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Faster List Iteration with RandomAccess Interface
Pages: 1, 2

Times shown are the average of three runs, and all times have been normalized to the first table cell; i.e., the time taken by the ArrayList to iterate the list using the List.get() method, using java -client.



Table 1: Access times for loop types and access methods

Loop type (loop test) and access method ArrayList
java -client
LinkedList
java -client
ArrayList
java -server
LinkedList
java -server
loop counter (i<n) and list.get() 100% too long 77.5% too long
iterator (Iterator.hasNext()) and Iterator.next() 141% 219% 109% 213%
iterator (i<n) and iterator.next() 121% 205% 98% 193%
RandomAccess test with loop from row 1 or 3 100% 205% 77.5% 193%

Note that HotSpot is capable of optimizing away accesses that are unnecessary, so the test accessed and operated on the list elements in a way that could not eliminate the list element access. The test code is available here.

The most important results are in the last two rows of the table. The last row shows the times obtained by making full use of the RandomAccess interface; the row before that shows the most optimal general technique for iterating lists, if RandomAccess were not available. The size of the lists I used for the test (and consequently, the number of loop iterations required to access every element) was sufficiently large that the instanceof test had no measurable cost in comparison to the time taken to run the loop. Consequently, we can see that that there was no cost (but also no benefit) in adding the instanceof RandomAccess test when iterating the LinkedList; whereas the ArrayList was iterated more than 20% quicker when the instanceof test was included.

Forward and backward compatibility

What should you do if you are implementing code now? Obviously, you can start developing with a 1.4 (beta) release, but this is not an option everywhere. There are three aspects to using RandomAccess if you are developing code now:

  1. You may want to include code referencing RandomAccess without moving to 1.4; many development environments cannot be upgraded rapidly or to a beta release.
  2. Many projects need their code to be able to run in any JVM, so the code needs to be backwards-compatible to run in JVMs using releases earlier than 1.4, where RandomAccess does not exist.
  3. You will want to make your code forward-compatible so that it will automatically take advantage of RandomAccess when running in a 1.4+ JVM.

Making RandomAccess available to your development environment is the first issue, and this can be as simple as adding the RandomAccess interface to your classpath. Any version of the SDK can create the RandomAccess interface. The definition for RandomAccess is

package java.util;
public interface RandomAccess {}

This interface can be created using javac, as follows:

  1. Create a directory called temp
  2. In temp, create a directory called java
  3. In java, create a directory called util
  4. In util, create a file called RandomAccess.java, containing the definition just given
  5. Compile RandomAccess.java, using javac: javac RandomAccess.java

Now including temp in your classpath should enable classes that refer to RandomAccess to be compiled.

Some Java integrated development environments (IDEs) can make it difficult to add a class to the core SDK packages. If this is the case for your IDE, your only hope is that it accepts an external classpath for compilation purposes, in which case the custom-generated RandomAccess class will need to be held in that external classpath.

We also need to handle RandomAccess in the runtime environment. For pre-1.4 environments, the test if (listObject instanceof RandomAccess) will generate a NoClassDefFoundError at runtime, when the JVM tries to load the RandomAccess class. For the instanceof test to be evaluated, the class has to be loaded; however, we can guard the test so that it is only executed if RandomAccess is available. The simplest way to do this is to check if RandomAccess exists, setting a boolean guard as the outcome of that test:

static boolean RandomAccessExists;
..

  //execute this as early as possible after the 
  //application starts
  try
  {
    Class c =  Class.forName("java.util.RandomAccess");
    RandomAccessExists = true;
  }
  catch (ClassNotFoundException e)
  {
    RandomAccessExists = false;
  }

Then, finally, we will need to change our instanceof tests to use the RandomAccessExists variable as a guard:

if (RandomAccessExists && (listObject instanceof RandomAccess) )

Now we have the solution for all three aspects mentioned at the beginning of this section:

  1. The RandomAccess interface can be created and compiled easily with any SDK, and this manually-compiled version can be used at compilation time as a stand-in, to compile any code which refers to RandomAccess.
  2. The guarded instanceof test will automatically revert to the Iterator loop if RandomAccess does not exist, and should avoid throwing a NoClassDefFoundError in pre-1.4 JVMs.
  3. The guarded instanceof test will also automatically use the faster loop branch when RandomAccess does exist and the list object implements it.

Further Resources

Jack Shirazi is the author of Java Performance Tuning. He was an early adopter of Java, and for the last few years has consulted mainly for the financial sector, focusing on Java performance.


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