Dane Cameron - A Software Engineer Learns Java and Object Orientated Programming

Programming languages can be classified in many ways; oneform of classification is whether they are compiled or interpreted.

In order for any software to run on a computer it must beconverted into machine code: the sequence of 0s and 1s representing a set ofinstructions that can be comprehended by a computer processor. A compiledlanguage is one where the source code is converted into machine code by a toolcalled a compiler, and packaged as a program that can be distributed directlyto end-users.

Because each operating system is different, compilers needto be written for each operating system. The beauty of compilers, however, isthat the same source code can be used to create executable programs fordifferent operating systems. Examples of compiled languages are C and C++.

Interpreted languages also need their source code to beconverted to machine code in order to be run on a specific operating system,but with interpreted languages this happens in real-time as the program executes.This means that the same code can be executed on any operating systemsupporting an interpreter for that language (which is the software thatconverts the source code into machine code in real time).

Interpreted languages are generally slower than compiledlanguages, because the compilation step is effectively happening while theprogram executes. Examples of languages that can be run by interpreters areJavaScript and Perl. Many interpreted languages can also be compiled, however,therefore the line between interpreted and compiled languages is sometimesblurred.

Java does not easily fit into either of these categories.Java is a compiled language: all source code must be compiled before it can beexecuted. As you will see, the Java Development Kit (JDK) contains a utilityprogram called javac that isresponsible for compiling Java source code.

Unlike traditional compilers however, the Java compiler doesnot produce machine code for a specific operating system. Instead, it producesJava bytecode.

Java bytecode does contain a set of instructions, just likemachine code. Unlike machine code, however, Java bytecode cannot be executeddirectly by a computer. Java requires an intermediary called the Java VirtualMachine which is provided by the Java Runtime Environment (JRE).

Any computer that wishes to run a program written in Javamust have a JRE installed, and this will perform the task of converting theJava bytecode into machine code for the specific operating system as theprogram executes.

If you use Windows or OSX you have probably noticed that youare often requested to install a new version of Java: this is the Java RuntimeEnvironment. Once Java is installed you can run any application written inJava.

You may be wondering why Java uses this approach? Javabytecode can be thought of as a halfway house between source code and machinecode. It contains a set of instructions that can be executed by an interpretermore efficiently than regular source code.

Java chose this approach as part of its write-once,run-anywhere philosophy. The same compiled version of the program can beexecuted on any computer that has a JRE installed: it is not necessary to builddifferent versions of the program for each platform. This has been immenselyvaluable, because JREs are available not just for desktop computers, but formobile phones, set-top boxes, and home appliances. In fact, Oracle claims some3 billion devices run Java.

This approach was also intended to make the language moresecure. Traditional compiled programs can do anything on the operating systemthat the user running them can do including deleting files, or sendingthem over the Internet to a hacker. The JRE by comparison provides a sandboxthat programs run inside of, and it is possible to control what the program cando inside this sandbox, or make it request the ability to perform certainoperations. This approach has become standard with Android and iOS in recentyears, but was built into Java from the very beginning.