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.Collections2$OrderedPermutationCollection<E> extends java.util.AbstractCollection<java.util.List<E>>
  minor version: 0
  major version: 59
  flags: flags: (0x0030) ACC_FINAL, ACC_SUPER
  this_class: com.google.common.collect.Collections2$OrderedPermutationCollection
  super_class: java.util.AbstractCollection
{
  final com.google.common.collect.ImmutableList<E> inputList;
    descriptor: Lcom/google/common/collect/ImmutableList;
    flags: (0x0010) ACC_FINAL
    Signature: Lcom/google/common/collect/ImmutableList<TE;>;

  final java.util.Comparator<? super E> comparator;
    descriptor: Ljava/util/Comparator;
    flags: (0x0010) ACC_FINAL
    Signature: Ljava/util/Comparator<-TE;>;

  final int size;
    descriptor: I
    flags: (0x0010) ACC_FINAL

  void <init>(java.lang.Iterable<E>, java.util.Comparator<? super E>);
    descriptor: (Ljava/lang/Iterable;Ljava/util/Comparator;)V
    flags: (0x0000) 
    Code:
      stack=3, locals=3, args_size=3
        start local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
        start local 1 // java.lang.Iterable input
        start local 2 // java.util.Comparator comparator
         0: .line 445
            aload 0 /* this */
            invokespecial java.util.AbstractCollection.<init>:()V
         1: .line 446
            aload 0 /* this */
            aload 2 /* comparator */
            aload 1 /* input */
            invokestatic com.google.common.collect.ImmutableList.sortedCopyOf:(Ljava/util/Comparator;Ljava/lang/Iterable;)Lcom/google/common/collect/ImmutableList;
            putfield com.google.common.collect.Collections2$OrderedPermutationCollection.inputList:Lcom/google/common/collect/ImmutableList;
         2: .line 447
            aload 0 /* this */
            aload 2 /* comparator */
            putfield com.google.common.collect.Collections2$OrderedPermutationCollection.comparator:Ljava/util/Comparator;
         3: .line 448
            aload 0 /* this */
            aload 0 /* this */
            getfield com.google.common.collect.Collections2$OrderedPermutationCollection.inputList:Lcom/google/common/collect/ImmutableList;
            aload 2 /* comparator */
            invokestatic com.google.common.collect.Collections2$OrderedPermutationCollection.calculateSize:(Ljava/util/List;Ljava/util/Comparator;)I
            putfield com.google.common.collect.Collections2$OrderedPermutationCollection.size:I
         4: .line 449
            return
        end local 2 // java.util.Comparator comparator
        end local 1 // java.lang.Iterable input
        end local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
      LocalVariableTable:
        Start  End  Slot        Name  Signature
            0    5     0        this  Lcom/google/common/collect/Collections2$OrderedPermutationCollection<TE;>;
            0    5     1       input  Ljava/lang/Iterable<TE;>;
            0    5     2  comparator  Ljava/util/Comparator<-TE;>;
    Signature: (Ljava/lang/Iterable<TE;>;Ljava/util/Comparator<-TE;>;)V
    MethodParameters:
            Name  Flags
      input       
      comparator  

  private static <E> int calculateSize(java.util.List<E>, java.util.Comparator<? super E>);
    descriptor: (Ljava/util/List;Ljava/util/Comparator;)I
    flags: (0x000a) ACC_PRIVATE, ACC_STATIC
    Code:
      stack=4, locals=6, args_size=2
        start local 0 // java.util.List sortedInputList
        start local 1 // java.util.Comparator comparator
         0: .line 462
            iconst_1
            istore 2 /* permutations */
        start local 2 // int permutations
         1: .line 463
            iconst_1
            istore 3 /* n */
        start local 3 // int n
         2: .line 464
            iconst_1
            istore 4 /* r */
        start local 4 // int r
         3: .line 465
            goto 12
         4: .line 466
      StackMap locals: int int int
      StackMap stack:
            aload 1 /* comparator */
            aload 0 /* sortedInputList */
            iload 3 /* n */
            iconst_1
            isub
            invokeinterface java.util.List.get:(I)Ljava/lang/Object;
            aload 0 /* sortedInputList */
            iload 3 /* n */
            invokeinterface java.util.List.get:(I)Ljava/lang/Object;
            invokeinterface java.util.Comparator.compare:(Ljava/lang/Object;Ljava/lang/Object;)I
            istore 5 /* comparison */
        start local 5 // int comparison
         5: .line 467
            iload 5 /* comparison */
            ifge 10
         6: .line 469
            iload 2 /* permutations */
            iload 3 /* n */
            iload 4 /* r */
            invokestatic com.google.common.math.IntMath.binomial:(II)I
            invokestatic com.google.common.math.IntMath.saturatedMultiply:(II)I
            istore 2 /* permutations */
         7: .line 470
            iconst_0
            istore 4 /* r */
         8: .line 471
            iload 2 /* permutations */
            ldc 2147483647
            if_icmpne 10
         9: .line 472
            ldc 2147483647
            ireturn
        10: .line 475
      StackMap locals: int
      StackMap stack:
            iinc 3 /* n */ 1
        11: .line 476
            iinc 4 /* r */ 1
        end local 5 // int comparison
        12: .line 465
      StackMap locals:
      StackMap stack:
            iload 3 /* n */
            aload 0 /* sortedInputList */
            invokeinterface java.util.List.size:()I
            if_icmplt 4
        13: .line 478
            iload 2 /* permutations */
            iload 3 /* n */
            iload 4 /* r */
            invokestatic com.google.common.math.IntMath.binomial:(II)I
            invokestatic com.google.common.math.IntMath.saturatedMultiply:(II)I
            ireturn
        end local 4 // int r
        end local 3 // int n
        end local 2 // int permutations
        end local 1 // java.util.Comparator comparator
        end local 0 // java.util.List sortedInputList
      LocalVariableTable:
        Start  End  Slot             Name  Signature
            0   14     0  sortedInputList  Ljava/util/List<TE;>;
            0   14     1       comparator  Ljava/util/Comparator<-TE;>;
            1   14     2     permutations  I
            2   14     3                n  I
            3   14     4                r  I
            5   12     5       comparison  I
    Signature: <E:Ljava/lang/Object;>(Ljava/util/List<TE;>;Ljava/util/Comparator<-TE;>;)I
    MethodParameters:
                 Name  Flags
      sortedInputList  
      comparator       

  public int size();
    descriptor: ()I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
         0: .line 483
            aload 0 /* this */
            getfield com.google.common.collect.Collections2$OrderedPermutationCollection.size:I
            ireturn
        end local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Collections2$OrderedPermutationCollection<TE;>;

  public boolean isEmpty();
    descriptor: ()Z
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
        start local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
         0: .line 488
            iconst_0
            ireturn
        end local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Collections2$OrderedPermutationCollection<TE;>;

  public java.util.Iterator<java.util.List<E>> iterator();
    descriptor: ()Ljava/util/Iterator;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=4, locals=1, args_size=1
        start local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
         0: .line 493
            new com.google.common.collect.Collections2$OrderedPermutationIterator
            dup
            aload 0 /* this */
            getfield com.google.common.collect.Collections2$OrderedPermutationCollection.inputList:Lcom/google/common/collect/ImmutableList;
            aload 0 /* this */
            getfield com.google.common.collect.Collections2$OrderedPermutationCollection.comparator:Ljava/util/Comparator;
            invokespecial com.google.common.collect.Collections2$OrderedPermutationIterator.<init>:(Ljava/util/List;Ljava/util/Comparator;)V
            areturn
        end local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Collections2$OrderedPermutationCollection<TE;>;
    Signature: ()Ljava/util/Iterator<Ljava/util/List<TE;>;>;

  public boolean contains(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.Collections2$OrderedPermutationCollection this
        start local 1 // java.lang.Object obj
         0: .line 498
            aload 1 /* obj */
            instanceof java.util.List
            ifeq 3
         1: .line 499
            aload 1 /* obj */
            checkcast java.util.List
            astore 2 /* list */
        start local 2 // java.util.List list
         2: .line 500
            aload 0 /* this */
            getfield com.google.common.collect.Collections2$OrderedPermutationCollection.inputList:Lcom/google/common/collect/ImmutableList;
            aload 2 /* list */
            invokestatic com.google.common.collect.Collections2.isPermutation:(Ljava/util/List;Ljava/util/List;)Z
            ireturn
        end local 2 // java.util.List list
         3: .line 502
      StackMap locals:
      StackMap stack:
            iconst_0
            ireturn
        end local 1 // java.lang.Object obj
        end local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    4     0  this  Lcom/google/common/collect/Collections2$OrderedPermutationCollection<TE;>;
            0    4     1   obj  Ljava/lang/Object;
            2    3     2  list  Ljava/util/List<*>;
    RuntimeVisibleTypeAnnotations: 
      METHOD_FORMAL_PARAMETER
        org.checkerframework.checker.nullness.qual.Nullable()
    MethodParameters:
      Name  Flags
      obj   

  public java.lang.String toString();
    descriptor: ()Ljava/lang/String;
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=3, locals=1, args_size=1
        start local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
         0: .line 507
            new java.lang.StringBuilder
            dup
            ldc "orderedPermutationCollection("
            invokespecial java.lang.StringBuilder.<init>:(Ljava/lang/String;)V
            aload 0 /* this */
            getfield com.google.common.collect.Collections2$OrderedPermutationCollection.inputList:Lcom/google/common/collect/ImmutableList;
            invokevirtual java.lang.StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
            ldc ")"
            invokevirtual java.lang.StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
            invokevirtual java.lang.StringBuilder.toString:()Ljava/lang/String;
            areturn
        end local 0 // com.google.common.collect.Collections2$OrderedPermutationCollection this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Collections2$OrderedPermutationCollection<TE;>;
}
Signature: <E:Ljava/lang/Object;>Ljava/util/AbstractCollection<Ljava/util/List<TE;>;>;
SourceFile: "Collections2.java"
NestHost: com.google.common.collect.Collections2
InnerClasses:
  private final OrderedPermutationCollection = com.google.common.collect.Collections2$OrderedPermutationCollection of com.google.common.collect.Collections2
  private final OrderedPermutationIterator = com.google.common.collect.Collections2$OrderedPermutationIterator of com.google.common.collect.Collections2