How to display Window in the center of the screen

In Java, many of the swings applications by default opens the window at the top left corner of the screen. In addition to setting the size and making it visible, we also need to set the coordinates where the new window has to be displayed. To show the window in the center of the given screen/monitor, I have written an example explained below.

This example automatically takes the current screen resolution and depending on the size (of the window) provided, it adjusts the start point of the screen.

Parsing Multiple XML Tags

How to parse XML with different multiple tags? For instance, I have a root tag element and few child tags with same name. For below is the snippet for the XML I will be providing parsers for

Multiple XML Tags

multiple-tags.xml

<?xml version="1.0" encoding="UTF-8"?>
<rootTag Name="Root">
 <ChildOne activityType="Task">
  <Name>P1</Name>
  <Incoming>0</Incoming>
  <Outgoing>P1P2</Outgoing>
 </ChildOne>
 <ChildOne activityType="Task">
  <Name>P2</Name>
  <Incoming>P1P2</Incoming>
  <Outgoing>P2G1</Outgoing>
 </ChildOne>
 <ChildOne activityType="GatewayParallel">
  <Name>G1</Name>
  <Incoming>P2G1</Incoming>
  <Outgoing>G1P3 G1P4</Outgoing>
 </ChildOne>
 <ChildTwo type="bpmn:SequenceEdge">
  <Name>P1P2</Name>
  <Source>P1</Source>
  <Destination>P2</Destination>
  <Path>P1P2</Path>
 </ChildTwo>
 <ChildTwo type="bpmn:SequenceEdge">
  <Name>P2G1</Name>
  <Source>P2</Source>
  <Destination>G1</Destination>
  <Path></Path>
 </ChildTwo>
 <ChildTwo type="bpmn:SequenceEdge">
  <Name>G1P3</Name>
  <Source>G1</Source>
  <Destination>P3</Destination>
  <Path>P2P3</Path>
 </ChildTwo>
</rootTag>

Difference in two Calendar Dates

There are many ways to calculate the difference between two Calendar dates in Year, Months, weeks or days by simply dividing them with a fixed constant. DateUtil.java below has a method for getting the difference in Days. this method can similarly be modified to get in Weeks, Months or Years.

But some times the challenge is to display the difference in two java.util.Calendar  or java.util.Date in more user friendly manner. Like in format X Years, X months, X weeks, X days. Where X is positive number greater than zero.

Example: If the number of Full Years < 1, the format should be X months, X weeks, X days. If the number of Full Years >= 1 but the number of Full Months < 1, the format should be X Years, X weeks, X days and so on. Values of 0 is not written. Like "2 months, 2 days" rather than "2 months, 0 weeks, 2 days"

How to read file without locking in Java

In java, we can read or write to a file without acquiring locks to it. These are mostly used for log files. Recently I was tying to mimic the tail command (of Linux) in java and wrote a simple program below.

Using java.io.RandomAccesFile we can read/write to a file without obtaining locks. Let me demonstrate with example step by step how it works.

RandomAccessFile accessFile = new RandomAccessFile(file, "r");

Create an instance of the RandomAccessFile by either passing java.io.File or a path of a file. The second constructor argument is the type of access needed. In this case it is read only "r". If you want to make it read/write pass "rw". Like Buffers of java.nio package, this class also works with the pointers. it reads the bytes and move the pointer as it progresses. The current pointer gives the information about how much of the file have bean read or has to be read.

public void print(OutputStream stream) throws IOException {
    long length = accessFile.length();
    while (accessFile.getFilePointer() < length) {
        stream.write(accessFile.read());
    }
    stream.flush();
}

Parsing complex nested XML with SAXParser and collections

In my previous example, I have explained how to parse simple XML with on and multiple elements or objects. This is beyond that. Have you ever encountered and XML which have nested elements and the nested elements again have same nested objects and the tree grows. I happen to encounter such huge XML of size in few MBs. Sometimes collections like java.util.Stack or java.util.Map are helpful and very useful in this case. While parsing, you have to add and object as a child to another object, but it becomes difficult to determine to which parent object the child has to be added. The image below shows a basic tree structure of the XML file in this demo example.

XML parsing with SAX Parser Demo

Java provides two ways of parsing XML. Namely SAXParser and DOM Parser. With DOM Parser complete XML document object is held in emory and uses the DOM approach with DOM elements. This approach is used when you need to get hold of elements from XML in any order. But using DOM parser is costly in term of memory. When you have a XML document and want to read it once and create in memory java object (POJO), go for SAXParser approach. SAXParser parses the document(XML) from top to bottom, tag by tag.
Using Sax parser is simple. All we need to do is create a Handler class which extends DefaultHandler class of org.xml.sax.helpers package (part of JDK). In this Handler class, we define how tags are processed and stored into in memory object.
Once the Handler is developed, Create a Parser class which uses SAXParser abstract class from javax.xml.parsers package (also part of JDK). Use this Parser class and parse the document with help of the Handler class which we created. Current example demonstrate simple XML, for parsing nested XML with hierarchy, read a another article as how to parse nested XML.
Below is a snippet of simple XML file which needs to be parsed.