Array of Object Refs

Question

I am doing exercises in a book (a couple books) to learn Java. I am a procedural pgmr and want to learn Java.
Question 3 in Chapter 4 says:
Create an array of object references of the class you created in Exercise 2, but don't actually create objects to assign into the array. When you run the program, notice whether the initialization messages from the constructor calls are printed.
In exercise 2, I created an created an overloaded constructor. There are two constructors one that takes an argument and one that doesn't. I think I understand that I am being asked to look to see the values of the variables before the object is actually created but after initialization.
My question is that I am not sure I understand what is being asked in question 3. I have to create an array of object references. I understand object references to be the varable names that hole the contents of the object. I don't understand what I am supposed to put into each (the two) elements of the array.
Here is program written for exercise 2:
//: c04:J402.java
/* This has overloaded constructors. However, the argument must be given. Without it, there is an exception at runtime. */
class Bird {
Bird() {
System.out.println("The object is an egg");
Bird(String age) {
System.out.println("The object is " + age + " months old");
public class J402 {
public static void main(String[] args) {
String age = args[0];
new Bird(age);
} ///:~
Thank you so much for any help.

Answer 1

My question is that I am not sure I understand what is
being asked in question 3. I have to create an array
of object references. I understand object references
to be the varable names that hole the contents of the
object. I don't understand what I am supposed to put
into each (the two) elements of the array.
You need to make three different concepts clear to yourself: objects, object references, and reference variables.
Objects are objects are objects. They are self contained entities that live somewhere in the memory. But, as you can't manipulate the memory directly (there are many good reasons for that), you can't manipulate objects directly.
To get to objects you need references (or pointers). A reference is like a memory address. It normally points to the memory location of an object, but if the reference can also be a special 'null' reference that doesn't point to any object (or to every possible object, depending on how you want to look at it). The JVM takes care of talking to the underlaying object through the reference.
An object can have many references pointing to it but a reference can point to only one object (or no / any object in the case of a null reference).
Reference variables, then, are the ones you have in source code. Here's an example of a reference variable:Bird tweety;
tweety = new Bird("I thought I saw a pussy cat!");(given a hypotetical class that can take a String object as an argument to the constructor)
"tweety" is a reference variable. A reference variable is a variable that can hold a reference -- surprise! :). If not set to any particular reference, it's value may be null. If you now do something likeBird bird2 = tweety;
tweety = new Bird();you first define a new ref. variable "bird2" and set its value to tweety's ref. Then you reset the var. tweety to point to a new Bird object. What happens to bird2? Nothing!! ... but if instead of "tweety = new Bird();" you had written "tweety.setChirp("TWEET!!!");" and then called bird2.chirp(), it would have chirped with "TWEET!!!" because the references of bird2 and tweety point to the same object.
But, you can have references outside reference variables. In reference arrays (or "object arrays", the terminology can be confusing), for instance. Then, the codeBird[] flock = new Bird[2];0) Creates, allocates and initializes a new reference array object.
1) Creates a new reference variable called "flock" and assigns it to a reference that points to the object that was created in 0).
2) Creates 2 more references to Bird objects, initially set to null. No further objects are created. These references can be used like reference variables through flock[0] and flock[1].
(not necessarily in this order)
Now this should answer the excersise 3. The references of flock[0] and flock[1] are left as null and they don't point to any objects - They will only if you do something like "flock[0] = new Bird("chirp!");" or "flock[1] = tweety;"