Question about Using Objects

Question

Hi Guys,
Need some help here with understanding the basics. Basically i'm getting a bit confused on how objects are instantiated and used in java. Firstly an object of a class is instantiated with the following piece of code;
ClassA obj = new ClassA();This would allow you to use the variable and method of ClassA. Say ClassA has the method set(). You could call it via the obj object with the code;
obj.set();that I understand. You can also declare an annonymous object which is an object that is only really going to be used once for the purposes of the statement. For example, something like;
System.out.println(new Date());That I understand too. However what is confusing me is this type of object instantiation;
public static ClassA obj; You cannot not call the set() method of the ClassA in the same way so why use this? Also this code is confusing me;
obj = new ClassA();Are you saying that a precreated object is now equal to ClassA. Meaning that you can call its set() method in the same way?
What do they mean? Why are they used? Can anyone give me an example of how they would be used?
Any help would be appreciated.

Answer 1

public static ClassA obj;This is a declaration, basically you are telling the
computer that you intend to make a "ClassA" object
and the program will allocate enough memory to hold a
"ClassA" object, although the object "obj" does not
actually "exist" at this time. This is why you can't
use the set() method, because obj doesn't have any
methods at all since it hasn't fully been created
yet.
obj = new ClassA();This is what actually creates the object, according
to the instructions in the object's "ClassA()"
method, which is known as a constructor.
ClassA obj = new ClassA();This statement simply combines the two previous ones
into one line. You are declaring "obj" to be a
"ClassA" which allocates memory to store it, and then
immediately executing the "ClassA()" constructor to
build a new ClassA and store it in the memory space
referenced by "obj". Does any of that make sense?
I'm not exactly a teacher, but I think I understand
your problem enough to explain it.Yes that does make sense. However i have some more questions now if you dont mind answering.
If the code
    public static ClassA obj;simply allocates the memory (instantiate) and does not name (declare) the object then why cant you do something like this afterwards;
obj = new ClassC();or this;
obj1 = new ClassA();The object doesn't yet exist you only putting memory aside for it. Its only at this point that your naming the object and setting the parameters for it.
Why bother even allocating memory for the object without actually declaring the object? Surely combing the two statements is more efficient and makes more sense than separating them. Why do you need to allocate the memory and then give it a name?
Also the following piece of code;
ClassA objA = obj;Since obj is already intialized as being a new ClassA object, then is this simply declaring and instantiating a new object of type ClassA? And what is the difference between this and;
ClassA objA = new ClassA();Thank You