$MapAdapter
  public abstract Entry = java.util.Map$Entry of java.util.Map