Maximum Performance with WebSphere Application Server V5.1 on iSeries

Chapter 9. Java and WebSphere application design 293

Table 9-3 Java 2 Collection Framework

When you use these collections, remember these points:

 The Collection interface is a group of objects, with duplicates allowed.

 Set extends the Collection but forbids duplicates.

 List extends Collection as well, but allows duplicates, and introduces positional indexing.

To choose the correct Collection, follow these guidelines:

 Sets

– Sets are slow in general.

– Usually Set is the fastest set.

– TreeSet is the slower set but provides iteration of keys in order.

 Lists

– Usually ArrayList is the fastest list.

– If the list is large and elements are frequently deleted or inserted, LinkedList is faster.

– Stack has the same speed as Vector and provides last in, first out (LIFO) queue

functionality.

 Maps

– Usually HashMap is the fastest map.

– TreeMap is the slower Map but provides iteration of keys in order.

One final tip regarding performance is to use iterators whenever you can to access the

elements of a list, rather than using positional access.

9.1.9 Java Native Interface

Native method invocation on an iSeries server may not perform as well as native method

invocation on other platforms. Java on the iSeries server has been optimized by moving the

JVM below the machine interface (MI). Native method invocation requires a call to above MI

code and may require expensive Java Native Interface (JNI) calls back into the JVM. Native

methods should not carry out small routines, which you can easily write in Java. Only use

native methods to start system functions that are relatively long running and are not available

directly in Java.

Interfaces

Implementations

Hash table Resizable array Balance tree Linked list

Set HashSet TreeSet

List ArrayList LinkedList

Map HashMap TreeMap

Attention: Use care when using Collections because they may become the beginning of

memory leaks issues. If you add elements to a Collection and never delete these

references, this becomes a memory leak. Imagine an application with a Collection variable

in the servlet to insert the customers that login the application. After a while, you have a list

of all the customers that logged in the application, but if you never delete these customers

from the Collection, it will grow endlessly.