Java Looking at our first console application in Eclipse Copyright © 2004-2015 Curt Hill

A moment on paradigms Java is well suited for either the console or GUI paradigm A console program views the keyboard/screen as nearly a single device We both read and write to it using strings A GUI program has a WIMP interface: Windows Input Menus Pointer The console paradigm is easier for starters, but we will do both Copyright © 2004-2015 Curt Hill

Eclipse Eclipse is the Interactive Development Environment that we will use Originally developed by IBM Donated to public domain Now maintained by the Eclipse Foundation www.eclipse.org Copyright © 2004-2015 Curt Hill

Terminology for Eclipse Resource Things such as projects, directories and files Editor A window pane that allows changes to a resource, often Java code View A window pane that shows a resource Perspective A group of editors and views Copyright © 2004-2015 Curt Hill

Eclipse Construction steps Create new project Create new class Customize as needed Debug and run Copyright © 2004-2015 Curt Hill

Create new project Click File|New|Java Project Use the New Project dialog to set some initial parameters Specify the project name This will become a directory tree The project is an XML file of name: .project in the directory The screen shots follow Copyright © 2004-2015 Curt Hill

Create New Project Copyright © 2004-2015 Curt Hill

Enter Project Name Copyright © 2004-2015 Curt Hill

A usually skipped screen Copyright © 2004-2015 Curt Hill

Projects and Classes A project is just a tool to manage pieces We now have to add the pieces Each Java program is a class Any class might use other classes as well What we now do is add the main or root class to our project Copyright © 2004-2015 Curt Hill

Create new class Click File|New|Class This generates a new class This new class will be your program Use java.lang.Object as the ancestor class Specify a main method Click on the box Copyright © 2004-2015 Curt Hill

Add an initial class Copyright © 2004-2015 Curt Hill

Name the class Copyright © 2004-2015 Curt Hill

Specified names In the prior screen there were three things that were specified: Package Class name Inclusion of the main method via checkbox The package name is usually all lower case The class name usually starts with a capital letter and each subsequent word is capitalized: FirstCon Copyright © 2004-2015 Curt Hill

Generated Code Copyright © 2004-2015 Curt Hill

Customize as needed At this point we have a Java console program with proper form, which does nothing Now add the desired functionality Insert into main: System.out.println(“Hello world”); Java is case sensitive so System must start with a capital That doesn’t sound like much work It will get worse Copyright © 2004-2015 Curt Hill

Customized Copyright © 2004-2015 Curt Hill

Debug and run Click the run button or use menu Run|Run Run time properties are displayed Specify the new class name Search may be easiest Console output will appear in the bottom pane First the command line Next the program output Copyright © 2004-2015 Curt Hill

Start Run Copyright © 2004-2015 Curt Hill

Run Copyright © 2004-2015 Curt Hill

Code package firstcon; public class FirstCon { /** * @param args */ public static void main (String[] args) { // TODO Auto-generated method stub System.out.println("Hello World"); } } Copyright © 2004-2015 Curt Hill

Comments A comment explains things for people /* and */ // Ignored by Java /* and */ Enclose multiline comments // Makes the rest of the line a comment Documentation uses special comments Program: Javadoc Copyright © 2004-2015 Curt Hill

Class Declaration public class FirstCon { Objects in Java are called Classes A class binds together variables and methods This class is named FirstCon Its visibility is public A private class in this case could not be executed Copyright © 2004-2015 Curt Hill

Classes and files A file is the compilation unit It must have an extension .java Each file must have one and only one public class The file name must match the public class name Even case, which may be a problem on MS Operating Systems Most of these issues are handled by Eclipse without our intervention Copyright © 2004-2015 Curt Hill

Compilation Units Public class names must match file name Other private or friendly classes may be present in the file Each class will have its own object (.CLASS) file after it is compiled Copyright © 2004-2015 Curt Hill

Main Function Most classes have both variables and methods Every application must have a method called main, which is where execution starts Main’s parameter is a string array Command line parameters Often, in a console program most of the work occurs in the main method Copyright © 2004-2015 Curt Hill

Main function public static void main(String args[ ]) { System.out.println("Hello World!"); } // end of main void indicates main returns nothing It is public Accessible to system Copyright © 2004-2015 Curt Hill

Main function It is static This main function has just one statement A static function has limited access to class data Can be called without having a class instance This main function has just one statement We will cover static more later Copyright © 2004-2015 Curt Hill

Writing to the console System.out.println("Hello world"); System is an instance of a class that contains numerous predefined classes and methods out is an output stream class with methods print and println Copyright © 2004-2015 Curt Hill

print and println Overloaded method names Accept one parameter There is a version for almost every parameter type Copyright © 2004-2015 Curt Hill

Classes and objects Sometimes these two terms are used interchangably A class is a type Defines possible values and actions An object is an instance of a class Example A person name would be a class “Curt Hill” would be an object, an instance of a class Copyright © 2004-2015 Curt Hill

Finally There are some other details that need to be considered: Handing in programs These will be covered in a subsequent presentation Copyright © 2004-2015 Curt Hill