Java Magazine - Premiere Issue 2011

looks like this:

public void act()
{
// Add your action code here.

}
This bit of code specifies what the
turtle does when it acts. The code
between the curly braces ({ and }) is
executed every time the turtle acts. In
this case, all that is written here is
the following:
you click the Act button, the act method
is executed once, so the turtle moves a
little bit ( 2 pixels) to the right. Clicking
the Run button calls the act method over
and over again (until you pause again),
so the turtle keeps moving.

turn( 2);

Errors

You might have noticed that you need to write your code very precisely. Getting even one character wrong makes the whole program not work. The compiler then reports an error message, and you need to fix your code.

If you have not yet seen an error message, try it now. For example, remove the semicolon after your instruction and try to compile. You’ll see what I mean.

Figure 7

JAVA IN ACTION

// Add your action code here.

The double slash at the beginning of the line marks this line as a comment. It is ignored by the Java system and is just a reminder for the human programmer. In other words, this act method contains no code at all. That’s why our turtle does not do anything.

Let’s change that.

3. Replace the comment with an instruction so that your act method looks like this:

Remember: Change the code, compile, create a new turtle, and then run. Experiment with different values here as well.

Sequences of Instructions You can write multiple instructions, as many as you like, into your act method. In fact, you can write one after the other:

public void act() {

move( 2); }

4. Compile, create a new turtle, and try clicking the Act button and the Run button.

So, what did we just do?

The instruction move( 2); tells the turtle to move two pixels forward. The turtle does this every time it acts. When

Methods and Parameters

So, what have we just done?

We have called (or invoked) a method called move and a method called turn, and we have passed a parameter, 2, to each of them.

A method is a bit of behavior that an object knows to execute, and all actors know the move and turn methods (meaning they know how to move and turn).

Both of these methods expect a parameter, which is a bit of additional information that tells them exactly how far to move or how much to turn. Both methods expect a number as a parameter, and we supply that number by writing it in parentheses after the name of the method. And each instruction in Java is ended with a semicolon.

public void act()
{
move( 4);
turn( 2);
}
in the object’s class.

ABOUT US

Try this out. Also, place multiple turtles into the world, and experiment with different parameter values for both the move and turn methods (see Figure 7).