1. Java Looking at our first console application in Eclipse
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2. 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
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3. 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
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4. 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
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5. Eclipse Construction steps
Create new project
Create new class
Customize as needed
Debug and run
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6. 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
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7. Create New Project
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8. Enter Project Name
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9. A usually skipped screen
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10. 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
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11. 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
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12. Add an initial class
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13. Name the class
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14. 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
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15. Generated Code
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16. 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
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17. Customized
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18. 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
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19. Start Run
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20. Run
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21. Code
package firstcon; public class FirstCon { /** args */ public static void main (String[] args) { // TODO Auto-generated method stub System.out.println("Hello World"); } }
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22. 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
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23. 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
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24. 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
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25. 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
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26. 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
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27. 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
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28. 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
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29. 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
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30. print and println Overloaded method names Accept one parameter
There is a version for almost every parameter type
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31. 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
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32. Finally There are some other details that need to be considered:
Handing in programs
These will be covered in a subsequent presentation
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