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Cheap Mobile Phones from dialaphone

Here we list some Cheap phones that are available to buy on a budget. All these Cheap Mobile Phones are sold SIM Free and they will work with all UK GSM cards such as Orange, O2, T Mobile, Virgin and Vodafone - both contract and on Pay As You Go. You can also use these phones in other countries by buying a local SIM card and simply inserting it onto your cheap mobile phone. This will allow you to save a lot on roaming fees.

What are Cheap Mobile Phones or Budget Phones?

To be clear, we need to define what we mean by a Cheap Phone or Budget Phone. These are simple to use phones from all the top manufacturer. They are cheap not because there is anything wrong with them. They are all good quality mobile phones, brand new and straight from the manufacturer. All our SIM Free phones have never been locked and come with a 12 month manufacturer's warranty.

They all come with a battery, UK charger and English manual as a minimum and are boxed in the original box. These phones are cheap because they do not have all the features that thier more expensive cousins have. Many cheap mobile phones will not have a colour screen or a camera, but some do! A budget phone might have also qualified for the cheap category due to its price drop as the phone model is about to become discontinued.

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Java Virtual Machine

At the heart of the Java platform lies the Java Virtual Machine, or JVM. Most programming languages compile source code directly into machine code, suitable for execution on a particular microprocessor architecture. The difference with Java is that it uses bytecode - a special type of machine code.

Java bytecode executes on a special type of microprocessor. Strangely enough, there wasn't a hardware implementation of this microprocessor available when Java was first released. Instead, the processor architecture is emulated by what is known as a "virtual machine"। This virtual machine is an emulation of a real Java processor - a machine within a machine (Figure One)। The only difference is that the virtual machine isn't running on a CPU - it is being emulated on the CPU of the host machine।

Inside Java offers a glimpse behind the Java platform, and related technologies। This month, we examine the Java Virtual Machine, which allows Java code to be executed on a wide variety of hardware and software environments।

The Java Virtual Machine is responsible for interpreting Java bytecode, and translating this into actions or operating system calls। For example, a request to establish a socket connection to a remote machine will involve an operating system call. Different operating systems handle sockets in different ways - but the programmer doesn't need to worry about such details. It is the responsibility of the JVM to handle these translations, so that the operating system and CPU architecture on which Java software is running is completely irrelevant to the developer

JVM handles ट्रांस्लाशन्स

The Java Virtual Machine forms part of a large system, the Java Runtime Environment (JRE). Each operating system and CPU architecture requires a different JRE. The JRE comprises a set of base classes, which are an implementation of the base Java API, as well as a JVM. The portability of Java comes from implementations on a variety of CPUs and architectures. Without an available JRE for a given environment, it is impossible to run Java software.

Differences between JVM implementations

Though implementations of Java Virtual Machines are designed to be compatible, no two JVMs are exactly alike. For example, garbage collection algorithms vary between one JVM and another, so it becomes impossible to know exactly when memory will be reclaimed. The thread scheduling algorithms are different between one JVM and another (based in part on the underlying operating system), so that it is impossible to accurately predict when one thread will be executed over another.

Initially, this is a cause for concern from programmers new to the Java language. However, it actually has very little practical bearing on Java development. Such predictions are often dangerous to make, as thread scheduling and memory usage will vary between different hardware environments anyway. The power of Java comes from not being specific about the operating system and CPU architecture - to do so reduces the portability of software.

Summary

The Java Virtual Machine provides a platform-independent way of executing code, by abstracting the differences between operating systems and CPU architectures. Java Runtime Environments are available for a wide variety of hardware and software combinations, making Java a very portable language. Programmers can concentrate on writing software, without having to be concerned with how or where it will run. The idea of virtual machines is nothing new, but Java is the most widely used virtual machine used today. Thanks to the JVM, the dream of Write Once-Run Anywhere (WORA) software has become a reality.

The Java Pogramming Language

Java - an island of Indonesia, a type of coffee, and a programming language. Three very different meanings, each in varying degrees of importance. Most programmers, though, are interested in the Java programming language. In just a few short years (since late 1995), Java has taken the software community by storm. Its phenomenal success has made Java the fastest growing programming language ever. There's plenty of hype about Java, and what it can do. Many programmers, and end-users, are confused about exactly what it is, and what Java offers.

Java is a revolutionary language

The properties that make Java so attractive are present in other programming languages. Many languages are ideally suited for certain types of applications, even more so than Java. But Java brings all these properties together, in one language. This is a revolutionary jump forward for the software industry.

Let's look at some of the properties in more detail: -

• object-oriented

• portable

• multi-threaded

• automatic garbage collection

• secure

• network and "Internet" aware

• simplicity and ease-of-use

Object-oriented

Many older languages, like C and Pascal, were procedural languages. Procedures (also called functions) were blocks of code that were part of a module or application. Procedures passed parameters (primitive data types like integers, characters, strings, and floating point numbers). Code was treated separately to data. You had to pass around data structures, and procedures could easily modify their contents. This was a source of problems, as parts of a program could have unforeseen effects in other parts. Tracking down which procedure was at fault wasted a great deal of time and effort, particularly with large programs.

In some procedural language, you could even obtain the memory location of a data structure. Armed with this location, you could read and write to the data at a later time, or accidentally overwrite the contents.

Java is an object-oriented language. An object-oriented language deals with objects. Objects contain both data (member variables) and code (methods). Each object belongs to a particular class, which is a blueprint describing the member variables and methods an object offers. In Java, almost every variable is an object of some type or another - even strings. Object-oriented programming requires a different way of thinking, but is a better way to design software than procedural programming.

There are many popular object-oriented languages available today. Some like Smalltalk and Java are designed from the beginning to be object-oriented. Others, like C++, are partially object-oriented, and partially procedural. In C++, you can still overwrite the contents of data structures and objects, causing the application to crash. Thankfully, Java prohibits direct access to memory contents, leading to a more robust system.

Portable

Most programming languages are designed for a specific operating system and processor architecture. When source code (the instructions that make up a program) are compiled, it is converted to machine code which can be executed only on one type of machine. This process produces native code, which is extremely fast.

Another type of language is one that is interpreted. Interpreted code is read by a software application (the interpreter), which performs the specified actions. Interpreted code often doesn't need to be compiled - it is translated as it is run. For this reason, interpreted code is quite slow, but often portable across different operating systems and processor architectures.

Java takes the best of both techniques. Java code is compiled into a platform-neutral machine code, which is called Java bytecode. A special type of interpreter, known as a Java Virtual Machine (JVM), reads the bytecode, and processes it. Figure One shows a disassembly of a small Java application. The bytecode, indicated by the arrow, is represented in text form here, but when compiled it is represented as bytes to conserve space.

Figure One - Bytecode disassembly for "HelloWorld"

The approach Java takes offers some big advantages over other interpreted languages. Firstly, the source code is protected from view and modification - only the bytecode needs to be made available to users. Secondly, security mechanisms can scan bytecode for signs of modification or harmful code, complimenting the other security mechanisms of Java. Most of all though, it means that Java code can be compiled once, and run on any machine and operating system combination that supports a Java Virtual Machine (JVM). Java can run on Unix, Windows, Macintosh, and even the Palm Pilot. Java can even run inside a web browser, or a web server. Being portable means that the application only has to be written once - and can then execute on a wider range of machines. This saves a lot of time, and money.

Multi-threaded

If you've ever written complex applications in C, or PERL, you'll probably have come across the concept of multiple processes before. An application can split itself into separate copies, which run concurrently. Each copy replicates code and data, resulting in increased memory consumption. Getting the copies to talk together can be complex, and frustrating. Creating each process involves a call to the operating system, which consumes extra CPU time as well.

A better model is to use multiple threads of execution, referred to as threads for short. Threads can share data and code, making it easier to share data between thread instances. They also use less memory and CPU overhead. Some languages, like C++, have support for threads, but they are complex to use. Java has support for multiple threads of execution built right into the language. Threads require a different way of thinking, but can be understood very quickly. Thread support in Java is very simple to use, and the use of threads in applications and applets is quite commonplace.

Automatic garbage collection

No, we're not talking about taking out the trash (though a computer that could literally do that would be kind of neat). The term garbage collection refers to the reclamation of unused memory space. When applications create objects, the JVM allocates memory space for their storage. When the object is no longer needed (no reference to the object exists), the memory space can be reclaimed for later use.

Languages like C++ force programmers to allocate and deallocate memory for data and objects manually. This adds extra complexity, but also causes another problem - memory leaks. When programmers forget to deallocate memory, the amount of free memory available is decreased. Programs that frequently create and destroy objects may eventually find that there is no memory left. In Java, the programmer is free from such worries, as the JVM will perform automatic garbage collection of objects.

Secure

Security is a big issue with Java. Since Java applets are downloaded remotely, and executed in a browser, security is of great concern. We wouldn't want applets reading our personal documents, deleting files, or causing mischief. At the API level, there are strong security restrictions on file and network access for applets, as well as support for digital signatures to verify the integrity of downloaded code. At the bytecode level, checks are made for obvious hacks, such as stack manipulation or invalid bytecode. The strong security mechanisms in Java help to protect against inadvertent or intentional security violations, but it is important to remember that no system is perfect. The weakest link in the chain is the Java Virtual Machine on which it is run - a JVM with known security weaknesses can be prone to attack. It is also worth noting that while there have been a few identified weaknesses in JVMs, they are rare, and usually fixed quickly.

Network and "Internet" aware

Java was designed to be "Internet" aware, and to support network programming. The Java API provides extensive network support, from sockets and IP addresses, to URLs and HTTP. It's extremely easy to write network applications in Java, and the code is completely portable between platforms. In languages like C/C++, the networking code must be re-written for different operating systems, and is usually more complex. The networking support of Java saves a lot of time, and effort.

Java also includes support for more exotic network programming, such as remote-method invocation (RMI), CORBA and Jini. These distributed systems technologies make Java an attractive choice for large distributed systems.

Simplicity and ease-of-use

Java draws its roots from the C++ language. C++ is widely used, and very popular. Yet it is regarded as a complex language, with features like multiple-inheritance, templates and pointers that are counter-productive. Java, on the other hand, is closer to a "pure" object-oriented language. Access to memory pointers is removed, and object-references are used instead. Support for multiple-inheritance has been removed, which lends itself to clearer and simpler class designs. The I/O and network library is very easy to use, and the Java API provides developers with lots of time-saving code (such as networking and data-structures). After using Java for awhile, most developers are reluctant to return to other languages, because of the simplicity and elegance of Java.

Summary

Java provides developers with many advantages. While most of these are present in other languages, Java combines all of these together into one language. The rapid growth of Java has been nothing short of phenomenal, and shows no signs (yet!) of slowing down. In next month's column, I'll talk more about the heart of Java - the Java Virtual Machine

History Of Java

History of Java

At first glance, it may appear that Java was developed specifically for the world wide web। However, interestingly enough, Java was developed independently of the web, and went through several stages of metamorphosis before reaching its current status of de facto programming language for the world wide web. Below is a brief history of Java since its infancy to its current state.

Oak

According the Java FAQ, Bill Joy, currently a vice president at Sun Microsystems, is widely believed to have been the person to conceive of the idea of a programming language that later became Java. In late 1970's, Joy wanted to design a language that combined the best features of MESA and C. In an attempt to re-write the UNIX operating system in 1980's, Joy decided that C++ was inadequate for the job. A better tool was needed to write short and effective programs. It was this desire to invent a better programming tool that swayed Joy, in 1991, in the direction of Sun's "Stealth Project" - as named by Scott McNealy, Sun's president.

In January of 1991, Bill Joy, James Gosling, Mike Sheradin, Patrick Naughton (formerly the project leader of Sun's OpenWindows user environment), and several other individuals met in Aspen, Colorado for the first time to discuss the ideas for the Stealth Project. The goal of the Stealth Project was to do research in the area of application of computers in the consumer electronics market. The vision of the project was to develop "smart" consumer electronic devices that could all be centrally controlled and programmed from a handheld-remote-control-like device. According to Gosling, "the goal was ... to build a system that would let us do a large, distributed, heterogeneous network of consumer electronic devices all talking to each other." With this goal in mind, the stealth group began work. [

Members of the Stealth Project, which later became known as the Green Project, divided the tasks amongst themselves. Mike Sheradin was to focus on business development, Patrick Naughton was to begin work on the graphics system, and James Gosling was to identify the proper programming language for the project. Gosling who had joined Sun in 1984, had previously developed the commercially unsuccessful NeWS windowing system as well as GOSMACS - a C language implementation of GNU EMACS. He began with C++, but soon after was convinced that C++ was inadequate for this particular project. His extensions and modifications to C++ (also know as C++ ++ --), were the first steps towards the development of an independent language that would fit the project objectives. He named the language "Oak" while staring at an oak tree outside his office window! The name "Oak" was later dismissed due to a patent search which determined that the name was copyrighted and used for another programming language. According to Gosling, "the Java development team discovered that Oak was the name of a programming language that predated Sun's language, so another name had to be chosen."

"It's surprisingly difficult to find a good name for a programming language, as the team discovered after many hours of brainstorming. Finally, inspiration struck one day during a trip to the local coffee shop" Gosling recalls. Others have speculated that the name Java came from several individuals involved in the project: James gosling, Arthur Van hoff, Andy bechtolsheim.

There were several criteria that Oak had to meet in order to satisfy the project objective given the consumer electronics target market. Given the wide array of manufacturers in the market, Oak would have to be completely platform independent, and function seamlessly regardless of the type of CPU in the device. For this reason, Oak was designed to be an interpreted language, since it would be practically impossible for a complied version to run on all available platforms. To facilitate the job of the interpreter, Oak was to be converted to an intermediate "byte-code" format which is then passed around across the network, and executed/interpreted dynamically.

Additionally, reliability was of great concern. A consumer electronics device that would have to be "rebooted" periodically was not acceptable. Another important design objective for Oak would then have to be high reliability by allowing the least amount of programmer-introduced errors. This was the motivation for several important modification to C++. The concepts of multiple-inheritance and operator overloading were identified as sources of potential errors, and eliminated in Oak. Furthermore, in contrast to C++, Oak included implicit garbage collection thereby providing efficient memory utilization and higher reliability. Finally, Oak attempted to eliminate all unsafe constructs used in C and C++ by only providing data structures within objects.

Another essential design criterion was security. By design, Oak-based devices were to function in a network and often exchange code and information. Inherently, security is of great concern in a networked environment, especially in an environment as network dependent as the conceived Oak-based systems. For this reason, pointers were excluded in the design of Oak. This would theoretically eliminate the possibility of malicious programs accessing arbitrary addresses in memory.

If Oak were to be widely accepted and used within the consumer electronics industry, it would have to be simple and compact, so that the language could be mastered relatively easily, and development would not be excessively complex. Some would argue that Oak/Java is C++ done right, but the jury is still out on that...

In April of 1991, Ed Frank, a SPARCstation 10 architect, joined the green project. He led the project's hardware development effort. In two months, they developed the first hardware prototype known as star-seven (*7). The name *7 was somewhat demonstrative of the project's objective. *7 was the key combination to press on any telephone to answer any other ringing telephone on their network. In the meantime, Gosling was beginning work on the Oak interpreter. By August of 1991, the team had a working prototype of the user interface and graphical system which was demonstrated to Sun's co-founders Scott McNealy and Bill Joy.

Development of Oak, the green OS, the user interface, and the hardware continued through the summer of 1992. In September of that year, the *7 prototype was complete and demonstrated to McNealy and Joy. The prototype was a PDA-like (personal digital assistant) device that Gosling described as a "handheld remote control." Patrick Naughton proclaimed that "in 18 months, we did the equivalent of what 75-people organizations at Sun took three years to do -- an operating system, a language, a toolkit, an interface, a new hardware platform, ..."

While impressive, the market was not conducive to this type of technology, as later demonstrated by Apple's Newton PDA. The Green project's business planner, Mike Sheradin, and hardware designer, Ed Frank had envisioned a technology similar to that of Dolby Labs which would become the standard for the consumer electronics products.