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	L	L
Queue	aC	aC	L	L	C	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 modifies 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 modifies 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.