Serge.V Lumber Barn

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[ [[tutorial-en-001 | Intro]]    | [[tutorial-en-002 | Begin]] |
[[tutorial-en-003 | if/else]]    | [[tutorial-en-004 | Loops]] |
[[tutorial-en-005 | Arrays]]     | [[tutorial-en-006 | Graphics]] |
[[tutorial-en-007 | Animation]]  | [[tutorial-en-008 | Mouse]] |
[[tutorial-en-009 | Game]]       | [[tutorial-en-010 | Real]] |
[[tutorial-en-011 | Methods]]    | [[tutorial-en-012 | Class]] |
[[tutorial-en-013 | Class 2]]    | [[tutorial-en-014 | Applet]] |
[[tutorial-en-015 | MouseClick]] | [[tutorial-en-016 | Thread]] |
[[tutorial-en-017 | Button]]     ]
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====== Java Methods ======

A java method is a series of statements that perform some repeated task.
Instead of writing 10 lines of code we can put those ten lines in a method
and just call it in one line. It is like a shortcut.

For example if we had to repeatedly output a header such as:
<code java>
    System.out.println("Feral Production");
    System.out.println("For all your Forest Videos");
    System.out.println("427 Blackbutt Way");
    System.out.println("Chaelundi Forest");
    System.out.println("NSW 2473");
    System.out.println("Australia");
</code>

We could put it all in a method like this:
<code java>
    public static void printHeader() {
	System.out.println("Feral Production");
	System.out.println("For all your Forest Videos");
	System.out.println("427 Blackbutt Way");
	System.out.println("Chaelundi Forest");
	System.out.println("NSW 2473");
	System.out.println("Australia");
    }
</code>

And to call it we simply write:
<code java>
    printHeader();
</code>

And it will print out:
<code>
    Feral Productions
    For all your Forest Videos
    427 Blackbutt Way
    Chaelundi Forest
    NSW 2473
    Australia
</code>

So lets write it in a java program:
<code java>
    public class Invoice {
	public static void main(String[] args) {
	    // call our method in the main method
	    printHeader();
	    System.out.println("You owe us $47.00");
	}

	// declare the method outside the main method
	// but inside the end curly bracket for the class
	public static void printHeader() {
	    System.out.println("Feral Production");
	    System.out.println("For all your Forest Videos");
	    System.out.println("427 Blackbutt Way");
	    System.out.println("Chaelundi Forest");
	    System.out.println("NSW 2473");
	    System.out.println("Australia");
	}
    }
</code>
Compile that (javac Invoice.java) and then run it (java Invoice).
What did you get? You should have printed out the header and then
the line "U owe us... blah blah"

===== Java Method Return Types =====
Notice that our first method had void in its declaration. Void means
that nothing is returned. A series of tasks or calculation may be
performed but nothing is equal to our method. I shall explain
what the last line means soon.

If we wanted something returned we would have to declare it in our
declaration for the method:
<code>
    public int getXposition()
</code>
will return an integer value. Here is an example of this:
<code java>
    public class Area {
	public static void main(String[] args) {
	    int length = 10;
	    int width  = 5;

	    // calling the method or implementing it
	    int theArea = calculateArea();
	    System.out.println(theArea);
	}

	// our declaration of the method
	public static int calculateArea() {
	    int methodArea = length * width;
	    return methodArea;
	}
    }
</code>

What was our output? should have been 50. The method calculateArea()
is equal to 50! We can put the result of a methods processing directly
into a variable or use it directly as in:
<code java>
    System.out.println("The area is " + calculateArea());
</code>
or:
<code java>
    System.out.println("The area is " + theArea);
</code>
Exactly the same thing. Same horse, different jockey.

Any type of datatype can be returned. We can have a double, a boolean,
a String. What else?

Some examples:
  * public int getXposition()
  * public double calculateWage()
  * public String getMyName()
  * public boolean isFinished()
  * public void paintEnemy()
  * public char getMyInitial()

===== Java Methods passing arguments =====
Methods can receive input through the use of arguments.
They are use like this:
<code java>
    public class Enemy {
	int posX;
	int posY;

        public void setXposition(int _x) {
	    posX = _x;
        }
    }
</code>
This method will accept or input an int as specified in
the declaration (int _x). Arguments can be of any java datatype.
They can be double, int, String, char, boolean, etc etc.
The method above allows a manager class to set the xposition of
the Enemy class, probably moving it.

It could be called in the Manager class like so:
<code java>
    Enemy yukky = new Enemy();
    yukky.setXposition(150);
</code>
This would place the yukky Enemy at the x position of 150. Another example:
<code java>
    public void calculateTax(double grossWage) {
	taxation = grossWage * 0.56;
    }
</code>
You would have the variable double taxation declared in the body of
the class and then you pass the grossWage to it and it will calculate
the taxation owing (yuk!). You could call it like so:
<code java>
    double taxation;
    calculateTax(350.00);
    System.out.println(taxation);
</code>
What would be the output of that couple of lines of code? A datatype
passed as an argument must be of the same type as that in the declaration
of the method. If a method is declared like so:
<code java>
    public void setValue(int amount);
</code>
you cant use it like so:
<code java>
    setValue(35.50); // no no this is passing it a double.. wrong, wrong!!
</code>
or:
<code java>
    setValue("Star Wars Sux");   // no no no. Star Wars is great!
</code>

Try to pass the wrong type of data to a method. See the error when you
compile it.

Dont believe me! Test it for yourself.

Also, passed arguments must be called in the order that they are declared:
<code java>
    // declare our method
    public void setAmounts(int _num1, String _name) {
	classesNum1 = _num1;
	classesName = _name;
    }
</code>
This setter methods takes an int first and then a String.
You have to pass the arguments in the correct order.
You MUST call the method like this: setAmounts(27,"Steve");

You will get an error if you try this: setAmounts("steve", 27);

You can have as many arguments as you like but they all still have
to be in the correct order.

You also can have a return value for methods that pass arguments.
For example:
<code java>
    public int calculateArea(int _width, int _height) {
	int myArea = _width * _height;
	return myArea;
    }
</code>

So if you called it like this:
<code>
    houseArea = calculateArea(20, 30);
</code>
houseArea would have a value of 600.

[[tutorial-en-012 | Next lesson]]
we will go on to learn about classes and how to use them.