HashMap log-debugged PDF Print E-mail
Written by Charles   
Thursday, 08 July 2010 22:03

The link below is to a version of Java's HashMap class with logback logging made initially for educational purposes, although this could be used during testing code to examine the Map internals.

logback is the latest version of what most people will know as log4j and is even more flexible than its predecessor. In the jar HERE, which is a fat jar (a single jar containing all dependencies needed to run the code) is another jar which holds the logback logging configuration file, logback.xml, which is the logback equivalent of log4j.xml.
By default, there are no fewer than five separate loggers in addition to the root logger:

 

<logger name="l_hashing" additivity="false">
    <level value="debug" />
    <appender-ref ref="STDOUT" />
</logger>

<logger name="l_find" additivity="false">
    <level value="debug" />
    <appender-ref ref="STDOUT" />
</logger>

<logger name="l_put" additivity="false">
    <level value="debug" />
    <appender-ref ref="STDOUT" />
</logger>

<logger name="l_get" additivity="false">
    <level value="debug" />
    <appender-ref ref="STDOUT" />
</logger>

<logger name="l_structure" additivity="false">
    <level value="debug" />
    <appender-ref ref="STDOUT" />
</logger>


These give control over various categories of operation inside the Map and you can turn them on and off by changing the level.
But before that can be done, the logback config file needs to be extracted from the fat jar:


 jar xvf hashmap-debug.jar lib/config.jar
 jar xvf  lib/config.jar logback.xml

You can now edit the config file as you see fit. In order for it to be used by the application, you need to ensure that it get activated using something like:

java -Dlogback.configurationFile=/tmp/logback.xml -jar hashmap-debug.jar

I found that the best results came from using an absolute location to the config file

The following is a small application to put the logged HashMap through its paces:

package test;

import static java.lang.System.out;
import net.proteanit.demo.util.HashMap;

public class MapTest {
        public static void main(String[] args) {
                HashMap<String, Integer> dbgMap = new HashMap<String, Integer>();

                for (int i = 0; i < 20; i++) {
                        // Generate random two-letter string
                        String s = net.proteanit.util.StringUtils.randomString(2);
                        dbgMap.put(s, i);
                }
                
                dbgMap.put("A", 100);
                // Dump Map to show entries
                dbgMap.dumpMap();
                // Calculate the degree of linearity in the Map
                out.printf("Map has a linearity of %.3f\n", dbgMap.getLinearity());
                out.println(dbgMap.get("A").toString());
                // Various ops to get the loggers working
                out.println(dbgMap.containsKey("A"));
                
                out.println(dbgMap.containsValue(2));

        }
}


As you can see, there are two additional methods: dumpMap and getLinearity.
The former shows the buckets of the map, which consist of 0 to n Entry objects. The best case scenario is that there be only one Entry in a bucket, giving a lookup order of O(1). Owing to the structure of the map and the implementation of the hashCode method of its key class however, there could be several Entry instances in the same bucket. A lookup operation might now encounter a degree of linearity, detracting from O(1), as entries (implemented as a linked list) need to be iterated to find the correct value. It's this linearity that getLinearity attempts to measure, by taking into account the number of multiple-entry buckets.
The source code is contained in /src in the jar.
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