Secure Java Development Concepts - Expert Oracle and Java Security - page 34

handing and setting memory pointers or references). The main() method is unique, and I will discuss it
more when I cover the static modifier, a little later.
public static void main( String[] args ) {
MyApp2 m = new MyApp2();
MyRef useRef = new MyRef();
m.setRef( useRef );
ARRAY mA = m.getArray();
myRef = new MyRef();
}
Objects
The words object , class , and instance are practically interchangeable. You have classes (theoretically)
before you create any instances. An instance only exists when you instantiate a new one in memory. You
have objects that you can move around and store even if you don't know what type they are. Objects can
be placed on disk or sent across the network, even when they don't exist as instances in memory.
So, you create an instance in memory of a class type, which is an object that can be moved out of
memory (stored on disk), at which point it is not an instance, but can still exist as an object.
Classes and Null
I have already used the term instance in our discussion. In Listing 3-3, there are three instances of classes
being created. To pick out the instances, look for the word new . Whenever you see the statement new , an
object is being instantiated.
Sometimes you will call other classes in order to get an instance of some type; however, even though
you don't use the word new in your code, that word was used somewhere down the line to get the
instance. In the example code for MyApp2 , we see the main() method setting the member mA equal to an
ARRAY that is returned from the method getArray() . You may notice that the getArray() method in
MyApp2 does not use the new statement, and when we define myArray at the class level (top), we do not say
new . Instead, we used the word null in the definition. When the method getArray() returns myArray , it is
returning a null instead of an instance of ARRAY . And when the main() method set mA equal to what's
returned from getArray() , it is setting mA equal to null . This is not very productive, but it is valid Java
code.
To explain null , we will consider what the following statements mean:
MyRef useRef = new MyRef();
new MyRef();
ARRAY myArray;
ARRAY myArray = null;
The first line instantiates a new instance of MyRef class and assigns a member reference (name) to it.
The name we will use is useRef .
We can instantiate a new instance without assigning it to a name as in the second line. This is often
done in graphical user interface (GUI) apps when they get started. The initial Frame is instantiated and
displays itself on screen—no name is required.
We can also create a new name for a certain type of class and not assign it to any instance, as in the
third line. The fourth line is identical in effect to the third line except that in the fourth line we are
specifically telling Java that we are assigning the member name myArray to a non-existent instance,
hence null . In effect, myArray points at no memory location.