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$PermutationCollection<E> extends java.util.AbstractCollection<java.util.List<E>>
  minor version: 0
  major version: 59
  flags: flags: (0x0030) ACC_FINAL, ACC_SUPER
  this_class: com.google.common.collect.Collections2$PermutationCollection
  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;>;

  void <init>(com.google.common.collect.ImmutableList<E>);
    descriptor: (Lcom/google/common/collect/ImmutableList;)V
    flags: (0x0000) 
    Code:
      stack=2, locals=2, args_size=2
        start local 0 // com.google.common.collect.Collections2$PermutationCollection this
        start local 1 // com.google.common.collect.ImmutableList input
         0: .line 588
            aload 0 /* this */
            invokespecial java.util.AbstractCollection.<init>:()V
         1: .line 589
            aload 0 /* this */
            aload 1 /* input */
            putfield com.google.common.collect.Collections2$PermutationCollection.inputList:Lcom/google/common/collect/ImmutableList;
         2: .line 590
            return
        end local 1 // com.google.common.collect.ImmutableList input
        end local 0 // com.google.common.collect.Collections2$PermutationCollection this
      LocalVariableTable:
        Start  End  Slot   Name  Signature
            0    3     0   this  Lcom/google/common/collect/Collections2$PermutationCollection<TE;>;
            0    3     1  input  Lcom/google/common/collect/ImmutableList<TE;>;
    Signature: (Lcom/google/common/collect/ImmutableList<TE;>;)V
    MethodParameters:
       Name  Flags
      input  

  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$PermutationCollection this
         0: .line 594
            aload 0 /* this */
            getfield com.google.common.collect.Collections2$PermutationCollection.inputList:Lcom/google/common/collect/ImmutableList;
            invokevirtual com.google.common.collect.ImmutableList.size:()I
            invokestatic com.google.common.math.IntMath.factorial:(I)I
            ireturn
        end local 0 // com.google.common.collect.Collections2$PermutationCollection this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Collections2$PermutationCollection<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$PermutationCollection this
         0: .line 599
            iconst_0
            ireturn
        end local 0 // com.google.common.collect.Collections2$PermutationCollection this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Collections2$PermutationCollection<TE;>;

  public java.util.Iterator<java.util.List<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.Collections2$PermutationCollection this
         0: .line 604
            new com.google.common.collect.Collections2$PermutationIterator
            dup
            aload 0 /* this */
            getfield com.google.common.collect.Collections2$PermutationCollection.inputList:Lcom/google/common/collect/ImmutableList;
            invokespecial com.google.common.collect.Collections2$PermutationIterator.<init>:(Ljava/util/List;)V
            areturn
        end local 0 // com.google.common.collect.Collections2$PermutationCollection this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Collections2$PermutationCollection<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$PermutationCollection this
        start local 1 // java.lang.Object obj
         0: .line 609
            aload 1 /* obj */
            instanceof java.util.List
            ifeq 3
         1: .line 610
            aload 1 /* obj */
            checkcast java.util.List
            astore 2 /* list */
        start local 2 // java.util.List list
         2: .line 611
            aload 0 /* this */
            getfield com.google.common.collect.Collections2$PermutationCollection.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 613
      StackMap locals:
      StackMap stack:
            iconst_0
            ireturn
        end local 1 // java.lang.Object obj
        end local 0 // com.google.common.collect.Collections2$PermutationCollection this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    4     0  this  Lcom/google/common/collect/Collections2$PermutationCollection<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$PermutationCollection this
         0: .line 618
            new java.lang.StringBuilder
            dup
            ldc "permutations("
            invokespecial java.lang.StringBuilder.<init>:(Ljava/lang/String;)V
            aload 0 /* this */
            getfield com.google.common.collect.Collections2$PermutationCollection.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$PermutationCollection this
      LocalVariableTable:
        Start  End  Slot  Name  Signature
            0    1     0  this  Lcom/google/common/collect/Collections2$PermutationCollection<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 PermutationCollection = com.google.common.collect.Collections2$PermutationCollection of com.google.common.collect.Collections2
  private PermutationIterator = com.google.common.collect.Collections2$PermutationIterator of com.google.common.collect.Collections2