Introduction to Java Tonga Institute of Higher Education

Programming Basics Program – A set of instructions that a computer uses to do something. Programming / Develop – The act of creating or changing a program Programmer / Developer – A person who makes a program Run / Execute – The act of using a program Every program was created by someone Computers use special languages Programmers use special languages to create or change a program

Machine Language Machine language – A language understood by computers When programs are run, machine language is used Machine languages are almost impossible for humans to understand Every operating system (OS) has its own machine language  Windows  Linux  Macintosh Hardware Machine Language High Level Language C++JavaVB.NET

High Level Languages High Level Language - A programming language that is understandable by people This enables a programmer to write programs High level languages must be translated into machine language before running on a computer Hardware Machine Language High Level Language C++JavaVB.NET

Compilers and Interpreters There are two main ways to change programs written in a high level language to machine language: 1. Use a compiler 2. Use an interpreter Source Code – Code written in a programming language by a developer.

Compilers Compiler – A program that transforms code from one format to another Source code Linux compiler Windows compiler Macintosh compiler Linux code Macintosh code Windows code Machine LanguageHigh Level Language Windows Linux Macintosh

Compilers vs. Interpreters Advantages  Programs run faster Disadvantages  Platform dependent - Programs only work on a specific operating system. Windows Linux Macintosh  Compiling a large program may take a long time

Interpreters Interpreter – A program that translates and executes code Usually, interpreters translate and execute source code. Source Code Interpreter High Level Language Code for Windows, Linux and Macintosh Machine Language Windows, Linux and Macintosh

Interpreters vs. Compilers Advantages  Platform Independent.  Don’t need to compile anything. Disadvantages  The interpreter program must be installed on the computer that the program will run on  Slower execution  You should only use services available on all platforms. Example: Windows has a cool sound library, but you can’t use it because it won’t run on Macintosh

Review Give a short description of:  Machine Language  High Level Language Give a short explanation of how an compiler is different from an interpreter

Introduction to Java Java is a programming language created by Sun Microsystems.  Java.sun.com Many programmers use Java Java is an object oriented language.

Java Setup Steps to install Java  Required 1. Install the Java Software Development Kit (SDK)  Find it at  Java 2 Standard Edition  Not Required but good 1. Install Java Documentation 2. Install Integrated Development Environment (IDE)  IDE – A program that helps you to write code

Demonstration Sun Website Java Documentation Java IDE

How Does Java Work? Java uses a compiler and interpreter! You can compile the program once and run it on each platform with an interpreter Source code Java Compiler Byte Code Windows Macintosh Linux Java Interpreter Windows Linux Macintosh

How to Create a Java Program 1. Create a source file. 2. Use the Java compiler to create byte code. 3. Run the byte code using the Java interpreter.

Create a Source File Write Java source code and save it in a file Use a text editor or Integrated Development Environment (IDE). Filename extension:.java

Demonstration Create a source file

Use the Java Compiler to Create Byte Code Source code is compiled by a Java compiler to byte code. Byte Code  A file that can run on any computer that has a Java Interpreter  Filename extension:.class Compiler program: javac.exe Java Compiler Javac.exe Source Code void main () …??this$0?!... Byte Code.java.class

Use the Java Compiler to Create Byte Code Important Details  Javac.exe.java Ex: Javac HelloWorld.java  A.class file is made for every class in the file. Common Errors  Unable to find the Compiler - Something is wrong with your Java setup. Windows can’t find the Java compiler. Is your access to Javac.exe and Java.exe setup properly?  Invalid Flag – One of the inputs are wrong. Did you include the.java extension? Are you using the right file?

Demonstration Compiling Source Code

Run the Byte Code Using the Java Interpreter Java Virtual Machine (JVM) – A program which interprets Java programs that have been compiled into byte-code and usually stored in a ".class" file. Interpreter performs actions  Creates windows  Prints  Etc. Interpreter program: java.exe Java Interpreter Java.exe Byte Code ??this$0?!....class

Run the Byte Code Using the Java Interpreter Important Details  Java.exe Ex: Java HelloWorld  The class name must have the correct case! Common Errors  NoClassDefFoundError – The class can’t be found. Is your class in the correct directory? Are you using the correct case?

Demonstration Running Byte Code