Introduction to Java

Virtual Machine

a software emulation of a physical computer that runs programs in an isolated environment

VMs handle memory allocation and garbage collection, reducing the risk of memory-related errors 

execution within a VM provides a layer of isolation

Virtual Machine Based Languages

Java (JVM), C# (CLR), Python (Cpython, Jython, IronPython), Kotlin, JS, Ruby…

instead of directly running on hardware, VM-based languages run on a software-based virtual environment

code written in VM-based languages can run on any system with the appropriate VM installed 

Java

A modern, platform independent, object-oriented programming language

• Developed by SUN microsystems in 1995 (James Gosling). Java is now owned by Oracle

• Overarching design goal: simplicity and reuse

• Originally designed for embedded devices… the first envisioned application was the Java toaster

• Next it was the web language of choice (applets) 

• Since evolved into a programming language of choice for high reliability and good performance. Now commonplace in: 

  • Enterprise Systems (IBM/Oracle)

  • Mobile Devices (Android) 

  • Embedded Devices (SmartTVs, IoT) 

  • Financial, Medical, and Automotive domains

Open

Java is a modern, platform independent, object-oriented programming language

• Open standards allow interoperation and promote innovation

Abstract

Java is a modern, platform independent, object-oriented programming language

• Java uses a virtual machine to abstract over device operating systems

Java Virtual Machine (JVM)

Abstracts over a device hardware

• Processor

• Memory

• Input / Output 

• Graphical Interfaces…

Contains a virtual computer processor

Virtual Computer Processor

• Executes its own machine language known as bytecode

• Very simple instructions, such as add, multiply, compare (c.f. machine language) 

• Java programs are compiled into bytecode by the developer

• The JVM interprets these into whatever the hardware understands at run time

• Java bytecode is an example of an intermediate language (neither something you write in, nor something that is directly executed by a computer)

Java Machine Language - Bytecode

C++ vs Java

• I ran a Bubble Sort algorithm on an array of 13 integers…

• Results shown in nanoseconds (1000 nsec = 0.001 msec)

  • Java: 4279 nsec

  • C++: 666 nsec (6 times faster)

Language

based on the syntax of C and C++

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

Classes

• Every Java program is made up of one or more

  • Java’s unit of modularity… they define objects 

• The class keyword defines a unique name for the class you’re writing. Curly braces define the code that is part of that class

• One class per file, with filename matching as the class name

public class HelloWorld
{

}

Methods

• The main method defines the start point for your program

  • This method always returns void in Java (they have no return value)

  • Parameter is an array of strings (command line arguments, like C)

• Methods are like functions in C

  • They are blocks of code that have names, parameters and return types 

public static void main(String[] args)
{
	//say what you want it to do
}

Comments

A single line comment begins with //

• All text on the line after the // characters are ignored

• Java also supports the C style /* */ comment blocks

//la la la

Statements

• Method invocation (like calling a function in C) statements end with a semicolon, as they do in C and C++

• For example: 

  • System.out is the name of an object (the console output stream) 

  • println is the name of a method – note there’s also a print method

System.out.println("Hello World");

Code Blocks

• The end of the method and end of the class (respectively)

• As with C, it's very important to maintain good code indentation

• In Java, methods are always defined inside classes

{
	{
	
	}
}

Compiling a Java Program

• Create a text file with your favourite editor (e.g. VS Code)

  • Filename must match the name of the class it contains

  • Filename must end in .java

• Open a command line prompt 

  • shell in Unix 

  • cmd in Windows 

• Change directory to the location of your file, then compile your program into bytecode:

  • javac HelloWorld.java 

• Now start a Java virtual machine that interprets your program: 

  • java HelloWorld