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

  void <init>(com.google.common.collect.ImmutableMultimap);
    descriptor: (Lcom/google/common/collect/ImmutableMultimap;)V
    flags: (0x0000) 
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // com.google.common.collect.ImmutableMultimap$Keys this
         0: .line 625
            aload 0 /* this */
            aload 1
            putfield com.google.common.collect.ImmutableMultimap$Keys.this$0:Lcom/google/common/collect/ImmutableMultimap;
            aload 0 /* this */
            invokespecial com.google.common.collect.ImmutableMultiset.<init>:()V
            return
        end local 0 // com.google.common.collect.ImmutableMultimap$Keys this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/ImmutableMultimap$Keys;
    MethodParameters:
        Name  Flags
      this$0  final

  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.ImmutableMultimap$Keys this
        start local 1 // java.lang.Object object
         0: .line 628
            aload 0 /* this */
            getfield com.google.common.collect.ImmutableMultimap$Keys.this$0:Lcom/google/common/collect/ImmutableMultimap;
            aload 1 /* object */
            invokevirtual com.google.common.collect.ImmutableMultimap.containsKey:(Ljava/lang/Object;)Z
            ireturn
        end local 1 // java.lang.Object object
        end local 0 // com.google.common.collect.ImmutableMultimap$Keys this
      LocalVariableTable:
        Start  End  Slot    Name  Signature
            0    1     0    this  Lcom/google/common/collect/ImmutableMultimap$Keys;
            0    1     1  object  Ljava/lang/Object;
    RuntimeVisibleTypeAnnotations: 
      METHOD_FORMAL_PARAMETER
        org.checkerframework.checker.nullness.qual.Nullable()
    MethodParameters:
        Name  Flags
      object  

  public int count(java.lang.Object);
    descriptor: (Ljava/lang/Object;)I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=2
        start local 0 // com.google.common.collect.ImmutableMultimap$Keys this
        start local 1 // java.lang.Object element
         0: .line 633
            aload 0 /* this */
            getfield com.google.common.collect.ImmutableMultimap$Keys.this$0:Lcom/google/common/collect/ImmutableMultimap;
            getfield com.google.common.collect.ImmutableMultimap.map:Lcom/google/common/collect/ImmutableMap;
            aload 1 /* element */
            invokevirtual com.google.common.collect.ImmutableMap.get:(Ljava/lang/Object;)Ljava/lang/Object;
            checkcast java.util.Collection
            astore 2 /* values */
        start local 2 // java.util.Collection values
         1: .line 634
            aload 2 /* values */
            ifnonnull 2
            iconst_0
            goto 3
      StackMap locals: java.util.Collection
      StackMap stack:
         2: aload 2 /* values */
            invokeinterface java.util.Collection.size:()I
      StackMap locals:
      StackMap stack: int
         3: ireturn
        end local 2 // java.util.Collection values
        end local 1 // java.lang.Object element
        end local 0 // com.google.common.collect.ImmutableMultimap$Keys this
      LocalVariableTable:
        Start  End  Slot     Name  Signature
            0    4     0     this  Lcom/google/common/collect/ImmutableMultimap$Keys;
            0    4     1  element  Ljava/lang/Object;
            1    4     2   values  Ljava/util/Collection<TV;>;
    RuntimeVisibleTypeAnnotations: 
      METHOD_FORMAL_PARAMETER
        org.checkerframework.checker.nullness.qual.Nullable()
    MethodParameters:
         Name  Flags
      element  

  public com.google.common.collect.ImmutableSet<K> elementSet();
    descriptor: ()Lcom/google/common/collect/ImmutableSet;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.ImmutableMultimap$Keys this
         0: .line 639
            aload 0 /* this */
            getfield com.google.common.collect.ImmutableMultimap$Keys.this$0:Lcom/google/common/collect/ImmutableMultimap;
            invokevirtual com.google.common.collect.ImmutableMultimap.keySet:()Lcom/google/common/collect/ImmutableSet;
            areturn
        end local 0 // com.google.common.collect.ImmutableMultimap$Keys this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/ImmutableMultimap$Keys;
    Signature: ()Lcom/google/common/collect/ImmutableSet<TK;>;

  public int size();
    descriptor: ()I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.ImmutableMultimap$Keys this
         0: .line 644
            aload 0 /* this */
            getfield com.google.common.collect.ImmutableMultimap$Keys.this$0:Lcom/google/common/collect/ImmutableMultimap;
            invokevirtual com.google.common.collect.ImmutableMultimap.size:()I
            ireturn
        end local 0 // com.google.common.collect.ImmutableMultimap$Keys this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/ImmutableMultimap$Keys;

  com.google.common.collect.Multiset$Entry<K> getEntry(int);
    descriptor: (I)Lcom/google/common/collect/Multiset$Entry;
    flags: (0x0000) 
    Code:
      stack=2, locals=3, args_size=2
        start local 0 // com.google.common.collect.ImmutableMultimap$Keys this
        start local 1 // int index
         0: .line 649
            aload 0 /* this */
            getfield com.google.common.collect.ImmutableMultimap$Keys.this$0:Lcom/google/common/collect/ImmutableMultimap;
            getfield com.google.common.collect.ImmutableMultimap.map:Lcom/google/common/collect/ImmutableMap;
            invokevirtual com.google.common.collect.ImmutableMap.entrySet:()Lcom/google/common/collect/ImmutableSet;
            invokevirtual com.google.common.collect.ImmutableSet.asList:()Lcom/google/common/collect/ImmutableList;
            iload 1 /* index */
            invokevirtual com.google.common.collect.ImmutableList.get:(I)Ljava/lang/Object;
            checkcast java.util.Map$Entry
            astore 2 /* entry */
        start local 2 // java.util.Map$Entry entry
         1: .line 650
            aload 2 /* entry */
            invokeinterface java.util.Map$Entry.getKey:()Ljava/lang/Object;
            aload 2 /* entry */
            invokeinterface java.util.Map$Entry.getValue:()Ljava/lang/Object;
            checkcast java.util.Collection
            invokeinterface java.util.Collection.size:()I
            invokestatic com.google.common.collect.Multisets.immutableEntry:(Ljava/lang/Object;I)Lcom/google/common/collect/Multiset$Entry;
            areturn
        end local 2 // java.util.Map$Entry entry
        end local 1 // int index
        end local 0 // com.google.common.collect.ImmutableMultimap$Keys this
      LocalVariableTable:
        Start  End  Slot   Name  Signature
            0    2     0   this  Lcom/google/common/collect/ImmutableMultimap$Keys;
            0    2     1  index  I
            1    2     2  entry  Ljava/util/Map$Entry<TK;+Ljava/util/Collection<TV;>;>;
    Signature: (I)Lcom/google/common/collect/Multiset$Entry<TK;>;
    MethodParameters:
       Name  Flags
      index  

  boolean isPartialView();
    descriptor: ()Z
    flags: (0x0000) 
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.ImmutableMultimap$Keys this
         0: .line 655
            iconst_1
            ireturn
        end local 0 // com.google.common.collect.ImmutableMultimap$Keys this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/ImmutableMultimap$Keys;

  java.lang.Object writeReplace();
    descriptor: ()Ljava/lang/Object;
    flags: (0x0000) 
    Code:
      stack=3, locals=1, args_size=1
        start local 0 // com.google.common.collect.ImmutableMultimap$Keys this
         0: .line 661
            new com.google.common.collect.ImmutableMultimap$KeysSerializedForm
            dup
            aload 0 /* this */
            getfield com.google.common.collect.ImmutableMultimap$Keys.this$0:Lcom/google/common/collect/ImmutableMultimap;
            invokespecial com.google.common.collect.ImmutableMultimap$KeysSerializedForm.<init>:(Lcom/google/common/collect/ImmutableMultimap;)V
            areturn
        end local 0 // com.google.common.collect.ImmutableMultimap$Keys this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/ImmutableMultimap$Keys;
    RuntimeInvisibleAnnotations: 
      com.google.common.annotations.GwtIncompatible()

  public java.util.Set elementSet();
    descriptor: ()Ljava/util/Set;
    flags: (0x1041) ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
    Code:
      stack=1, locals=1, args_size=1
         0: .line 1
            aload 0
            invokevirtual com.google.common.collect.ImmutableMultimap$Keys.elementSet:()Lcom/google/common/collect/ImmutableSet;
            areturn
      LocalVariableTable:
        Start  End  Slot  Name  Signature
}
Signature: Lcom/google/common/collect/ImmutableMultiset<TK;>;
SourceFile: "ImmutableMultimap.java"
NestHost: com.google.common.collect.ImmutableMultimap
InnerClasses:
  Keys = com.google.common.collect.ImmutableMultimap$Keys of com.google.common.collect.ImmutableMultimap
  private final KeysSerializedForm = com.google.common.collect.ImmutableMultimap$KeysSerializedForm of com.google.common.collect.ImmutableMultimap
  public abstract Entry = com.google.common.collect.Multiset$Entry of com.google.common.collect.Multiset
  public abstract Entry = java.util.Map$Entry of java.util.Map