collections

Performance Characteristics

The previous explanations have made it clear that different collection types have different performance characteristics. That’s often the primary reason for picking one collection type over another. You can see the performance characteristics of some common operations on collections summarized in the following two tables.

Performance characteristics of sequence types:

  head tail apply update prepend append insert
immutable              
List C C L L C L -
Stream C C L L C L -
Vector eC eC eC eC eC eC -
Stack C C L L C C L
Queue aC aC L L L C -
Range C C C - - - -
String C L C L L L -
mutable              
ArrayBuffer C L C C L aC L
ListBuffer C L L L C C L
StringBuilder C L C C L aC L
MutableList C L L L C C L
Queue C L L L C C L
ArraySeq C L C C - - -
Stack C L L L C L L
ArrayStack C L C C aC L L
Array C L C C - - -

Performance characteristics of set and map types:

  lookup add remove min
immutable        
HashSet/HashMap eC eC eC L
TreeSet/TreeMap Log Log Log Log
BitSet C L L eC1
ListMap L L L L
mutable        
HashSet/HashMap eC eC eC L
WeakHashMap eC eC eC L
BitSet C aC C eC1
TreeSet Log Log Log Log

Footnote: 1 Assuming bits are densely packed.

The entries in these two tables are explained as follows:

   
C The operation takes (fast) constant time.
eC The operation takes effectively constant time, but this might depend on some assumptions such as maximum length of a vector or distribution of hash keys.
aC The operation takes amortized constant time. Some invocations of the operation might take longer, but if many operations are performed on average only constant time per operation is taken.
Log The operation takes time proportional to the logarithm of the collection size.
L The operation is linear, that is it takes time proportional to the collection size.
- The operation is not supported.

The first table treats sequence types–both immutable and mutable–with the following operations:

   
head Selecting the first element of the sequence.
tail Producing a new sequence that consists of all elements except the first one.
apply Indexing.
update Functional update (with updated) for immutable sequences, side-effecting update (with update for mutable sequences.
prepend Adding an element to the front of the sequence. For immutable sequences, this produces a new sequence. For mutable sequences it modified the existing sequence.
append Adding an element and the end of the sequence. For immutable sequences, this produces a new sequence. For mutable sequences it modified the existing sequence.
insert Inserting an element at an arbitrary position in the sequence. This is only supported directly for mutable sequences.

The second table treats mutable and immutable sets and maps with the following operations:

   
lookup Testing whether an element is contained in set, or selecting a value associated with a key.
add Adding a new element to a set or key/value pair to a map.
remove Removing an element from a set or a key from a map.
min The smallest element of the set, or the smallest key of a map.
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