[ [[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]]     ]
----
====== Java Classes and Constructors (continued) ======

As we learnt in our last lesson, a java class can be thought of an object
and copies of that class can be made and called separate names.
This is called ** instantiation **.
Constructors are used to instantiate members of a class. You can use
the default one supplied or write your own. When you
overwrite the default constructor the default one is no longer available.

===== Scope =====
Scope in java programming is the portion of a program within which
a variable exists. Whatever set of curly brackets a variables is declared
in then it cannot exist outside of those curly brackets. Try this:
<code java>
    public class TestScope {
	public static void main(String[] args) {
	    private int myNum = 5;

	    // scope is inside the main method
	    System.out.println(myNum);

	    // what do you get?
	}
    }
</code>
Then put myNum inside a method outside of the main method. Its scope
is only the doSomething() method. It does not exist outside the curly
brackets in which it was created. When a variable goes out of scope
it is destroyed.

<code java>
    public class TestScope {
	public static void main(String[] args) {
	    System.out.println(myNum);
	    // what do you get now?
	}

	public void doSomething() {
	    private int myNum = 5;
	    // scope is inside the getNumber method
	}
    }
</code>

===== Java Multiple Constructors =====
We can always use the default constructor of a class if it meets our needs.
It is automatically there when a class is declared. Here are some examples
to recap:
<code java>
    Circle redCircle = new Circle();
    Circle blueCircle = new Circle();
    Employee mechanic = new Employee();
    Employee welder = new Employee();
</code>

As we saw last lesson, we can overwrite our default constructor with our own:
<code java>
    public class Circle {
	int xcenter;
	int ycenter;
	int radius;

	// our overwritten constructor
	public Circle() {
		radius = 1;
	}
    }
</code>

When the above constructor is used the radius has a value of 1.
If we wanted to make a Constructor that allowed us to give our own value
to its radius, for example, we could supply a constructor as follows:
<code java>
    public Circle(int _radius) {
	radius = _radius;
    }
</code>
And it would be called like so:
<code java>
    Circle smallCircle = new Circle(2);
    // radius = 2
</code>
or
<code java>
    Circle bigCircle = new Circle(569);
    // radius = 569
</code>

We could go even further and allow the users of our class to set
every variable of our Circle class by supplying a further Constructor
with 3 arguments:
<code java>
    public Circle(int _x, int _y, int _radius) {
	xcenter = _x;
	ycenter = _y;
	radius = _radius;
    }
</code>

Let us see what our class looks like with our added constructor now:
<code java>
    public class Circle {
	int xcenter;
	int ycenter;
	int radius;

	// our overwritten default constructor
	public Circle() {
	    radius = 1;
	}

	// Constructor with 1 argument
	public Circle(int _radius) {
	    radius = _radius;
	}

	// our constructor with 3 arguments
	public Circle(int _x, int _y, int _radius) {
	    xcenter = _x;
	    ycenter = _y;
	    radius = _radius;
	}
    }
</code>

So now we have 3 choices when we instantiate our Circle class. We can go:
<code java>
    Circle defaultCircle = new Circle();	// radius is 1
</code>
or:
<code java>
    Circle uniCircle = new Circle(27);		// radius is 27
</code>
or:
<code java>
    Circle customCircle = new Circle(100, 75, 14);
</code>
This custom Circle will be created at position 100:75, and have a radius of 14.

Of course, this Circle class would need set and get access methods
to be written, especially if you wanted to move them around or
change them someway.

=== Java Programming Exercise ===
Create your circle class using the above constructors and create all
the access methods as well (getx, setx, getRadius, setRadius, etc..).
Create a class called TestCircle with a main method to test all
your constructors and access methods.

What did you come up with?