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. 
final class com.google.common.collect.Sets$SubSet<E> extends java.util.AbstractSet<E>
  minor version: 0
  major version: 59
  flags: flags: (0x0030) ACC_FINAL, ACC_SUPER
  this_class: com.google.common.collect.Sets$SubSet
  super_class: java.util.AbstractSet
{
  private final com.google.common.collect.ImmutableMap<E, java.lang.Integer> inputSet;
    descriptor: Lcom/google/common/collect/ImmutableMap;
    flags: (0x0012) ACC_PRIVATE, ACC_FINAL
    Signature: Lcom/google/common/collect/ImmutableMap<TE;Ljava/lang/Integer;>;

  private final int mask;
    descriptor: I
    flags: (0x0012) ACC_PRIVATE, ACC_FINAL

  void <init>(com.google.common.collect.ImmutableMap<E, java.lang.Integer>, );
    descriptor: (Lcom/google/common/collect/ImmutableMap;I)V
    flags: (0x0000) 
    Code:
      stack=2, locals=3, args_size=3
        start local 0 // com.google.common.collect.Sets$SubSet this
        start local 1 // com.google.common.collect.ImmutableMap inputSet
        start local 2 // int mask
         0: .line 1498
            aload 0 /* this */
            invokespecial java.util.AbstractSet.<init>:()V
         1: .line 1499
            aload 0 /* this */
            aload 1 /* inputSet */
            putfield com.google.common.collect.Sets$SubSet.inputSet:Lcom/google/common/collect/ImmutableMap;
         2: .line 1500
            aload 0 /* this */
            iload 2 /* mask */
            putfield com.google.common.collect.Sets$SubSet.mask:I
         3: .line 1501
            return
        end local 2 // int mask
        end local 1 // com.google.common.collect.ImmutableMap inputSet
        end local 0 // com.google.common.collect.Sets$SubSet this
      LocalVariableTable:
        Start  End  Slot      Name  Signature
            0    4     0      this  Lcom/google/common/collect/Sets$SubSet<TE;>;
            0    4     1  inputSet  Lcom/google/common/collect/ImmutableMap<TE;Ljava/lang/Integer;>;
            0    4     2      mask  I
    Signature: (Lcom/google/common/collect/ImmutableMap<TE;Ljava/lang/Integer;>;I)V
    MethodParameters:
          Name  Flags
      inputSet  
      mask      

  public java.util.Iterator<E> iterator();
    descriptor: ()Ljava/util/Iterator;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=3, locals=1, args_size=1
        start local 0 // com.google.common.collect.Sets$SubSet this
         0: .line 1505
            new com.google.common.collect.Sets$SubSet$1
            dup
            aload 0 /* this */
            invokespecial com.google.common.collect.Sets$SubSet$1.<init>:(Lcom/google/common/collect/Sets$SubSet;)V
            areturn
        end local 0 // com.google.common.collect.Sets$SubSet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Sets$SubSet<TE;>;
    Signature: ()Ljava/util/Iterator<TE;>;

  public int size();
    descriptor: ()I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.Sets$SubSet this
         0: .line 1528
            aload 0 /* this */
            getfield com.google.common.collect.Sets$SubSet.mask:I
            invokestatic java.lang.Integer.bitCount:(I)I
            ireturn
        end local 0 // com.google.common.collect.Sets$SubSet this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Sets$SubSet<TE;>;

  public boolean contains(java.lang.Object);
    descriptor: (Ljava/lang/Object;)Z
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=3, locals=3, args_size=2
        start local 0 // com.google.common.collect.Sets$SubSet this
        start local 1 // java.lang.Object o
         0: .line 1533
            aload 0 /* this */
            getfield com.google.common.collect.Sets$SubSet.inputSet:Lcom/google/common/collect/ImmutableMap;
            aload 1 /* o */
            invokevirtual com.google.common.collect.ImmutableMap.get:(Ljava/lang/Object;)Ljava/lang/Object;
            checkcast java.lang.Integer
            astore 2 /* index */
        start local 2 // java.lang.Integer index
         1: .line 1534
            aload 2 /* index */
            ifnull 2
            aload 0 /* this */
            getfield com.google.common.collect.Sets$SubSet.mask:I
            iconst_1
            aload 2 /* index */
            invokevirtual java.lang.Integer.intValue:()I
            ishl
            iand
            ifeq 2
            iconst_1
            ireturn
      StackMap locals: java.lang.Integer
      StackMap stack:
         2: iconst_0
            ireturn
        end local 2 // java.lang.Integer index
        end local 1 // java.lang.Object o
        end local 0 // com.google.common.collect.Sets$SubSet this
      LocalVariableTable:
        Start  End  Slot   Name  Signature
            0    3     0   this  Lcom/google/common/collect/Sets$SubSet<TE;>;
            0    3     1      o  Ljava/lang/Object;
            1    3     2  index  Ljava/lang/Integer;
    RuntimeVisibleTypeAnnotations: 
      METHOD_FORMAL_PARAMETER
        org.checkerframework.checker.nullness.qual.Nullable()
    MethodParameters:
      Name  Flags
      o     
}
Signature: <E:Ljava/lang/Object;>Ljava/util/AbstractSet<TE;>;
SourceFile: "Sets.java"
NestHost: com.google.common.collect.Sets
InnerClasses:
  private final SubSet = com.google.common.collect.Sets$SubSet of com.google.common.collect.Sets
  com.google.common.collect.Sets$SubSet$1