Arrays in Java and how they are stored in memory

Question

I'm trying to understand the array setup in java. Why must you initialize space for each each object in the array, after you have created the array. How is it stored in memory like this:

[object][object]

or like this:

[*class]->[object]  
[*class]->[object]

In other words, what is actually being done in memory. Does array[0] = new class() just return a reference to a reserved location in memory, and the class[] array = new class[10] statement create something along the lines of 10 pointers, which are later assigned to by the new statements?

Answer 1

If you are familiar with C/C++ you can think of Java object references as pointers to objects (or pointers to structs). So:

Person p = new Person();
p.setName("Helios");

is:

• declare a p pointer to a Person struct (in the stack)
• reserve memory for and initialize Person struct
• assign its address to p
• execute method setName on object referenced by p

So when you are doing:

Person[] ps = new Person[5];

you are reserving an array of 5 references to Person. Next you will have to create each real person and assign each reference to a place in the array.

Edit: the (almost) C/C++ version of the previous code

class Person { ... };
typedef PersonStruct* Person; // I don't remember if this declaration is ok
Person p = new PersonStruct();
p -> setName(...);

Person[] ps = new Person[5]; 
// ps is a variable in the stack pointing to the array in the heap
// (being the array five references to the PersoStruct)

and you could do

ps[3] = p;