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Introduction to Java What is Java? • Java is an OOP (Object Oriented Programming) language developed by Sun Microsystems in 1990. • It is widely used for the development of Internet (and other distributed) software. What is Java? • Its features include neutrality (i.e. it doesn’t depend on a specific type of machine), security, robustness and multi-threading. • It offers extensive application programmer interfaces (APIs) including support for graphical user interfaces (GUIs), utilities, database access, networking etc. What can you do with Java? Amongst many other things, you can • Produce stand-alone applications which run directly on your computer. • Produce applets on a world-wide web server and embed them in your web pages to be run in a browser such as Microsoft Internet Explorer. • Produce servlets on a world-wide web server. Unlike applets, these will run on the web server machine; typically these are used to communicate with web databases. Creating simple Java applications This involves the following steps: • Writing the Java source code - saved (by you) as Filename.java in a suitable folder. You can use any editor, even NotePad, for this purpose. • Compiling the Java source code using the javac command in a command shell to create Filename.class in the same folder. • Running the program using the command java Filename. (N.B. not java Filename.class) Example – Hello World • Create a folder J:\java\Hello • Using NotePad or EditPlus, type the following: // this is my first java program class Hello1 { public static void main(String[] args) { System.out.println("Hello world"); } } NB the first line is a Java comment Example – Hello World • Save this as Hello1.java in J:\java\Hello • Note that: – Java is case sensitive. For example Hello1 is not the same as hello1. – Spaces are significant. For example Hello 1 is not the same as Hello1. – The file name must be the same as the class name (class Hello1, file Hello1.java here) A Java program consists of a number of classes, which contain methods, which contain statements Here we have a single class // this is my first java program class Hello1 { public static void main(String[] args) { System.out.println("Hello world"); } } This class has a single method // this is my first java program class Hello1 { public static void main(String[] args) { System.out.println("Hello world"); } } This method has a single statement // this is my first java program class Hello1 { public static void main(String[] args) { System.out.println("Hello world"); } } This statement prints Hello world in the command shell window Compiling Hello1.java • Open up a command shell • Move to the folder where you saved Hello1.java • Type the command javac Hello1.java If you get error messages, go back to Notepad and correct the typing errors, save and try again Running Hello1 • If / when you don’t get any error messages, the java compiler has created the bytecode file Hello1.class • Type command java Hello1 to run your application. Note that you must NOT include the .class extension here (See next slide) Hello world with input In Notepad or EditPlus, change the program: // this is my second java program class Hello2 { public static void main(String[] args) { String name; name = args[0]; System.out.println("Hello " + name); } } and (important!) save it as Hello2.java. Compile and run it. 1 class Hello2 2 { 3 public static void main(String[] args) 4 { 5 String name; 6 name = args[0]; 7 System.out.println("Hello " + name); 8 } 9 } At line 5 we have introduced a String variable name. Variables hold values (in this case a String like "Don" or "Kate") within a program, and the values of the variables can change as the program runs. 1 class Hello2 2 { 3 public static void main(String[] args) 4 { 5 String name; 6 name = args[0]; 7 System.out.println("Hello " + name); 8 } 9 } Line 6 is an assignment statement which takes the first command line argument args[0] ("Don" or "Kate" or whatever the user typed) and puts it into the variable name on the left hand side. 1 class Hello2 2 { 3 public static void main(String[] args) 4 { 5 String name; 6 name = args[0]; 7 System.out.println("Hello " + name); 8 } 9 } At line 7 the expression "Hello " + name ‘glues’ (or concatenates) String "Hello " and whatever is in the name variable. This is then printed out in the command shell window, so if name has the value "Don" the output is Hello Don Using EditPlus • NotePad is fine for simple programs but provides absolutely no support for Java. • There are many Java development environments like NetBeans for serious Java programming but these are quite complicated. • EditPlus provides quite a lot of help for the Java programmer and is as easy to use as NotePad. • See the notes for an explanation of how to use EditPlus