Java Performance Tuning

Java 2 Collections

The collections framework introduced with Java 2 comes with a set of collection classes in the JDK. Each class has its own performance strengths and weaknesses, which I cover here. The collection implementations use the synchronized-wrapper framework to provide synchronized classes; otherwise, the implementations are unsynchronized (except for two exceptions noted shortly). Collection classes wrapped in synchronized wrappers are always slower than unwrapped, unsynchronized classes. Nevertheless, my recommendation is generally to use objects within synchronized wrappers. You can selectively “unwrap” objects when they have been identified as part of a bottleneck and when the synchronization is not necessary. (The performance aspects of thread-safe collections are discussed in detail in Chapter 10. Synchronized wrappers are also discussed in that chapter, in Section 10.4.1.)

Table 11-1 summarizes the performance attributes of the collection classes.

Table 11-1. Performance Attributes of Java 2 Collection Classes

Interface	Class	Synchronized?
Set	HashSet	No	Fastest Set; slower than HashMap but implements the Set interface (HashMap does not)
	TreeSet	No	Slower than HashSet; provides iteration of keys in order
Map	HashMap	No	Fastest Map
	Hashtable	Yes	Slower than HashMap, but faster than synchronized HashMap
	TreeMap	No	Slower than Hashtable and HashMap; provides iteration of keys in order
List	ArrayList	No	Fastest List