A programming guide: A simple “Hello World” program in BlueJ

1. Click Start>All Programs>BlueJ>BlueJ.
The BlueJ Environment will then come into view.

2. Click the Project menu>New project. In the Look in Listbox, select My documents> Enter “Example” (no quotes) in the Folder Name textbox then click Create.

3. Click the New Class button>Type “Hello” (no quotes) in the Class Name textbox> Ensure that Class is selected in the class type radio buttons then click OK.

A class icon with a caption “Hello” will then appear in the BlueJ workarea.

4.Double click the Hello class icon, the following source code window will then appear:

Before creating our “Hello World” program, let us first dissect the pre-made code bit by bit. If you noticed, I’ve enabled line numbers by clicking Options>Preferences>Display line numbers, for discussion purposes.

Line 8: Is the class declaration section. It simply tells the BlueJ compiler the name of our class and to create class file based on it.
Line 9: Indicates the beginning of our class.
Line 10-12: Is the instance variable declaration section. This is where we declare the variables that will be needed in our application.
Line 16-20: Is the variable initialization section. This is where we initialize the values of our variables.
Line 28-32: Is the method declaration section. This is where we specify the actions that our application is capable of performing.
Line 34: Simply specifies the end of our class.

5. Now that we’ve understood (somehow) Blue’s code structure, let us now make our hello world program. Go to the instance variable declaration section (Line 10-12) and change “private int x;” to “private String strtext;” no quotes. The reason why we are declaring a string variable instead of the default integer variable is because we will be storing text i.e. “Hello World” to this variable in the variable initialization section.

6. Go to the variable initialization section (Line 16 to 20) and assign a “Hello World” value to our previously declared variable. This can be done by erasing ‘x=0;’ and changing it to ‘strtext=”Hello World”;’.

7. Go to the method declaration section (Line 28-32). Since we will not be accepting a numeric value from other methods nor we will be passing numeric value, change :

8. Your code will now look like this:

/**
* Write a description of class Hello here.
* 
* @author (your name) 
* @version (a version number or a date)
*/
public class Hello
{
// instance variables - replace the example below with your own
private String strtext;

/**
* Constructor for objects of class Hello
*/
public Hello()
{
// initialise instance variables
strtext="Hello World";
}

/**
* An example of a method - replace this comment with your own
* 
* @param  y   a sample parameter for a method
* @return     the sum of x and y 
*/
public void sampleMethod()
{
//clears the previous outputs
System.out.println("\u000c");
//retrieves the value of the strtext variable and display it on the screen.
System.out.println(strtext);
}
}

9. Click the compile button to convert your Bluej source code into bytecode.

10. Click the Close Button.

11. Create an Object based on your Hello class by right-clicking the Hello class icon>new Hello.