Turtlegrafik

Simple drawing (Sequence)

The simplest programs have of a sequence of actions which are executed in the exact order of appearance. The following example shows a program in which the Turtle draws a square.

// Tu1.java

import ch.aplu.turtle.*;

public class  Tu1
{
  Turtle joe = new Turtle();

  public Tu1()
  {
    joe.forward(30);
    joe.right(90);
    joe.forward(30);
    joe.left(90);
    joe.fd(30);
    joe.rt(90);
    joe.fd(30);
    joe.lt(90);
    joe.fd(30);
  }

  public static void main(String[] args)
  {
    new Tu1();
  }
}

Explaining the program code:

import ch.aplu.turtle.*;	imports the clas library Turtle
Turtle joe = new Turtle();	generates a new object from the class Turtle
Tu1 () { }	Tu1() is called the constructor of the class Tu1. The constructur always has the same name as the class itself.
joe.forward(100);	Turtle joe move forward 100 steps.
joe.right(90);	Turtle joe rotates 90 degrees to the right.
public static void main(String[] args)	Each Java application needs the method main() which is always defined with the same structure as shown above. The method main() is the first method exectued when running the program. It defines the start of the program.
new Tu1 () ;	This generates a new object of the class Tu1. All defined properties and methods from its constructor are executed automatically.

Remarks about the syntax:

Java is case sensitive and therefor it is necessary to pay great importance to upper and lower case writing. Class names and constructors always start with a capital letter. Keywords such as (new, import ...), variables and methods are written in only small letters.
Each command line ends with a semicolon.
Indentation and right positioning of curly brackets make the program code easier to edit and understand.
Two spaces are used for the indentation and curly brackets are placed on a new line
Comments on one line //
Comments over several lines /* ....... */
Spaces are used as in normal written language