BNWF / JAVA / 01
Java for teens
Objects first, then whole programs.
Open the syllabusRunning main
How to run a program, with arguments, in BlueJ
BlueJ does not have a single Run button for main, which confuses students coming from other editors. The FAQ explains the BlueJ way.
- Compile the class. The FAQ notes that gray diagonal stripes across a class box disappear once it is compiled, so stripes mean it still needs compiling.
- Right-click the class box in the class diagram. Static methods, including main, appear in the menu.
- Choose void main(String[] args). A dialog asks for the array of strings main takes.
- Leave it as { } for no arguments, which is the default, an empty array.
- Or type arguments as an array, for example { "one", "two", "three" }, which passes three strings exactly as a command line would.
- Watch the terminal. Output appears there; tick Clear screen at method call in the terminal’s Options menu if old output gets in the way.
Why BlueJ works this way
BlueJ is designed around objects rather than whole programs. You can create an object from any compiled class and call its methods one at a time, without writing a main method at all, and inspect the values of its fields as you go.
For learning what a constructor does, or how two objects share data, that interactive object bench is genuinely useful, and it is the main reason teachers choose BlueJ for a first course.
Libraries and the standard classes
If your class gives you a jar file, the FAQ lists three ways to add it: the Libraries tab in Preferences, the userlib folder inside the BlueJ installation, or a folder named +libs inside your project.
To experiment with a standard class such as String or java.awt.Point, the Tools menu has Use Library Class, which lets you create objects from it directly on the bench.
