How does a computer run a program?

The program is first loaded into RAM. The computer reads the program one instruction at a time. As instructed the CPU reads data, manipulates it and stores it back. The CPU may also interact with the monitor, printer or speakers as instructed.

Translating Programs to Machine Code

The CPU understands machine language. The machine language instructions are encoded as strings of bits. The instruction set is dependent on the type of CPU. The instructions are primitive like load, add, store, jump.

• add 3 to register D1
• store the contents of register D1 into memory location 16
• jump to the instruction sequence starting at location 208 if register C is zero.

The machine instruction that adds 3 to register D1 might be encoded as the bit string:

0101 011 000000 001

Writing and understanding machine language code is error prone. The first step was to have short mnemonics for the primitive operations instead of binary strings. The above instruction could be written as:

ADD 3, D1

This is assembly language programming. An assembler translates the mnemonics to machine code. But even the assembly language code is unwieldy. The time to write, debug, understand, and maintain a piece of code is proportional to the number of lines of instruction.

A high level language allows common operations such as expression evaluation, repetition, assignment, and conditional action to be invoked in a single high-level statement. Examples of high level languages - Fortran, Pascal, C, C++, Lisp, SmallTalk, Basic, Java.

A computer does not understand a program that is written in a higher level language. A compiler translates a high level program into machine code for that particular CPU.

Why Java?

• Simple - Java is similar to C and C++ but with fewer features. It is cleaner and more elegant.
• Object-Oriented - Java was designed to be an object-oriented language from the start. It is easier to implement the object oriented features in Java.
• Distributed - Java is designed to support applications on networks. Java supports various levels of network connectivity. With Java it is as easy to open a remote file as it is to open a local file.
• Interpreted - The Java compiler generates byte-code for the Java Virtual Machine rather than native machine code. The Java interpreter than executes the byte code.
• Robust - Java has eliminated certain features like pointers that make for unsafe programming. It is strongly typed language. There is an exception handling facility that allows the user to deal with error conditions gracefully.
• Secure - Java was designed for applications running over the Internet. There is a security mechanism in place to make difficult to write malicious code.
• Architecture Neutral - The byte code that the Java compiler generates can be executed on any platform that has the Java Virtual Machine.
• Portable - Because the byte code can run on any platform, Java programs are portable.
• High-performance - Java does not have the same type of performance as compiled C code. However, in interactive applications where time is spent in user input, Java is on par with C programs.
• Multithreaded - Java provides support for multiple threads of execution that can handle different tasks.
• Dynamic - Java can load classes as they are needed, even from across a network as the program is running.

Your first Java program

public class HelloWorld
{
  public static void main ( String args[] )
  {
    System.out.println ("Hello World!");
  }
}

Your program has to be saved in a file called HelloWorld.java. To compile the code for the Java Virtual Machine do:

javac HelloWorld.java

This will produce a file called HelloWorld.class which is the byte code for the Java Virtual Machine. To run this code do:

java HelloWorld