TheCodersCorner.com items tagged with java.
Using Future and ConcurrentMap for a lazy cache
In this example I demonstrate how to use ConcurrentMap along with Future to generate a lazy cache. Concurrent maps are a good choice in some situations, where absolute atomicity can be traded off for performance, some level of control over ordering of events has been traded for performance. Let's assume for the sake of example that we have an object that...
Writing a zip file in java using ZipOutputStream.
Following on from one of our popular articles on Reading a zip file from java using ZipInputStream we've put together a new article on how to create a zip archive using Java. The below example uses ZipOutputStream to create a zip file from all the items in a directory. Zip files are written slightly differently to a normal stream in that...
Old Generation GC and cache performance?
This is a question as much as a discussion, I can’t find a lot of detail on the state of play in this area and would really welcome any feedback or corrections. Please don’t read this as a negative article as that is not how it is intended; it’s in the optimisation section but I’m not sure what impact it really...
Simple uses of CountDownLatch
CountDownLatch provides a means of waiting for a number of asynchronous events before proceeding. In order to do this one constructs a latch providing the event count. Then one thread would normally call whilst the other thread calls . Once the count reaches zero the call returns and the latch is set. If the call to happens after...
Formatting dates in Java with DateFormat
There are several ways to format dates in Java, but by far the easiest is to use DateFormat. Creating a DateFormat is very similar to NumberFormat that we saw on the previous page. Here are the static factory methods called directly on the DateFormat class: getDateInstance(..) getDateTimeInstance(..) getTimeInstance(..) There are several overloaded versions of each method above. We will not cover...
The background, string formatting in java
Over the years there have been no shortage of ways to format a string in java. What with the + operator, , , and various specialised formatters for numbers and dates we sometimes feel a little spoilt for choice. But how do they all work and what are their advantanges / disadvantages? StringBuffer - a hang up from times gone...
Using Java's XMLStreamReader PULL XML Parser
In this entry I show how to use the inbuilt Java XMLStreamReader PULL parser class to read an XML file. The XML stream libraries are PULL based XML parsers that do not load the whole document into a memory structure, so therefore are more suited to large volumes of XML. Below is an example XML file for a zoo, it contains...
Using Semaphore and CyclicBarrier in Java applications
In this example I show an usage of two more concurrent classes and ; which are both provided in the core Java library. There's a wealth of concurrency classes built directly into the JVM that can really simplify multi-threaded development. CyclicBarrrier to make threads wait for alignment. In the example below, I use a cyclic barrier to make several threads...
BlockingQueue producer consumer example
In this BlockingQueue example we show how to write a very simple producer - consumer with a blocking queue. This example generates SimpleAddition objects that require an addition of two numbers to be performed on the consumer thread. In this case the two values to be added are generated using java.util.Random's nextInt call. They are stored on the queue as a...
Watching files in Java 7 with WatchService
In this article I show how to watch files and directories for additions/changes/deletions using the new file system support in Java 7. Finally, after years of native solutions, it is now possible to listen for file changes without resorting to OS specific solutions. In the code below, first we use the new file system support class to get a ...
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