I will begin our blog on Java Tutorial with an incredibly important aspect of java development:  memory management.  The importance of this topic should not be minimized as an application's performance and footprint size are at stake.

From the outset, the Java Virtual Machine (JVM) manages memory via a mechanism known as Garbage Collection (GC).  The Garbage collector

• Manages the heap memory.   All obects are stored on the heap; therefore, all objects are managed.  The keyword, new, allocates the requisite memory to instantiate an object and places the newly allocated memory on the heap.  This object is marked as live until it is no longer being reference.
• Deallocates or reclaims those objects that are no longer being referened. 
• Traditionally, employs a Mark and Sweep algorithm.  In the mark phase, the collector identifies which objects are still alive.  The sweep phase identifies objects that are no longer alive.
• Deallocates the memory of objects that are not marked as live.
• Is automatically run by the JVM and not explicitely called by the Java developer.  Unlike languages such as C++, the Java developer has no explict control over memory management.
• Does not manage the stack.  Local primitive types and local object references are not managed by the GC.

So if the Java developer has no control over memory management, why even worry about the GC?  It turns out that memory management is an integral part of an application's performance, all things being equal.  The more memory that is required for the application to run, the greater the likelihood that computational efficiency suffers. To that end, the developer has to take into account the amount of memory being allocated when writing code.  This translates into the amount of heap memory being consumed.

Memory is split into two types:  stack and heap.  Stack memory is memory set aside for a thread of execution e.g. a function.  When a function is called, a block of memory is reserved for those variables local to the function, provided that they are either a type of Java primitive or an object reference.  Upon runtime completion of the function call, the reserved memory block is now available for the next thread of execution.  Heap memory, on the otherhand, is dynamically allocated.  That is, there is no set pattern for allocating or deallocating this memory.  Therefore, keeping track or managing this type of memory is a complicated process. In Java, such memory is allocated when instantiating an object:

String s = new String();  // new operator being employed
String m = "A String";    /* object instantiated by the JVM and then being set to a value.  The JVM
calls the new operator */

Checking to see if a file exists is a two step process in Python. Simply import the module shown below and invoke the isfile function:

import os.path
os.path.isfile(fname)

The field of information technology is in many ways perfectly suited for entrepreneurship. Many highly successful enterprises started with a lone IT professional venturing out on their own and starting up their own company. If you have computer science skills and want to explore alternative options outside the corporate arena you should seriously consider going into business for yourself. Businesses may be more willing to hire you as a contractor rather than as a full-time worker. There are certain IT jobs that are perfect for individuals who want to be self-employed, they include:

• Working as a Consultant
Large IT departments are not as necessary for corporations as they were at the start of the internet era; this is partly due to the trend towards cloud computing. Consultants are often brought in to handle the need for tech expertise when companies downsize or eliminate their IT departments. A consultant may work for several different clients at the same time, be on call for various disciplines or be commissioned for specific projects.

• Web Entrepreneurship
The ease of building a website and the fact that web hosting is relatively affordable means that it does not take a lot of know-how to start your own online empire. You can sell products or services, or start your own online community. Another option is to start selling goods via auction sites or on sites that sell advertising space. You will need an understanding of marketing and of search engine optimization so that you can draw visitors to your site.

• Programming Apps for Mobile Devices
The future of the Internet is in mobile devices. Statistics show that much of the world will be using mobile devices and smart phones to handle their surfing needs in the near future. If you have the skills to program the apps used on these devices, you could be among those riding the wave of this trend.

It is not impossible to start an Information Technology company with very little startup capital. Getting it off the ground in terms of online visibility will require focus to detail, knowing your target market, a consistent campaign to build a client list and a solid reputation.

Below are two pdf's which discuss and outline the Java Message Service, JMS, tutorial.

One Hour Java Messaging Service Tutorial
Advanced One Hour Java Messaging Service Tutorial