Getting it right from the beginning : Object vs Reference

[leo donahue]

Would someone be willing to tell me if I have forgotten anything in this example code? I'm trying to comment on everything so that I get this right from the get-go.

class Fruit {  //the name of the class is "Fruit"
 
//variables can be either primitive type or class type
 
//class type variables are called "reference variables"
 
//primitive type variables are either:
// byte, short, int, long, char, float, double, boolean
 
int grams;//a primitive type Data FIELD declaration
 
Fruit apple;//a class type Data FIELD declaration
 
//apple has a "reference to a Fruit" type object.
//class type variables always hold the address
//or "reference" of some object and not the object itself.
//Object variables contain a null pointer upon declaration,
//meaning that they don't point to any created object.
//Fields and Methods are called class MEMBERS
//and in this example the members belong to the Fruit object
 
int totalCalories() {//a METHOD called "totalCalories"
return grams+10;
}//end method totalCalories
 
 //a METHOD called "main"
public static void main (String [] args) {
 
Fruit lemon = new Fruit();
 //instantiate a new Fruit object called lemon
 
System.out.println(lemon);
//lemon is now a reference to a Fruit object, 
 //so it prints the value of the lemon reference 
 //to the Fruit object.
 
System.out.println(lemon.apple);
 //apple still has a null pointer, so it prints "null".
 
System.out.println(lemon.totalCalories());
 //call the "totalCalories" method on our lemon 
 //reference to a Fruit object
 
//System.out.println(grams);
 //non-static variable grams cannot be referenced 
 //from a static context
 
//System.out.println(apple);
 //non-static variable apple cannot be referenced 
 //from a static context
 
//System.out.println(apple.totalCalories());
 //non-static variable apple cannot be referenced 
 //from a static context
 
System.out.println(lemon.apple.totalCalories()); 
//tying to call a method on an object with a null 
//pointer gives a "Null Pointer Exception"

//we used an instance of the Fruit class with a 
//reference variable called "lemon" to access the 
//apple field.

}//end method main
}//end class Fruit

I really appreciate everyone's help.
Leo
[ edited to reduce the left-right scrolling -ds ]
[ January 31, 2004: Message edited by: Dirk Schreckmann ]

[Dirk Schreckmann]

Writing similar example programs is exactly what I started doing when I first began learning Java. It's a practice I still hold today, as I learn some knew API or technology.
It looks like you're understanding things.
//Object variables contain a null pointer upon declaration,
//meaning that they don't point to any created object.
Not exactly. Instance member variables, static class member variables, and array members are always automatically initialized upon declaration. Had your example apple variable been simply declared in a method, it would not have been initialized automatically to null as it is when it's an instance member.
If you haven't already stumbled across it, I'd recommend a read of a couple JavaRanch Campfire Stories - "Cup Size" and "Pass-by-Value Please" - just to help clarify the situation.